Computers / Tips and Solutions

The Numbers on Your Memory Card Explained

Memory cards are as ubiquitous as memory itself—almost every mobile device you own has some type of built-in storage, from cameras to iPods to tablets, and almost every device has a slot for adding additional storage space. Storage on a device can range from a couple of gigabytes of memory to dozens of gigabytes. You’ll soon find, though, that it’s never enough.

Photographers Love CF

Photographers are a rare breed. From amateur to professional, photographers depend on gear to make a difference in their art, and some of them produce astounding works of art. Like painters, photographers don’t skimp on materials—they need memory that matches their instincts when shooting—fast, in the moment, and reliable. Photographers like to have their pictures as uncompressed as possible, and to view those pictures in brilliant colors and high resolution. A memory card to match that would also have to perform in the field, away from the studio. And that’s where CompactFlash® came in.

CompactFlash® used to be the standard for high-end DSLRs, some years ago. During the infancy of SD, SD cards could not handle the speeds or capacity needed to store RAW files. Many also believed that the substantial weight and bulk of CF cards made them tougher and more resilient, and you will find CF cards that have higher temperature resistances. But all things change, and today the CompactFlash association backs XQD cards as the official replacement for CompactFlash cards.

As such, the speed of CompactFlash cards has been stagnant for some time, and while they still are very fast cards, they aren’t the fastest around anymore. Still, many camera manufacturers continue to include CompactFlash card slots in their equipment because they are good enough, and CFast and XQD cards are still competing for dominance in post-CompactFlash-card world. UDMA 7 CompactFlash cards are the fastest, and have maximum read speeds of 167 MB/s. This is more than fast enough for most photography and videography uses. Also, because UDMA 7 has been around for a while, pretty much all CompactFlash cameras and card readers support it, which isn’t the case with the fastest SD cards.

  1. UDMA rating  The UDMA rating determines the maximum bus speed at which a card can read, assuming the memory in the card is fast enough to match it. UDMA 1 supports speeds up to 16.7 MB/s. UDMA 7 is ten times that speed, maxing out at 167 MB/s.
  2. This is the minimum sustained write speed—the slowest the card will write. This speed (represented in MB/s) is most important for videographers, since sudden drops in writing speed can cause dropped frames.
  3. This is a rather outdated way of expressing the max read speed.  It is based on the read speed of audio CDs  at 150 KB/s. You can figure out how fast an 800x card is in KB/s by multiplying 150 by 800 and convert KB/s to MB/s by dividing by 1,000 (the answer is 120 MB/s) you could also just go by a card’s stated 120 MB/s speed in figure 5.
  4. Max Read Speed  this is the maximum read speed of the card usually given in Megabytes per second (MB/s). Note that cards rarely are able to sustain these speeds for long periods of time.
  5. Capacity  CF cards range from 2GB to 512GB. Pick a card that can hold everything you need for a prolonged shoot; you won’t be misplacing these chunky memory cards easily.


The Rise of SD Cards

Next on the block were SD cards. SD stands for secure digital, and these cards quickly rose to the top of the memory heap due to their high capacities and improved speeds. In the beginning, however, a standard SD card could only handle a capacity of 128MB to 2GB. Standard SD cards are a rare breed these days, as the industry has moved on to SDHC and SDXC cards. The file format for SD cards was FAT16, originally. They can be used in any device that has an SD card slot.

So what are SDHC and SDXC? SDHC cards use a different file system, FAT32, which can handle capacities between 4GB and 32GB. SDXC cards use exFAT, and support capacities between 64GB and 2TB, though 2TB cards aren’t on the market yet. Hardware-wise, there are no differences between SDHC and SDXC cards; however, older SDHC card readers that aren’t able to understand the newer exFAT file format won’t read SDXC cards. SDXC-compatible card readers and cameras will always be backward compatible with SDHC and normal SD cards reader, though.

Traditionally, SD cards were always a few steps behind CompactFlash cards when it came to performance, but since CompactFlash cards haven’t been updated in a while, SD cards have recently passed them with UHS-II cards. UHS-II SD cards offer read speeds up to 312 MB/s, almost twice the speed of UDMA 7 CompactFlash cards. However, because they are so new, not many cameras and card readers support UHS-II cards, and while UHS-II cards are backward compatible, they won’t work at full speed with older card readers. In fact, most high-end DSLR cameras are still unable to write at UHS-II speeds.

microSD cards are essentially just miniaturized versions of full-size SD cards, and share all the same classifications. Like their full-size brethren, there are microSDHC and microSDXC cards; there are even UHS-II microSD cards, and they have the same speed-class ratings. However, due to their small size, they typically are a little slower and available in lower capacities than the top-of-the-line, full-size SD cards.

But do you need that much storage?

It depends. A 4GB SD card can hold about 280 RAW images and 1,500 high-quality JPEGs. A 128GB card would be a nightmare to manage, with almost 9,000 RAW images and 48,000 high-quality JPEGs. Many photographers prefer smaller-capacity cards that can be easily labeled and managed.

For non-photographers, adding SD memory can be a blessing, especially if your device is low on internal memory. Many tablets, for instance, ship with low internal memory (and is mostly used by the system software and internal processes) and having an extra slot to increase memory could mean the difference between watching a lot of HD movies and being stuck with low-res home video of your sister’s Sweet 16 birthday party.

  1. Maximum Read Speed  This is the maximum read speed of the card usually given in Megabytes per second (MB/s). Note that cards rarely are able to sustain these speeds for long periods of time.
  2. This is another (rather outdated) way of expressing the max read speed.  It is based on the read speed of audio CDs at 150 KB/s. You can figure out how fast a 1000x card is in KB/s  by multiplying 150 by 1,000 and converting KB/s to MB/s by dividing by 1,000 (the answer is 150 MB/s). You could also just go by a card’s stated 150 MB/s speed, in figure 2.
  3. Type  This is the type of card; different card types use different file formats and newer cards won’t work in older card readers. SDHC cards range from 2GB to 32GB, SDXC cards range from 64GB to 2TB.
  4. UHS Speed Class Rating  This is the minimum sustained writing speed of the card; important for video recording. UHS Speed class 3 cards will never write slower than 30 MB/s, UHS Speed class 1 cards never slower than 10 MB/s.
  5. Speed-Class Rating  This is an older speed-class rating. It is redundant of the UHS speed class, but many card manufacturers include it, as well, since many consumer products still recommend products based on the old standard. A class 10 is the fastet of the old speed class ratings and a class 10 card is verified to never write slower than 10 MB/s, class 4 would be never slower than 4 MB/s.
  6. UHS Rating  The UHS rating of a card determines the maximum bus speed at which a card can read, assuming the memory in the card is fast enough to match it. Non-UHS cards max out at 25 MB/s, while UHS-I cards support up to 104 MB/s, and UHS-II cards support up to 312 MB/s. Both the card reader and card must support the same standard to benefit from the increased speeds, but UHS cards are backward compatible with older readers—they just won’t be as fast in them.

So that is everything you need to know—for now—about the memory cards that you use every day. A couple of minor notes: SD cards are easier to lose, especially microSD cards, so get yourself a memory card wallet to hold your cards. Also, SD cards have a unique physical lock on the front of the card that prevents accidental erasure of its contents. Go forth and spread the news; memory cards are as intrinsic a part of your tech life as anything else, and shopping smartly can shave dollars from your tech budget.


Editor's note: This article has been revised as of September 1, 2015. 

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Thanks for sharing this information!
There is a mistake in SD drawing, there are two number 4, one pointing at the 256GB and the other pointing at the 1000x.

Thanks for pointing that out! We changed it to the correct number and callout.

On point 7 there is a mistale regarding the UHS classes: you say that for the end user there is no difference...  In fact there is a huge difference for the end user between Uhs I and II if your camera has the Uhs II pins (a second row of pins, easily recognizable): I have an Olympus omd Em5 mkII that accepts those cards and I can record the large Hi-res raw+jpg photos (about 130mb for 3 files for every shot) at a speed that is almost double compared to that of the Uhs I.  To be correct, I have to say that my camera doesn't support UHS I at the full speed, so Uhs II is mandatory.

UHS-II class SD cards are significantly faster than all other SD cards, writing and reading. If you are cranking on your "motor drive" a UHs-II equipped camera will write faster and allow you to shoot longer before filling the camera buffer. Reading from a UHS-II card with a matching card reader is 40% faster. Well worth the extra $$ and much more forgiving when inserted into a reader the CF. 

I want to buy a 64 GIG SD card for my canon 70D for video recording.  What card speed is the right one to buy? 



Hi Charl -

The 64GB Extreme UHS-I Speed Class 3 SDXC Memory Card from SanDisk features UHS Speed Class 3 compatibility to allow for both Full HD (1080p) and 4K (3840 x 2160p) video recording. With data read speeds of up to 60 MB/s and write speeds of up to 40 MB/s you can also save and transfer other files such as photos, music, and more quickly and efficiently.

Please contact us via e-mail if you have additional questions:

There's also an error for the CF cards, as #3 and #5 are switched.  The graphic says #3 is the read speed and #5 is the write speed, but the narrative has that switched.  So, which is it?

Yes  -  Someone should edit/proof these articles BEFORE they are released.

What is the use life of an SD card? Is it indefinite, or a year, or somewhere in between?

I do not tihnk the useful life is known.

However- I have heard many electronics can suffer from "rot" after say 10 or 12 years.

Practically speaking- I would use only 5 year old or younger memory - and I now own cameras that can record to 2 cards at the saem time for safety,

There is no hard answer to your above inquiry.  The SD Association states that the lifespan of a memory card depends on how it is manufactured. They state that, under normal usage and with the current technology, an SD/SDHC/SDXC memory card will have a lifespan of approximately 10 years or more.  Most people prefer to state write / erase cycles when expressing life expectancy of flash media, and state they are capable of approximately 100,000 write / erase cycles over the life of the card.  While it is subjective, many working professional photographers who shoot 100 or more events a year (with many more write / erase cycles) may prefer, for security and business purposes, to replace their cards every 2-3 years.  For their needs and their heavy usage, the normal dropping price of cards compared to the card size and the increase in card speed over that time frame makes it condusive for their business practices to do this.  More recreational shooters or pros who shoot less events may choose to update their cards every 3-5 years, or as they update cameras.  New generations of cameras usually require an updated card, so updating cards when updating cameras is a good idea.  For the more casual shooter, every 7-10 years (or, being cautious, every 5-7 years), would be sufficient to keep up with technology and to prevent issues with data loss from age.

I've had 2 SD cards (Sandisk) fall apart after about 2 years of use. Luckily is was able to snap together & download images but the cads  were pretty much toast. I always keep spares just in case.

Superbly done – thank you for the excellent explanations.

How long does a memory card last and how many times can you re format a card before problems *****? What happens if the card is dropped?

You have to pick it up...I have been using mine for about 7 years now, reformatting a couple times a month.

I reformat after every off-load of data.  Yeah, it may be a bit paranoid... but I know the card is fresh every time.

As mentioned in a previous reply, the SD Association states that the lifespan of a memory card depends on how it is manufactured. They state that, under normal usage and with the current technology, an SD/SDHC/SDXC memory card will have a lifespan of approximately 10 years or more.  Most people prefer to state write-then-erase (reformat) cycles when expressing life expectancy of flash media, and state they are capable of approximately 100,000 write-then-erase (reformat) cycles over the life of the card.  I still recommend replacing the card before you reach the 10-year mark or the 100,000 cycle mark for safety purposes and to stave against data loss/corruption.

Flash memory have no moving parts within them, and as such, do not incur much damage from dropping.  Most CompactFlash/SD/SDHC/SDXC memory cards are laboratory-tested to have an operating shock rating of up to 2,000 Gs, the equivalent to a 10-foot drop to the floor.  That is not to say drop your cards at a whim, as depending on where/how/into what the card is dropped, the shape of the card can be damaged, causing issues with inserting the card in the camera, or with SD/SDHC/SDXC cards with exposed contact surfaces, where these cards are dropped can cause scratches on the contact surfaces and make the card unable to be read.  The cards can handle more abuse than a drive with spinning disks and moving parts (as some flash drives have accidentally been washed and dried or cameras recovered from lakes with cards that were still working and read-able), but as with any item, try not to purposely inflict damage to the cards.

Thanks for the info. I have been a Press Photographer for nearly 43 years and using Nikon digital cameras since they came out. I have only experienced a card failure once but we do treat our cards fairly rough. I reformat my card after every job, up to 4 times a day. 

Let's not fail to recognize the real advantage to buying fast cards - download time.  When you are out on a job or back at the gore in the evening the last thing I want to do is watch data transfer, therefore I tend to buy the fastest cards possible, even if the cameras don't require the speed, so I can get to reviewing images sooner.  The reality is time is money and investing in fast media will pay off over and over again.

This article has an odd bias leaning toward the older CF technology. Granted, they are a little less easy to misplace, but that is all. The fact that SD cards come in two sizes is irrelevant, as the smaller one is obviously for smaller devices and is a clear advantage for over CF. My phone uses a micro SD card, and I can only imagine how big my phone would be to accept a CF card(or for that matter, a full size SD). Also, this statement is misleading: "SD cards, meanwhile, use their own protocol, which changes almost yearly and, with each change, may make your memory card incompatible with your devices."  - While newer format SD cards may not work in your old device, the older cards will work in newer devices, as the devices are backward compatible. 
 I shoot professionally with a Mico 4/3 camera, and do not "Love CF", my camera would also have to be much bigger to accept that old clunky CF card, over the SDXC card that it uses. And I don't expect my camera to be compatible with some future SD card format that can go 100x faster and store 10TB of data, but I would expect a new SD compatible camera to handle that and be backward compatible with SDXC (even though it might be too slow for it, or not have the capacity I need)

This article should lose it's bias and just explain the numbers, as the title suggests, but clearly the author is having trouble moving on and letting go.

Oh, and by the way, it is NTFS, not NFTS.  

That has nothing to do with "Compatibility", but rather has to do with needed capability. It can still access and save to the card, but not to the needed speed for certain functions. Just like I mentioned that a future camera could be backward compatible, " (even though it might be too slow for it, or not have the capacity I need)", that is true for current cameras and older cards. It is true as well for older CF cards, they can also not handle the speeds needed for newer cameras. 

It's not completely true that Ed cards are backward compatible. The Sony a7xx series of cameras, for example, will not write their better video codecs to older SD cards. And by older, I mean anything but the latest SDXC cards.

When Canon added an SD card slot to their pro camera, the 1D Mark II. I compared two Sandisk cards one CF and one SD both rated for 10MB/sec. I filled the camera buffer with a burst of raw dots and the SD took ten times as long to clear the buffer. Probably a computer could read both cards at same speed, but SD has more processor overhead than CF. That's why pro cameras now are only CF. SD is OK for Micro 4/3, but their is a reason you don't see pro sports shooters with Micro 4/3 cameras. When speed matters, go CF. My 1DX has two CF card slots. My   My 7D, non-pro camera has a CF and an SD because it is a compromise camera. Just makes different compromises than the little 4/3 sensors.

Here, here!

My thoughts as well.

"Oh, and by the way, it is NTFS, not NFTS." It was a typo it is correct later in the paragraph

What micro SD card do you recommend for my Gopro 4 silver. I want to be able to shoot some 4k video.

Hi Cal -

SanDisk - 64GB Extreme UHS-I microSDXC Memory Card (U3, Class 10)

Please contact us via e-mail if you have additional questions:

I've always heard that the "X" factor was times 150 KB to determine the CF Card speed.

Where did the 6.666 come from?

Divide 1000kb, which is 1MB, by 150kb

Norman D's comment is correct.  Divide 1000kb, which is 1MB, by 150kb.

Some of the assertions are, well, odd.

  • CF cards can be formatted as NTFS, EXT3, or anything else that could be put on a IDE disk drive.
  • CF cards with SATA are not CompactFlash, but are CFast. Totally dfifferent, cannot be interchanged.
  • No mention of the CF "Types"? If you try to put a Type II card into a Type I device, you'll be unpleasantly surprised.
  • An SDXC device is required to be backward compatible with SDHC and SD, similarly SDHC devices must work with SD.
  • Some devices try to read the performance of a card, and act accordingly. Famously, a Canon HD camcorder (mentioning no model numbers) chose to interpret a "Class 10" card as "Class 1", and refused to work with it, claiming it was too slow. Class 6 worked fine...
  • Bigger issues when selecting cards (CF or SD or anything else) are the issues of NAND flash management: wear-leveling, over-provisioning, etc. It's always safer to buy a high-temperature rated card, because if it will operate well at high temps, it will preserve your data better at normal temps.

You have another mistake to correct.  Please note, MB/s is *not* equivalent to Mb/s.  If they were, then there wouldn't really be much of a speed increase for point #2 on CF cards.

Also, people are asking about lifetime of flash memory.  Unfortunately, there isn't really a good answer.  It turns out, every piece of electronics will eventually fail.  How long?  The more you use it, the closer it gets to end of life.  The older it is, the closer it gets to end of life.  So if it sits on a shelf, it will still fail, just a lot longer than if it gets used.  The more you use it, the more likely it is to fail.  Plus, keep in mind that static discharge can destroy some flash memory, or at least shorten it's lifetime.

Lastly, I use both CF and SD cards daily.  Please remember that CF cards have more shielding.  And to me, size is not an issue, as each are small enough to not really matter.  Granted, I would not want a CF card for a phone, but outside of that, I actually prefer CF cards.  I have found that the good CF cards (SanDisk Professional or Lexar Professional) do last longer than SD cards or even gardent variety CF cards.  They appear to have more shielding and more electronics for static discharge protection.  (Just my opinion here.)  So if what you put on the flash card is important to you, then the device you buy should matter just as much.  Don't be surprised if that super cheap flash card (CF or SD) fails and loses all of your precious memories.  You did remember to back that information up, right??

Nice article on an important issue. Thanks.

I have many CF cards that do not show the write speed. When I look at the Sandisk or Lexar websites, most of the specs give the Read Speed, then say "Wrrite slower"  so I never know when buying a card which is better card for the camera where writing speed can be critical.

'The formatting protocol for SD cards was FAT16 a few light years ago'
Since when are lightyears a unit of time…?

It's also interesting how the article mentions NTFS (or NFTS as it's typoed at one point) as filesystem, while the standard for SDXC (as far as I know) is exFAT and not NTFS. doesn't even mention NTFS.

Thanks for this information.  Since my nature is to nit-pick, the CompactFlash “hover over” information for bubbles 3 and 5 is reversed.

Thanks for the great explanation! Small typo, you have NFTS instead of NTFS in paragraph 7. Thanks again!

A light year is a unit of distance, not a unit of time. "Light years ago" is meaningless. Would you also say that something occurred "meters ago"?!

As pointed out below, there are numerous errors in this article.  So many, it calls into question the credibility of the author.

This is an article aimed to non-techies, who want to know the technical details, about the information on these memory cards.  When writing an article of this type, the author needs to take great care to carefully and accurately define ALL acronyms used, then be consistent in their use.  Neither was done here.  One example: The use of “MB” & “Mb”; they are NOT interchangeable.  “MB” = megaBYTE, “Mb” = megaBIT. A BYTE = 8-BITs; MBs are NOT equal to Mbs

Additionally he offers, without explanation, a formula for calculation of Read Speed by dividing the speed multiplier number on the card by 6.6666.  It would have been just as simple to so say multiply by 0.15. The “X” value is a constant based on the Read Speed of the original Audio CDs of 150kB/s (0.15MB/s).

All-in-all these errors, as I said above, bring into question the credibility of the entire article.  At the very minimum the article was poorly worded and sloppily written, and should have been proof read/edited for clarity & technical accuracy before publishing.

A better description can be found at: &

I enjoyed your article about card nomenclature but I think that there is an error.On point #2 you say that the VDMA 7 is ten times the speed of the 16.7 MB/s transfer speed maxing out at 167Mb/s.

I think that that would make it 167 Mega bits per second instead of 16.7 MegaBytes per second.

One Mega Byte(MB) is made up of 8 MegaBits (Mb).

Thanks ,


Excellent point, as it sure does get confusing and one can't know if/when the writer knows the difference. In this piece, I think he does, just some typos. But more generally the size of that "b" can lead to a lot of trouble.

The only obvious difficulty is that developments in card technology are rushing ahead of those in camera technology. Older camers won't accpet newer cards, if I got the gist here. Great info, by the way. But then what, and SD cards are becoming scarce? Would that manufacturers could produce legacy cards that will also perform with the latest equipment. Otherwise, one's camera might have a lifespan of just a few years, or so it sounds like to me. I shoot very few photos and need only one, relatively low-capacity card to get the job done before I upload. I wonder now how long until my equipment is obsolete? Can't shake that feeling of being hustled, like when the I-Pod came out rendering one's CDs obsolete, or CDs vinyl obsolete.

can i get a faster writing speed card?

Very possibly.  In order for us to confirm, can you please reply back and state the specific camera model you are purchasing a memory card for, and also please indicate the current fastest card you are using in it (please list both the make/model and speed), and if there are any faster compatible options, we will offer recommendations for you to regard.  Thank you in advance. 

The IEEE defines reliability as the ability of an item to perform a required function under stated conditions for a stated period of time.  In the long run, everything wears out.  Do not assume that a product will exhibit a constant failure rate.

Bottom line: back up your data, and implement the military maxim “one is none; two is one”

People have calculated empirical population failure rates as units age over time and repeatedly obtained a graph or failure rate curve, the shape of which has become widely known as the "Bathtub" curve.  The statistical model that describes the bathtub curve is the Weibull probability distribution.  A decent explanation is at:

The Bathtub Curve and Product Failure Behavior Part One

The Bathtub Curve and Product Failure Behavior  Part Two

Thank you, thank you, thank you!  Those numbers on SD cards have been mysterious for some time now, and it's been a bit difficult to get a straight answer about them.  This was very, very helpful!

Hovering over the numbers doesn't show anything. That and the corrections by readers might encourage readers to look elsewhere than B&H, or any dealers, for more exact information.  I buy almost all my camera gear from B&H and am pleased with their customer service.  Although all the major sellers mysteriously seem to have the exact same price for major items.  This also makes me suspicious.

Hello and thank you for sharing such information with clarity. Much needed.

Regarding the toughness of SD vs CF. I use SD exclusively now and I have noticed the plastic tines over the contacts can become damaged too easily. I'm not sure how this affects the contacts themselves, but if the tines had been double the width they would stand up better. The CF package is clearly very tough, but I've heard of people accidently bending pins in the reader section.

Great helpful information.  Thank you.

I have found the CF cards to be more durable than the SD cards.  I keep my CF cards in a pocket sized vault so as not to be exposed to the elements.  I also back up the photos to my portable hard drive (1TB) just in case of damage to the card itself.  I keep a spare CF card in my case just in case I start shooting and forget how much memory I really need.  I also have in my case an SD adaptor of a CF card.  I have founf that CF cards are hard to come by.  So I can buy a SD and insert it into the CF adaptor just in case.

I use a sharpie to number the cards and to put my name and number just to keep track since they all look alike after a while.

Happy photographing!        Steven Gewirtz