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. 
  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.
  7. Capacity  This is the card's capacity: SD cards range up to 2GB, SDHC cards range from 2GB to 32GB, and SDXC cards range from 32GB to 2TB.

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. 

Discussion 77

Add new comment

Add comment Cancel

Can you suggest a card to be used for small bluetooth speaker? TF's were mentioned. 

We would need to know the specific brand and model of the speaker in question. Please send us an email,, and we'll be happy to make a recommendation for you. 

how do CFast cards and XQD cards differ from SD cards?

I would read our more updated article The Fastest Memory Cards Money Can Buy which will more directly compare these three different card formats. 

I would read our more updated article The Fastest Memory Cards Money Can Buy which will more directly compare these three different card formats. 

Great info. I would refer to the numbering that you have on the diagram in the text for easier referencing of the explanation. It would make it much easier to follow along especially for people who are fairly new to the technical lingo. 


I have a fujifilm FinePix 825 camera and need an SD card recommendation for routine storage and usage for taking several hundred photos before download to my laptop for editing etc.  What do you recommend??

Hi Robert:

The Delkin 2GB SD 115x Memory Card features a maximum read speed of 17MB/s and a maximum write speed of 9MB/s. It prevents against accidental data loss with built-in write-protection. To ensure the highest quality possible, Delkin's memory cards are put through several rigorous quality assurance tests before being released to the consumer. 

I just purchased a Canon Powershot SX60HS camera.. What is the best memory card ratings for taking landscape photos as well as videos ?

While there is no specific recommendation for a memory card for landscape usage, if you are looking for the fastest performing card compatible for use with your camera, the Canon PowerShot SX60 HS Digital Camera's instruction manual indicates the camera supports UHS-I memory cards, which would be the fastest card compatible for use in the camera.  As such, I would recommend a UHS-I card for both video and your still landscape photo needs. At the time of this reply, I would state the SanDisk 32GB Extreme UHS-I SDHC Memory Card (2-Pack), B&H # SAESD32GBCP, would be the best offer, and the SanDisk 16GB Extreme UHS-I SDHC Memory Card, B&H # SAESD16GBC, would be an economical option that will also work for your needs.

searches answer everything i did not ask and nnothing that i do. I need a cf card of 256 mb no faster or slower than a class four. all the technical snow storm driven in my face tells me how do do complex math to deterimine read and write speeds, but nothing defines a class four in those terms. can someone, PLEASE tell me where I can buy a class four 256 mb cf card? Please?

Unfortunately, there are no "Class 4" 256MB CompactFlash memory cards for two reasons: 1) Due to the larger megapixel sizes of cameras and other electronic devices, requiring larger storage capacity, smaller cards under 1GB are no longer availabe (to my knowledge -- I cannot predict what may still be available in someone's inventory on eBay, Craigslist, or sold used or through vendors on  The smallest CompactFlash card we currently carry is 4GB (which is 4000MB, over 15x times larger than a 256MB card).

And 2) When 256MB cards were originally created (more than a decade ago), video was not being recorded in digital cameras, and Speed Class ratings had not yet been invented.  Also, "Speed Class" designations only apply to SD/SDHC/SDXC memory cards.

SD/SDHC/SDXC cards have additional symbols to defind speed, which are found on their card lablel, one being the the Speed Class of the card. While CompactFlash cards list the card's capacity, and some indicate the read/write speed or "X-speed", with newer CompactFlash cards also indicating a VPG (Video Performance Guarantee) symbol, CompactFlash cards do not have "Speed Classes," which is dedictad to SD/SDHC/SDXC memory cards, just as the "VPG" is dedicated to CompactFlash cards. As such, you will not find a CompactFlash card with a Speed Class rating or a Class 4 card, as it does not exist with CompactFlash cards.

While most all current CompactFlash cards should be faster than a Class 4 SD/SDHC/SDXC memory card, if you need a card to have a particular listed speed performance for video needs, then you would have to find a card with a VPG rating. At this time, there are only two ratings, which is either VPG-20, which would have a sutained write speed of 20MB/sec, or VPG-65, which would have a sustained write speed of 65MB/sec. In comparison, a SD/SDHC/SDXC card with a Class 4 rating only maintains a write speed of 4MB/sec, so the sustained write speed of even the slowest VPG-20 card is still 5x times faster than a Class 4 SD/SDHC/SDXC memory card.

Hi, I need to buy a 32 GB memory for my DJI Mavic Pro. Which one is the best or which you recommend? Thanks

You might checkout the SanDisk 32GB Extreme PLUS UHS-I microSDHC Memory Card.  It would support all the resolutions of the Mavic's camera. 

which is more faster class 10 or class 4.. im really confused

A class 10 card will be faster than a class 4 card.  Class 10 would be the fastest of the older speed class ratings. 

Yes excellent article. I'm wondering about a SD to CF adapter as CF is a lot more $$$ than SD for teh same speed and capacity. Any ideas? It's for my Canon 5D MK II. Thanks!

You could look at the DigiGear Extreme SD-HC-XC to CF Adapter.  Though, I would go with an actual CF card.  The adapters can be hit or miss, and will likely have a negative impact on performance.

Hey this was very helpful but i was wondering about Micro Sd cards...

As Cris Silvestri stated in this article, “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.”  If you have specific questions about microSD cards, I would suggest sending us an email:

Well written, extremely helpful, thanks!

Thanks for reading, Alan!

Thank you very much, for years i was confused about the numbers on the memoy cards. But still confused which card is best for my Canon 650D DSLR camera. If you guide me I will be very oblidged. Thank you once again

You might look at a 32GB SDHC memory card from Lexar or SanDisk.  They make some of the better cards on the market.  A UHS-1 U1 or Class 10 option both would be plenty fast for your 650D (T4i).

My DMC - TZ3 takes great photos. I cannot buy a memory card that works in my camera. My card that works is a SD  2.0 GB  The ones that I bought was SDHC and SD X.  What am I doing wrong ?

It looks as though the DMC-TZ3 is compatible with up to 32GB capacity SDHC memory cards.  So, it wouldn’t be compatible with SDXC memory cards.  If you have been using 32GB or smaller SDHC cards and haven’t been able to get them to work, I would suggest sending us an email letting us know which cards you have tried.  We would then be able to research what the issue might be, and recommend a card that could work.

Great info. Thanks. Especially for the demistifying the 100X. I could never figure that out. Question, have you read about the fact that SDs can break easily. I had this problem a week ago. The store gave me a replacement

but the one I had bought was out of stock so they gave me another brand... I was lucky. The Samsung was a better choice. The casing is a bit thicker on the sides. I recommend them and will only buy SDs which have a better thickness. I am glad to own a camera with 2 SD slots.

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 

Great helpful information.  Thank you.

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.

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

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.

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!

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

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 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.

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.

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: &

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"?!

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

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

'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.

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.

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??

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.

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.

Show older comments