Symbols on SD Cards and their meaning

If you were planning on getting yourself a new SD card, you are probably wondering: what on earth all of these numbers and symbols on SD cards mean? Maybe besides the obvious ones like the capacity number.

We have you covered! Here is a full list of all the symbols on an SD card, along with what they stand for.

SD card storage capacity

Let’s get this one out of the way first, as most people do understand this one. The number that sits next to GB is the capacity of the card. Usually, it’s the largest written element on a card. What you see is what you get, for the most part.

SD Card Type

Among all memory cards these days, there are only two types of SD cards. SDHC and SDXC. There used to be the classic SD, but it fell out of favor due to a reason that will immediately become apparent. See, the card type doesn’t affect the form factor or speed of the card (at least not directly). It affects the capacity size of both the card itself and the maximum size of a file on that card.

• SD (Secure Digital) cards use the FAT12 or FAT16 file system and have a capacity between 64MB to 2GB. The max file size when using FAT16 is technically 4GB, but most of its cards are smaller than that.
• SDHC (Secure Digital High Capacity) cards use the FAT32 file system and have a capacity between 4GB and 32GB. The max file size when using FAT32 is 4GB.
• SDXC (Secure Digital Extended Capacity) cards use the exFAT file system and have a capacity between 64GB and 2TB. The max file size when using exFAT is 128 PB.

Read speed

Usually, the only number next to MB/s is read speed. If not, it may be signified by the letter “R” or the word “Read”. It’s the second biggest element on a card, besides maybe the company/card name. Unlike capacity, you aren’t guaranteed to get the number shown here. This is due to a variety of factors, so be sure to take it with a grain of salt and just treat it as the maximum potential speed you will get out of the SD card.

Better read speed will allow you to read/open files faster. Usually less helpful than write speeds for photographers/videographers.

Write speed

It is not always mentioned on a card, but if it is, the write speed will be found next to the read speed and distinguishable by the letter “W” or the word “Write”.

Better write speeds will allow you to create more files faster.

Bus interface and camera compatibility

You can find a Roman “I” or “II” on your card. That is the bus interface. It determines the maximum bus speed at which a memory card can transfer data. Modern SD cards are usually split into one of two categories of bus interface:

• UHS-I – allows data transfer speeds up to 104MB/s
• UHS-II – allows data transfer speeds up to 312MB/s

You can use UHS-II cards in UHS-I devices and vice versa, but your speed will be capped to UHS-I.

Speed Class Ratings

This is a useful marking that defines the minimum sequential write speed of a memory card. This reading is probably the most helpful one to those who wish to know if their card will suffice their needs or not.

It is, however, a bit confusing. See the way SD card companies named speed class ratings changed with the years. In the beginning, we had five ratings: C2, C4, C6, C8, and C10. C10 was guaranteed to sustain a write speed of 10MB/s. You can recognize what level your card was by finding a number inside a “c” letter on the card.

Later, we got the UHS speed class. It only has two ratings: U1 and U3. Similar to the original rating series, you can tell which level your card is by finding a number inside the letter “u” on your card. This time, you multiply the number by ten, and you get your minimum sequential write speed. U1 for 10 MB/s and U3 for 30 MB/s.

Lastly, we got the video speed class. Comprised of V6, V10, V30, V60, and V90, they give you a minimum write speed according to the name. Meaning V60 will have a minimum read of 60 MB/s, v90 will be 90 MB/s, and so on and so forth.

You might then notice there is some overlap between these ratings. This would be fine if companies stopped using old ratings when new ones came out, but that’s not the case. Sadly, you would need to remember how all rankings work to guarantee you won’t buy the slower card by accident.

How to choose a memory card?

Now that you know what the symbols mean, you better understand what speeds your card will have. But how much speed do you actually need?

To answer that question, it’s important to ask, “What are you going to shoot with your camera?” If all you are doing is shooting a couple of photos at a time, then you don’t need a fast SD card, even if you’re shooting RAW.

If, on the other hand, you use something a bit more demanding, like 4K video recording, for example, you might need a faster card. That depends on what format you’re recording in and what camera you’re using.

Demanding video formats usually mention the amount of megabits they use per second. Do not be confused with megaBYTES you find on SD card speeds. See, one megabyte (MB) roughly equals eight megabits (MB).

What that means is that if the 4k format in your camera was listed for 400 megabits per second, you should probably get a card with a V60 rating. Why? Because 400/8=50 and a V60 SD card has a minimum write speed of 60 MB/s, you are guaranteed to handle that format with that card.

Just be wary not to pick up a fake memory card, and you’ll be good to go.

Conclusion

These are all the major symbols you are likely to find as you’re going through SD cards. Hopefully, this will help you next time you’re going shopping for a new SD card. If you need a decent one, it’s hard to go wrong with the SanDisk Extreme PRO series, which you can get here, Lexar professional series (here), or ProGrade digital (here).