Confused with all the numbers, letters on SD cards? Here’s what they mean

Q: Can you help me understand all the numbers on SD camera cards?

Today’s memory cards comprise a confusing batch of geek speak, but once you understand some of the basics, it might make it easier to digest.

Physical size

Most of today’s devices use either SD or microSD cards, which you can determine by looking at the card slot on your camera.

Storage capacity

The easiest specification to digest is the storage capacity, which is represented in gigabytes, abbreviated as GB. It’s easy to assume that the bigger the number, the better, but this isn’t necessarily the case.

Not only can it be cheaper to use multiple smaller cards, but then, you won’t live and die with just one card — they do degrade, get lost or experience data corruption.

SD vs. SDHC vs. SDXC

A memory card’s capacity is determined by the file system being used to store data designated by the type of card.

SD (Secure Digital) cards are the oldest and least-used, and are limited to 2 GB of storage.
SDHC (High Capacity) cards can store up to 32 GB of data.
SDXC (Extended Capacity) cards can store up to 2 Terabytes (2000 GB).

Older devices may not be able to use the SDXC format, so make sure your device supports these larger cards before buying one.

Rated speed

This is where things can get a bit confusing, as various designations can refer to the card’s speed. It’s represented in MB/s (megabytes per second), or a large number followed by an “x” — or sometimes both.

The “x” designation is a marketing term: 1x represents 150 kb/s, so 600x is the same as a 90 MB/s rating (600 x 150 = 90,000). This number typically represents the “read” speed, which is generally higher than the “write” speed.

Speed Class

This specification is represented on the card as a number inside of the letter “C.” It represents the minimum write speed and is most important to those shooting video or very large images in succession (burst mode).

There are four classes: 2, 4, 6 and 10. The number represents the minimum sustained megabytes per second (MB/s) write speed — the higher the number, the faster the sustained speed.

UHS Speed Class

Faster SDXC memory cards will have the UHS rating, or Ultra High Speed, represented by a number inside the letter “U.” U1 means it’s 10 MB/s; U3 means it’s rated at 30 MB/s.

UHS Bus Class

Different from the UHS Speed Class, this rating refers to the “bus interface” and is represented by a roman numeral. Think of the bus interface as the number of lanes on a freeway, while the UHS Speed Class represents the speed of individual cars.

Video Speed Class

If you’re trying to shoot extremely high-resolution video (4 or 8K), this class of cards, designated by the letter V and a number that ranges from 6 to 90, offers the fastest sustained write speeds.

Overkill vs. future-proofing

If you have older equipment that you plan on upgrading soon, getting a faster card than you currently need may make sense. Otherwise, don’t spend more money than you need to — there’s no performance benefit.

To help you figure out what you actually need, check out SanDisk’s web tool. Once you figure out the specs for your desired card, you can easily comparison-shop around the internet.

Ken Colburn is founder and CEO of Data Doctors Computer Services. Ask any tech question on Facebook or Twitter.