SD Card: Speed Classes, Sizes, and Capacities Explained
Getting yourself in a memory card muddle and not sure which card to buy? We look at memory card speeds and the fastest memory card on the market to help explain the differences so you can find out what’s the best card for you.
Not sure what the difference is between a £50 Class 2 SD and a £450 Class 10 SDHC memory card? We’ve split them up into their categories and broken down the speed jargon by translating it into real speed ratings so you can decide if a certain memory card is worth the extra money.
SD card types
SD, SDHC and SDXC
As SD cards have begun to physically shrink down in size over the years, the SD card can be considered the ‘full-size’ variant. It’s typically what is used in most modern digital cameras and nearly all laptops will come equipped with an SD card reader slot.
Where things can become confusing is when you see ‘SDHC’ and ‘SDXC’. The first stands for ‘Secure Digital High Capacity’. This was initially introduced to cover SD cards with a capacity above 2GB and below 32GB. The latter stands for ‘Secure Digital eXtra Capacity’, which go far beyond 32GB in size. You can buy SDXC cards with a whopping 2TB of storage.
SD, SDHC and SDXC all physically look the same, therefore they’ll all fit in a regular SD card slot. Problems might arise depending on the device you’re looking to use them in. Older devices that pre-date SDXC’s introduction might therefore not be compatible, so be sure to check your product’s specifications. Devices are backwards compatible, so an SDXC-compatible device can use SDHC and SD cards. An SDHC-compatible device can use SDHC and SD cards, but will not be able to use SDXC cards.
MiniSD and MiniSDHC
The MiniSD was the first miniaturisation of the SD card and is around half the height of the original SD card. You can also find miniSDHC cards, which expanded storage to 4GB. Again, you’ll need to make sure your device specifically supports miniSDHC rather than just miniSD. The miniSD card never gained much traction, however, so devices that use it are relatively uncommon. Often, miniSD cards will come with an adaptor that can convert it into a full-size SD card, which makes it easier to use with laptop card readers.
MicroSD, MicroSDHC and MicroSDXC
The microSD is the smallest version. You might have also seen it referred to as TransFlash or abbreviated as a TF card. MicroSD cards, as the name implies, are physically tiny and came about predominantly to be used in smartphones that would benefit from a much smaller card.
Like the full-size SD card, there are also microSDHC and microSDXC variants that expanded the storage beyond the initial limitations. Right now, the largest microSDXC available is 128GB, which isn’t as much as the largest SDXC card but then you’re limited by the physical size of the card. When buying a microSD, microSDHC or microSDXC card you’ll usually get a full-size SD card adaptor as well. Again, SDXC-compatible devices will be backwards compatible with microSDHC and microSD cards, and microSD cards can be used in microSDHC devices.
 SD card speed classes
The next area of confusion is around speed classes. These are how different SD, miniSD and microSD cards are rated in terms of read and write speeds. These are important particularly when the cards are used in camcorders or action cameras as the speed of the card will limit the video resolution and bit rate you’re able to record. High-resolution and high bit rate video requires a lot of data to be written to the card very quickly. Still cameras with high resolutions and fast burst shooting will also take modern cards to their limits.
The SD Association devised a way to standardise the speed ratings for different cards. These are defined as ‘Speed Class’ and refer to the absolute minimum sustained write speeds. Cards can be rated as Class 2 (minimum write speed of 2MB/s), Class 4 (4MB/s), Class 6 (6MB/s) or Class 10 (10MB/s). It’s important to note that these are the minimum, so it’s entirely possible a card can achieve faster speeds but these give you an impression of the least you can expect.
Many SD card manufacturers will also list a specific speed alongside the Class rating. This means a card can be Class 10 but also be listed as ‘up to 80MB/s’. The wording is important, as that’s the best you can expect but not necessarily what you’ll always achieve. You might also see a description such as ‘533x’. This refers to a multiplication of the speed of an old CD-ROM (150KB/s). So in this case 533 x 0.15 = 80MB/s (as there are 1,000KB in a MB). On any type of SD card, the Class rating is denoted by a number inside of a C symbol.
UHS Speed Class
The next rating is the UHS Speed Class. This stands for Ultra High Speed and refers to minimum sustained writing performance for recording video. UHS came about due to 4K-capable video devices needing faster write speeds.
The SD Association has two UHS Speed Classes, UHS Speed Class 1 and UHS Speed Class 3. UHS Speed Class 1 supports a minimum 10MB/s write speed, whereas UHS Speed Class 3 supports at least 30MB/s write speed. The UHS Speed Class is denoted by either a 1 or 3 inside of a bucket U symbol. As a rule of thumb, 4K-capable camcorders will usually require at least a U3 rated SD card.
UHS Bus Speed
Things get a little more confusing as UHS Speed Class-rated devices will also use one of two UHS Bus Interfaces that indicates the theoretical maximum read and write speeds. They’ll be listed as either UHS-I or UHS-II to show which interface is used. UHS-I devices have a maximum read speed of 104MB/s, whereas a UHS-II card has a maximum read speed of 312MB/s. Note, that unlike the UHS Speed Class, these are not sustained speeds.
Buying the right SD card
It might sound obvious, but the safest way to make sure you buy the correct card is to first consult your product’s user guide or specifications, at least to be sure if your device takes full-size SD or microSD cards. Checking before you buy an SD card is particularly important if your device has specific requirements, such as many 4K camcorders. These will usually explicitly state what type of memory card is required in order to record at the top resolution or bit rate. The Sony FDR-X1000V action camera, for example, clearly states that it needs a Class 10 U3 microSD card in order to record in 4K.
If you’re not recording in 4K, the speed might not be so important. A faster-rated card will still be useful if you’re using the card to expand the storage in a smartphone or laptop, as a faster read and write speed will make it more responsive. Faster cards in a still image camera will also mean being able to take more or faster photos in burst mode, as these can be written to the card far quicker. Class 10 cards are usually not much more expensive so it’s worth paying the extra.