SD Card Speeds and What They Mean (2023): UHS

Jun 28, 2023

Eric Ravenscraft

You might not think too much about how fast your SD cards are, but picking the right memory card can upgrade your gaming experience, help you take better photos, and even make it possible to record super high-resolution video. If you’ve never given thought to the speed ratings on your cards, we’re here to help make the terms easier to digest—and we have a few recommendations too.

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First, it's worth exploring why you might care about SD card speeds. We’ve reached a point where most SD cards you can buy off the shelf are good enough, and if you never pay attention to speeds, you’ll probably be OK. So, does it make a difference?

Depending on what you use your SD card for, yes. A lot. For example, the Nintendo Switch has limited storage, and getting an SD card can let you install more and bigger games. Any SD card will help, but the faster the card, the quicker those games on it will load. If you’re getting sick of staring at loading screens, a faster SD card might be just what you need.

There are also things you just can't do with slower SD cards. Photographers who use extremely fast continuous shooting modes (where you might capture many photos every second) will need faster cards to keep up with storing so many photos at once. Likewise, many cinema cameras that can shoot in 4K or even 6K can't function without extremely high-speed SD cards. (In the case of 6K, they may even need something faster, like a CFexpress card or even a solid-state drive.)

The label on SD cards is a mishmash of different specs and signifiers, all crammed into a tiny space. But when it comes to speeds, there are only a few that really matter—and if companies provide the speeds of their cards directly, you might be able to ignore them entirely.

First, there's Speed Class. This is indicated by a C symbol surrounding a number. On most modern SD cards, that number is 10, which is the highest classification for this class. Class 10 cards are capable of at least 10 megabytes (MB) per sec. Since most cards are capable of much, much faster speeds, this symbol is a bit outdated. However, it's worth keeping an eye out in case you come across an older card with a lower Speed Class number.

Next, there's the more useful UHS Speed Class. There are only two of these: U1 and U3. U1 cards have a minimum speed of 10 MB per second, while U3 must be at least 30 MB/s. In both cases, they’re often capable of much, much more. These are usually indicated by either the number 1 or 3 inside a U symbol.

Brenda Stolyar

WIRED Staff

Medea Giordano

Adrienne So

Confusingly, there's also the UHS bus type. For ultra-fast SD cards, these are probably more relevant. UHS-I cards have a theoretical maximum of 104 MB/s, while UHS-II cards top out at 312 MB/s. These aren't necessarily their actual speeds, just what the connector is capable of. If you’re doing something heavy-duty, like recording 6K or 8K video, you’re probably going to be looking for a UHS-II card. These are usually indicated by the Roman numeral I or II.

Finally, there's the Video Speed Class. These are denoted by a V symbol, followed by a number representing the minimum write speed, in MB/s. The speed classes are V6, V10, V30, V60, and V90. For example, a V6 means the card has a minimum write speed of 6 MB/s. Cards can obviously be faster than that, but if you need to hit a minimum speed, this is a good symbol to look out for.

It's important to keep in mind that all of these symbols are just baselines. Most of the time, you can find actual speeds on a card's listing page or its packaging, if not on the card itself. Be sure to check because sometimes the gap between minimum ratings and the card's actual speed can be quite large.

Additionally, keep in mind that SD cards often have different speeds for reading and writing. If you need to take a dozen photos every second, it won't matter if your card can read data really fast, it needs to write fast as well. Unfortunately, this can mean that despite the bevy of symbols and icons on your SD card, the best way to make sure it meets your needs is to just look up that card's speeds directly.

Your natural instinct, after reading all this, might be to just buy the fastest SD card you can. But there is a bit of a trade-off. Faster SD cards generally cost more for the same amount of storage, so consider your use case carefully. If you need to transfer large amounts of data at once—either by storing lots of media to the card or reading huge amounts of data from the card—a faster card may help. However, if storage capacity is more important, a slower SD card may be more cost-effective.

For example, here's a 128-gigabyte SD card designed for video work that can record up to 8K footage (depending on the format) and can read data at speeds up to 280 MB/s. It normally costs $150. But here's another 128 gigabyte SD card that can "only" read data at up to 170 MB/s—it usually costs $45. You might end up paying a hefty premium for higher speeds, so it's better to balance out what's best for you.

You also need to consider what your device itself can do. For example, the Nintendo Switch is only capable of transfer speeds up to around 95 MB/s, so faster SD cards will be useless on it. Besides, you’ll get more use out of additional storage at that speed anyway. Speed is important, but it isn't the only factor that matters.