Buy a microSD card and you get a soup of symbols on the package: C10, U3, V30, A2, UHS-I. Each one measures something different, and the big “up to 200MB/s” on the front is often the number that matters least. This guide explains what each symbol means and which one to check for what you plan to do with the card.
Why there are so many ratings
Card speed is not one number. Recording a steady stream of video is a different job from opening hundreds of tiny app files, and a card can be great at one and weak at the other. That is why the SD Association, the group that sets the card standards, created separate classes for each kind of use.
It also helps to separate two things: a class is a minimum performance guarantee, while an “up to X MB/s” figure is a maximum read speed that depends on the card, the reader and the device.
The five ratings, one by one
1. Speed Class (C2, C4, C6, C10)
The oldest one. It states the minimum sequential write speed. C10 guarantees at least 10MB/s. It works as a baseline and says little if you need more speed.
2. UHS Speed Class (U1 and U3)
Also a minimum sequential write speed, on devices that support UHS. U1 guarantees at least 10MB/s and U3 at least 30MB/s. U1 overlaps with C10, so the two are not interchangeable in every case.
3. Video Speed Class (V6, V10, V30, V60, V90)
Built for recording high-resolution video, including 4K and 8K. The number is the minimum sustained write speed in MB/s: V30, for example, guarantees 30MB/s. For cameras and camcorders, this is the one that counts. V6 and V10 work on High Speed and UHS buses, V30 needs UHS, and V60 and V90 require UHS-II or UHS-III.
4. Application Performance Class (A1 and A2)
This one measures small, scattered operations, the kind apps produce when they run from the card. The minimums are:
| Class | Random read | Random write | Sequential |
|---|---|---|---|
| A1 | 1,500 IOPS | 500 IOPS | 10MB/s |
| A2 | 4,000 IOPS | 2,000 IOPS | 10MB/s |
IOPS means read or write operations per second. One catch: A2 only delivers all of that if the device supports the A2 features too. Without that support, the card won’t reach its full performance.
5. SD Express (E150, E300, E450, E600)
The newest class, and the numbers are minimum performance in MB/s: 150, 300, 450 and 600. It applies to microSD Express cards, which only show that speed in devices built for the standard. The Nintendo Switch 2, for example, requires microSD Express for expandable game storage and doesn’t ask for a specific E class. In an original Switch, an Express card falls back to ordinary microSD speeds.
The Roman numeral: UHS-I, II and III
The Roman numeral names the bus, the “road” between card and device. It sets the speed ceiling, not the card’s guaranteed performance:
- UHS-I: up to 104MB/s;
- UHS-II: up to 312MB/s (half-duplex mode);
- UHS-III: up to 624MB/s.
Real speed depends on the card, the reader and the device. Some UHS-I cards advertise reads above 104MB/s using the maker’s own technology, and that only works with compatible hardware.
If your real problem is running out of room rather than speed, our guide to transferring data to a new phone covers moving files around first. And if the card is going into a security camera such as the Tapo D260 doorbell, which records to microSD, the same rule applies: check what the maker recommends.
How to choose: step by step
- Find out what your device supports. Check the manual or support page for UHS-I, UHS-II, A2 or microSD Express. A rating above what the device supports won’t speed anything up.
- Decide the main use. Video, apps and games, or just storing photos and files.
- For video: if the camera manual asks for a V rating, buy that exact rating. Sustained writes matter more than peak read speed.
- For apps on a phone or tablet: look for A1 or A2, preferring A2 if the device supports it.
- For the Switch 2: the card must be microSD Express. Don’t worry about the E class.
- For copying photos and files: look at the bus and the read speed, as long as your card reader can keep up.
- Be skeptical of 190 or 200MB/s on the front. That figure measures something else and doesn’t guarantee the sustained writes video needs.
Which rating for which job
- Action camera or 4K camcorder: the V rating the camera maker specifies, often V30 or higher;
- Phone or tablet with apps on the card: A1 or A2;
- Handheld console: whatever the maker asks for, such as microSD Express on the Switch 2;
- Storing photos and music: C10 or U1 is enough.
Frequently asked questions
Can I use a card with a higher rating than my device supports?
Yes. The card works, but it performs at whatever the device can handle, so you pay extra for nothing.
Are U3 and V30 the same thing?
Both guarantee 30MB/s of minimum sequential writing, but the V class is the one designed for video and includes features for that use. If the camera manual says V30, buy V30.
What is A2 for if my phone doesn’t support it?
Nothing beyond what A1 already does. A2 shows its full performance only on devices that support its features.
Is “up to 200MB/s” the write speed?
No. That number is usually a maximum read speed and depends on the card, the reader and the device. For video, trust the V class.
