Guide · Updated 2026-10-08
MicroSD Speed Classes Explained: U3, V30 and A2
What the symbols on a microSD card mean, how to work out the write speed and size you need for 4K video, a dashcam or a console, and mistakes that corrupt footage.
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A microSD card has several marks on the front, and they do not all mean the same thing. Some describe how quickly the card writes video, and others describe how well it runs apps and games. Choosing the wrong one is a common cause of dropped frames and slow consoles.
The marks and what they mean
| Mark | What it describes | Minimum guaranteed |
|---|---|---|
| Class 10 / U1 | Sustained write speed for HD video | 10 MB/s |
| U3 / V30 | Sustained write speed for 4K video | 30 MB/s |
| V60 | Sustained write speed for high bitrate 4K and 8K | 60 MB/s |
| V90 | Sustained write speed for professional video | 90 MB/s |
| A1 | App performance: small random reads and writes | about 1,500 read and 500 write operations per second |
| A2 | Higher app performance | about 4,000 read and 2,000 write operations per second |
How fast does video need to be?
Video is recorded at a bitrate in megabits per second (Mbps). Divide by 8 for megabytes per second, and then add a margin of about 50 percent because speeds fluctuate. A camera recording 4K at 100 Mbps writes about 12.5 MB/s, so a card with a 30 MB/s minimum (U3 or V30) is comfortable.
How big a card do you need?
Gigabytes per hour = bitrate in Mbps ÷ 8 × 3,600 ÷ 1,000.
| Recording | Typical bitrate | Data per hour | 3 hours plus 20% spare | Card to buy |
|---|---|---|---|---|
| Full HD at 30 fps | 20 Mbps | 9 GB | 32 GB | 64 GB (32 GB is just too small) |
| 4K at 30 fps | 100 Mbps | 45 GB | 162 GB | 256 GB |
| 4K at 60 fps | 150 Mbps | 68 GB | 243 GB | 256 GB |
| Dashcam | 12 Mbps | 5.4 GB | 19 GB | 32 GB (high endurance) |
Bitrates vary by device and setting. Use the figure in your camera's menu or manual if you can find it.
Dashcams, consoles and handhelds
- Dashcams rewrite the card constantly, so use a card sold as "high endurance" or for dashcams rather than a standard one.
- Games consoles and handheld PCs usually want UHS-I speeds, and an A2 rating helps with loading. Check the maker's requirements for the maximum capacity and speed.
- Phones and tablets use A1 or A2 cards for apps and photos.
Avoid these mistakes
- Buying from an unknown seller: counterfeit cards report a large capacity but hold far less.
- Ignoring the device's maximum capacity: some devices stop at 32 or 128 GB. Check the manual.
- Filling the card to the brim: leave 10 to 20 percent free for smooth recording.
- Formatting in a computer when the device wants to format it: format in the device.
Try your own numbers
The MicroSD Card Size and Speed Calculator turns your recording type and hours into a speed class and a card size. After you shoot, check pictures are sharp enough to print with the Print-Ready Image Checker.
Frequently asked questions
Is a faster card always better? Not for your device: a card faster than the device can use gives no benefit. Buy what the device needs with a safety margin.
What is the difference between SDHC and SDXC? SDHC cards go up to 32 GB and SDXC cards run from 64 GB up to 2 TB, using the exFAT file system. Devices must support the type you choose.
Specifications and requirements differ by device. Always check the device manual.
Try the tools
MicroSD Card Size and Speed Calculator
Work out the card capacity and write speed you need for video recording or a console, and read what the speed marks mean.
DPIPrint-Ready Image Checker
Check whether an image has enough pixels for a print size: effective DPI, crop and the largest size you can print.
mAhPower Bank Capacity and Flight Rules Calculator
Work out how big a power bank you need for your devices, how long it takes to charge, and whether it is allowed on a plane.