# Decode the symbols on your microSD card

Type what your card says — SDXC, UHS-I, U3, V30, A2 — and get what speed each mark guarantees and which ones say the same thing.

Canonical: https://mariuscomper.uk/coduri-card-microsd/en/

Two kinds of numbers. Classes (C, U, V, E) are floors: the minimum write speed you get for sure. The bus (I, II, III, Express) and the speed on the packaging are ceilings: what you can reach in good conditions, with a matching reader and host. And A1/A2 measure something else: random access for apps.

## Capacity

| Family | Max | Format |
|---|---|---|
| SD | 2 GB | FAT12/16 |
| SDHC | 32 GB | FAT32 |
| SDXC | 2 TB | exFAT |
| SDUC | 128 TB | exFAT |

## Bus (ceiling)

| Bus | Ceiling |
|---|---|
| UHS-I (I) | 104 MB/s |
| UHS-II (II) | 312 MB/s |
| UHS-III (III) | 624 MB/s |
| SD Express | 985 MB/s |

You need a matching reader and host to reach the ceiling.

## Write classes (floor)

| Class | Floor |
|---|---|
| C2 | 2 MB/s |
| C4 | 4 MB/s |
| C6 | 6 MB/s |
| V6 | 6 MB/s |
| C10 | 10 MB/s |
| U1 | 10 MB/s |
| V10 | 10 MB/s |
| U3 | 30 MB/s |
| V30 | 30 MB/s |
| V60 | 60 MB/s |
| V90 | 90 MB/s |
| E150 | 150 MB/s |
| E300 | 300 MB/s |
| E450 | 450 MB/s |
| E600 | 600 MB/s |

## Apps

| Class | Random read | Random write | Sequential |
|---|---|---|---|
| A1 | 1,500 IOPS | 500 IOPS | 10 MB/s |
| A2 | 4,000 IOPS | 2,000 IOPS | 10 MB/s |

## Frequently asked questions

**What does V30 mean on a memory card?**

Minimum 30 MB/s sustained sequential write — the bar of the V30 video class, designed for demanding video recording, including 4K. The U3 mark on the same card guarantees exactly the same floor.

**U1 vs U3 — what's the difference?**

Both are UHS speed classes and both guarantee minimum sequential write: U1 means 10 MB/s, U3 means 30 MB/s.

**V30 or V60 — which one?**

V30 guarantees 30 MB/s, V60 guarantees 60 MB/s. V30 covers ordinary 4K; V60 and V90 are for heavier streams — 8K, production, long bursts.

**A1 or A2? What does the A actually mean?**

A measures random access for apps, not copy speed: A1 guarantees 1,500/500 read/write IOPS, A2 guarantees 4,000/2,000 IOPS, both plus 10 MB/s sequential. Full A2 speed also needs an A2-capable host.

**SDHC or SDXC?**

SDHC means capacity up to 32 GB (usually FAT32); SDXC means over 32 GB, up to 2 TB (exFAT). A 128 GB card is always SDXC.

**Why do cards print both U3 and V30 if they mean the same thing?**

They come from different generations of standards and are read by different hosts. Vendors print several symbols so one card reads clearly everywhere — the floor stays a single number: 30 MB/s.

**The packaging says 200 MB/s but the class is V30. Which speed is real?**

Both, but they measure different things: 200 MB/s is the vendor-declared maximum, usually read, in ideal conditions. V30 is the guaranteed write minimum. Between them, the weakest link decides: card, bus, reader, host.

**Do I need A2?**

Only if you run apps or an operating system off the card — phone, console, Raspberry Pi. For plain photos and video, the A changes nothing.

## Sources and limits

- [https://www.sdcard.org/consumers/about-sd-memory-card-choices/speed-class-standards-for-video-recording/](https://www.sdcard.org/consumers/about-sd-memory-card-choices/speed-class-standards-for-video-recording/)
- [https://www.sdcard.org/consumers/about-sd-memory-card-choices/bus-interface-speed-standards-for-large-size-data-transfer/](https://www.sdcard.org/consumers/about-sd-memory-card-choices/bus-interface-speed-standards-for-large-size-data-transfer/)
- [https://www.sdcard.org/consumers/about-sd-memory-card-choices/application-performance-class-for-running-smartphone-apps/](https://www.sdcard.org/consumers/about-sd-memory-card-choices/application-performance-class-for-running-smartphone-apps/)
- [https://www.sdcard.org/consumers/about-sd-memory-card-choices/sd-sdhc-sdxc-and-sduc-card-capacity-choices/](https://www.sdcard.org/consumers/about-sd-memory-card-choices/sd-sdhc-sdxc-and-sduc-card-capacity-choices/)
- [https://en.wikipedia.org/wiki/SD_card](https://en.wikipedia.org/wiki/SD_card)