Decode the symbols on your microSD card
Type the marks on your card — SDXC, UHS-I, U3, V30, A2 — to see each class minimum, each bus ceiling and which marks overlap.
Example filled in above: — type your own marks over it.
- MICROSDXCcapacity up to 2 TB (usually exFAT)
- UHS-Ibus reaches up to 104 MB/s — a ceiling, not a guarantee
- U3minimum 30 MB/s sustained write
- V30minimum 30 MB/s sustained write
- A2apps: 4,000/2,000 IOPS + 10 MB/s
- 128 GBthe declared capacity
- microSD means the same specifications as SD in a small physical format.
- U3 and V30 say the same thing: minimum 30 MB/s sustained write. They do not add up.
- Full A2 speed also needs an A2-capable host.
Numbers with different meanings. C, U and V classes give minimum sustained write speeds; E150–E600 set minimum sequential-write thresholds in SD Express mode when the card and host support the class and bus. The bus (I, II, III, Express) has a theoretical ceiling, while a packaging speed is a vendor-declared maximum — neither is the card's guaranteed actual speed. A1/A2 measure random access for apps.
- microSDXC capacity up to 2 TB (usually exFAT)
- I UHS-I · bus reaches up to 104 MB/s — a ceiling, not a guarantee
- 128GB the declared capacity
- ⑩ U3 V30 same floor: minimum 30 MB/s write.
- A2 apps: 4,000/2,000 IOPS + 10 MB/s
Every class, in brief¶
C, U and V give minimum sustained write speed; E150–E600 give minimum sequential-write thresholds in SD Express mode when the card and host support the class and bus. Highlighted rows have different marks with the same threshold.
| Family | Up to | Usually formatted |
|---|---|---|
| SD | 2 GB | FAT12/16 |
| SDHC | 32 GB | FAT32 |
| SDXC | 2 TB | exFAT |
| SDUC | 128 TB | exFAT |
| Bus and mode | Theoretical ceiling |
|---|---|
| UHS-I (I) | 104 MB/s |
| UHS-II (II) | 312 MB/s |
| UHS-III (III) | 624 MB/s |
| SD Express · PCIe Gen3 ×1 (SD and microSD) | 985 MB/s |
| SD Express · PCIe Gen4 ×1 (SD and microSD) or Gen3 ×2 (SD) | 1,970 MB/s |
| SD Express · PCIe Gen4 ×2 (full-size SD) | 3,940 MB/s |
These are interface limits, not speeds guaranteed by each card. Actual speed depends on the card, reader, host and operation. microSD v8 supports only PCIe Gen4 ×1; the two-lane modes in the table apply to full-size SD cards.
| Class | Standard minimum |
|---|---|
| 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 |
| 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 |
Full A2 speed also needs an A2-capable host.
Why so many symbols on such a small card?¶
Because each answers a different question: how much fits (SDXC), how fast data goes in when writing (U3, V30), how fast it comes out through the bus (UHS-I), and how apps behave (A2). And the classes overlap because they come from different generations of standards: vendors print several symbols so one card reads clearly on old and new hosts alike.
Real speed is set by the weakest link: the card, the negotiated bus mode, the reader, and the host. A V30 card in an old reader never reaches 30 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?
They measure different things: 200 MB/s is a vendor-declared maximum, often for read speed; V30 guarantees at least 30 MB/s sustained sequential write. E150–E600 set minimum sequential-write thresholds of 150–600 MB/s under SD Express conditions, with a compatible card and host. The bus ceiling — up to 1,970 MB/s for microSD v8 — is separate and is not the card's actual speed, which depends on the card, reader, host and operation.
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¶
- SD Express bus modes follow the official SD Association table, checked 17 October 2026. The official speed-class definition says numbered class marks indicate minimum writing speed and E classes are used in SD Express mode; the SD Association brochure lists E150–E600 as minimum sequential-write speeds. Both were checked 17 October 2026. Capacity and app-performance figures use SD Association pages and checks from 18 September 2026.
- Historical capacity, class and app-performance figures are documented in the sources below; the Wikipedia SD card article was used only as a cross-check for information retained from the earlier edition.
- All thresholds are minima under standard conditions; real speed is set by the weakest link: card, negotiated bus mode, reader, and host.
- A combination that does not exist in the standard (say 512 GB next to SDHC) is flagged as such — the page does not rule whether a card is counterfeit.