Why microSD Speed Ratings Are Confusing (And How to Read Them)
Buy a microSD card and you'll see a cluster of logos and numbers on the label: a number inside a C, a Roman numeral I or II, a U with a number, a V with a number. It looks like alphabet soup — but each rating tells you something specific about how fast the card can read or write data.
Here's a plain-English breakdown of every speed class you'll encounter.
Speed Class (The C Symbol)
The original speed class system uses a number inside the letter C. It defines the minimum sustained write speed:
- Class 2 — 2MB/s (SD video only)
- Class 4 — 4MB/s
- Class 6 — 6MB/s
- Class 10 — 10MB/s (minimum for Full HD video)
Class 10 is the baseline you should expect on any card sold today. If a card doesn't show at least Class 10, skip it.
UHS Speed Class (The U Symbol)
UHS (Ultra High Speed) is a newer standard that sits on top of the Class system. The U symbol contains either a 1 or a 3:
- U1 — minimum 10MB/s sustained write (same floor as Class 10)
- U3 — minimum 30MB/s sustained write
U3 is the minimum you need for 4K video recording. Most modern action cameras, drones, and mirrorless cameras require U3 to record 4K without dropped frames.
UHS Bus Interface (The Roman Numeral)
Separate from the speed class, the Roman numeral (I, II, or III) tells you the bus interface — essentially the maximum theoretical throughput the card's hardware supports:
- UHS-I — up to ~104MB/s
- UHS-II — up to ~312MB/s (requires a UHS-II compatible slot)
- UHS-III — up to ~624MB/s
For most users, UHS-I is more than sufficient. UHS-II cards are faster but cost significantly more and only deliver their full speed in UHS-II compatible card readers — which most cameras and phones don't have.
Video Speed Class (The V Symbol)
The V rating was introduced specifically for video workloads. It guarantees minimum sustained write speeds under continuous recording conditions:
- V6 — 6MB/s
- V10 — 10MB/s
- V30 — 30MB/s (4K video)
- V60 — 60MB/s (4K high-bitrate / 8K)
- V90 — 90MB/s (8K RAW)
V30 is the practical standard for 4K shooters. V60 and V90 are for professional cinema cameras shooting uncompressed or RAW formats — overkill for most use cases.
Application Performance Class (A1 / A2)
If you're using a microSD card in an Android phone to run apps, look for the A rating:
- A1 — 1500 IOPS read / 500 IOPS write
- A2 — 4000 IOPS read / 2000 IOPS write
IOPS (Input/Output Operations Per Second) matters for app performance, not video recording. For cameras and drones, ignore this rating.
What Rating Do You Actually Need?
| Use Case | Minimum Rating |
|---|---|
| Full HD (1080p) video | Class 10 / U1 / V10 |
| 4K video (standard bitrate) | U3 / V30 |
| 4K high-bitrate / slow motion | V60 |
| 8K RAW / cinema | V90 |
| Android app storage | A1 or A2 |
The AXYON Pro Performance microSD: Built for 4K
The AXYON Pro Performance High-Speed microSD is rated UHS-I Class 10 — delivering the sustained write performance needed for 4K video recording across cameras, drones, and action cams.
Available in capacities from 8GB to 512GB, with an SD adapter included in every pack. Whether you're filling a 64GB card on a weekend shoot or running a 512GB card for a multi-day production, the Pro Performance has you covered.
