Pick up any microSD card and it’s crowded with cryptic markings: a 10 inside a C, a 3 inside a U, “V30,” “A2,” a Roman numeral, and a big bold “170MB/s.” None of it is decoration — each symbol is a different rating from a different system, and buying a card that ignores the one your device needs means dropped 4K footage or an apps drive that crawls. Here’s what every marking means, and exactly which card to buy for what.
Two ratings matter most. The minimum write class — shown as U3 or V30 (both mean 30MB/s, the floor for smooth 4K video) — and, if you’ll run apps or a system off the card, the App class (A2). The big “170MB/s” on the front is peak read speed, not the guaranteed write speed that actually prevents dropped recordings. Match the class to your use and ignore the marketing number.
The symbols, decoded
| Marking | What it rates | Meaning |
|---|---|---|
| C10 (number in a C) | Speed Class | Min 10MB/s sustained write |
| U1 / U3 (number in a U) | UHS Speed Class | Min 10 / 30MB/s write |
| V30 / V60 / V90 | Video Speed Class | Min 30 / 60 / 90MB/s write |
| A1 / A2 | App Performance Class | Fast random read/write for apps |
| I / II (Roman numeral) | UHS Bus | Max read: ~104 / 312MB/s |
Speed Class, UHS Class and Video Class — all the same idea
Three of those systems measure the same thing — the guaranteed minimum sustained write speed — they were just introduced in different eras as cards got faster. The original Speed Class tops out at C10 (10MB/s). The UHS Speed Class adds U1 (10MB/s) and U3 (30MB/s). The Video Speed Class goes furthest with V6, V10, V30, V60 and V90, the number being the megabytes-per-second floor. Here’s the part that confuses everyone: U3 and V30 are identical — both guarantee 30MB/s. A card often prints both simply to be readable by older and newer devices.
The number that fools everyone
That eye-catching “170MB/s” or “200MB/s” on the front of the card is the peak read speed in ideal conditions — how fast you can copy files off the card. It tells you almost nothing about recording. What stops a 4K video from stuttering is the guaranteed minimum write speed, and that’s what the little U3/V30 symbols promise. A card can boast 170MB/s read and still only be rated V30 (30MB/s write). Always buy on the class symbol, not the headline number.
The App class (A1/A2) — only matters sometimes
A1 and A2 rate something different again: random read and write performance, measured in operations per second (IOPS), which is what matters when a card stores and runs apps rather than big sequential video files. A1 guarantees 1,500 read and 500 write IOPS; A2 raises that to 4,000 and 2,000. This is the spec to care about for Android “adoptable storage,” a Raspberry Pi, a retro-gaming handheld or a Steam Deck. For a camera or dashcam, the App class is irrelevant. Note that A2’s full benefit needs a host device that supports it.
The bus: UHS-I vs UHS-II
The Roman numeral (I or II) is the bus interface — the pipe the data travels through. UHS-I caps around 104MB/s, while UHS-II adds a second row of pins for up to ~312MB/s. The catch: UHS-II only helps if your camera or reader also has UHS-II pins; in a UHS-I device, a UHS-II card just runs at UHS-I speeds and the extra cost is wasted. Match the bus to your hardware.
Which card should you actually buy?
Phone storage / 1080p video: U1 or V10 is plenty. 4K video (phones, action cams, most cameras): U3 / V30 — this is the everyday sweet spot. High-bitrate 4K, 6K or 8K: step up to V60 or V90 on a UHS-II card. Running apps, a Pi or a handheld: prioritize A2, alongside U3/V30. Dashcam or security camera: choose a “High Endurance” card rated at least V30, because those write continuously for years. And always buy a known brand from a reputable seller — counterfeit cards that lie about their capacity and class are common. If you’re juggling storage on your phone, our guide to freeing up Android storage pairs well with adding a card.
Frequently asked questions
What do the symbols on a microSD card mean?
They are separate speed ratings. The number in a ‘C’ is the Speed Class, the number in a ‘U’ is the UHS Speed Class (U1 = 10MB/s, U3 = 30MB/s minimum write), ‘V’ numbers are the Video Speed Class (V30/V60/V90 MB/s), ‘A1/A2’ rate app performance, and the Roman numeral (I/II) is the bus interface that sets maximum read speed.
What is the difference between U3 and V30 on an SD card?
There is none in terms of speed — both guarantee a minimum sustained write of 30MB/s, which is the floor for smooth 4K recording. They come from two different rating systems (UHS Speed Class and Video Speed Class), and cards often print both so older and newer devices can read the rating.
Does the big MB/s number on a microSD card matter?
Not for recording. The large ‘170MB/s’ or ‘200MB/s’ figure is the peak read speed (copying files off the card) in ideal conditions. What prevents dropped video is the guaranteed minimum write speed shown by the U3/V30 class symbol, so buy based on the class, not the headline number.
What does A1 or A2 mean on a microSD card?
A1 and A2 rate random read/write performance for running apps, measured in operations per second. A1 guarantees 1,500 read and 500 write IOPS; A2 doubles that to 4,000 and 2,000. It matters for Android adoptable storage, a Raspberry Pi, or a gaming handheld, but is irrelevant for cameras and dashcams.
Which microSD card do I need for 4K video?
A card rated U3 or V30, both of which guarantee a 30MB/s minimum write speed — the everyday sweet spot for 4K. For very high-bitrate 4K, 6K or 8K, step up to a V60 or V90 card on the faster UHS-II bus, and make sure your device supports UHS-II to benefit from it.