U1 vs U3 vs V30 — Memory Card Speed Classes Explained
Speed class markings measure minimum sustained write performance, not peak read speed. C-class (Class 10) and U-class (U1, U3) are minimum sequential write rates in MB/s. V-class (V6, V10, V30, V60, V90) is the video speed class, designed for recording workloads. A-class (A1, A2) measures random read and write IOPS for running applications from the card. For video recording, the V-class is the meaningful one. For a device running apps from the card, A-class matters. Peak read speed on the packaging is the number least relevant to either.
The markings
| Marking | Means | Measured | Relevant for |
|---|---|---|---|
| C10 | 10 MB/s minimum sequential write | Sequential write | Basic recording |
| U1 | 10 MB/s minimum sequential write | Sequential write | Full HD recording |
| U3 | 30 MB/s minimum sequential write | Sequential write | 4K recording |
| V6 / V10 / V30 / V60 / V90 | Minimum sustained write in MB/s | Sustained write under video workload | Video — the class to specify for cameras |
| A1 / A2 | Random IOPS performance | Random read/write IOPS | Running apps from the card |
Which one to specify
- Dashcam or IP camera recording → specify a V-class, matched to your bitrate
- A device that boots and stores data → sustained write and controller compatibility matter more than class markings
- A card used to run applications → A1 or A2
- General file storage → the class is rarely the constraint; capacity and grade are
The trap
A card can meet a class marking on a bench test and still fail in a device, because the class is a minimum sustained rate under defined conditions, not a guarantee of behaviour over months of continuous overwrite. For recording workloads, test sustained write on the actual device for hours, not minutes.
How to read a card label
Read the marking against the workload, not against the price. V30 means a guaranteed 30 MB/s sustained write — enough for a 4K dashcam at typical bitrates, which need only a few MB/s. A card advertised on a high peak read speed may still stumble on sustained writes if the controller or cache is weak.
| You are building | Marking to specify |
|---|---|
| Full HD dashcam / camera | U1 or C10 (10 MB/s) |
| 4K recording | U3 or V30 (30 MB/s) |
| Running apps from the card | A1 or A2 (random IOPS) |
| File storage / giveaway | Class rarely the constraint — capacity and grade lead |
The mistake to avoid
Do not trust the peak read speed printed on the packaging as a measure of recording performance. Read and write, sequential and random, are different figures; for video the only marking that matters is the sustained-write class (V30 or U3). A card can meet its class on a bench for minutes and still stumble under months of continuous overwrite if the controller or cache is weak. Verify it on the actual device over hours, recording the real workload, not on a benchmark loop.
Not sure which applies to your product?
Send your workload — what writes, how often, at what temperature — and we will propose a configuration with the reasoning. We reply to business enquiries within 1 business day.
Request a quoteFrequently asked questions
Is U3 the same as V30?
They both indicate 30 MB/s, but they are measured differently — U3 is sequential write, V30 is sustained write under a video workload. For cameras, V30 is the more meaningful specification.
Does a higher class mean a faster card overall?
It means a higher guaranteed minimum sustained write. Read speed is a separate figure, and random performance is a third. They do not move together.
Can you supply a card to a specific speed class?
Yes. Tell us the workload and the class you need and we will specify the silicon and controller to meet it, stated in the quotation.