An SD or microSD card label can carry half a dozen symbols: a C inside a circle, a U with a number, a V30, an A2, a Roman numeral, and a headline speed like "170 MB/s". Each one is a separate promise defined by the SD Association, and the headline number is usually the least important of them. Knowing which symbol matters for which device is the difference between smooth 4K recording and dropped frames, or between a responsive handheld and one that stutters.
Headline speed vs guaranteed minimum
The large number printed on a card is almost always the maximum sequential read speed measured under ideal conditions. Cameras and recorders care about the opposite: the minimum sustained write speed that the card can hold for minutes at a time without buffering. That guarantee is what the speed class symbols describe. Two cards both labelled "100 MB/s" can behave completely differently when a camera writes a long 4K clip.
The three speed class families
| Symbol | Minimum sustained write | Typical use |
|---|---|---|
| Class 2 / 4 / 6 / 10 (C2–C10) | 2 / 4 / 6 / 10 MB/s | Legacy devices, SD-quality video, Full HD on older cameras |
| UHS Speed Class U1 | 10 MB/s | Full HD video, general use |
| UHS Speed Class U3 | 30 MB/s | 4K video on most consumer cameras and drones |
| Video Speed Class V6 / V10 | 6 / 10 MB/s | SD and Full HD video |
| Video Speed Class V30 | 30 MB/s | 4K video, action cameras, drones |
| Video Speed Class V60 | 60 MB/s | High-bitrate 4K, 6K, burst photography |
| Video Speed Class V90 | 90 MB/s | 8K, high-bitrate 4K/6K, professional cinema cameras |
The three families overlap: a U3 card also meets V30 and Class 10, which is why manufacturers often print all three. When a camera manual asks for a specific class, match or exceed that exact symbol.
A1 and A2: speed for apps, not video
Video recording writes large files in long, sequential streams. Running apps and games from a card is the opposite: thousands of tiny random reads and writes. The Application Performance Class measures exactly that, in input/output operations per second (IOPS).
| Class | Random read | Random write | Sustained sequential write |
|---|---|---|---|
| A1 | 1,500 IOPS | 500 IOPS | 10 MB/s |
| A2 | 4,000 IOPS | 2,000 IOPS | 10 MB/s |
A2 reaches its figures using command queuing and a card-side cache, and the host device has to support those features. In an older phone or reader, an A2 card falls back to roughly A1 behaviour. It never performs worse, but the extra speed only appears on compatible hosts.
UHS-I, UHS-II and SD Express: the bus decides the ceiling
Separate from the speed classes is the bus interface, shown as a Roman numeral (I, II, III) or the SD Express logo. It sets the maximum possible transfer rate between card and device.
| Bus | Maximum bus speed | How to recognise it |
|---|---|---|
| UHS-I | 104 MB/s | Roman numeral I; single row of contacts |
| UHS-II | up to 312 MB/s | Roman numeral II; second row of contacts |
| UHS-III | up to 624 MB/s | Roman numeral III; rarely used |
| SD Express | about 985 MB/s and above | SD Express logo; uses PCIe and NVMe |
A UHS-II card works in a UHS-I slot, but only at UHS-I speed. Some UHS-I cards advertise read speeds above 104 MB/s; those figures rely on a manufacturer-specific mode and are only reached with a matching card reader. SD Express cards run in legacy mode in ordinary slots, and some newer handheld devices now require them.
SDHC, SDXC and SDUC: capacity and file system
- SDHC: over 2GB up to 32GB, formatted FAT32.
- SDXC: over 32GB up to 2TB, formatted exFAT.
- SDUC: over 2TB up to 128TB.
Older devices that only support SDHC will not read a 64GB SDXC card. For fleets of older equipment such as dashcams, industrial HMIs and point-of-sale terminals, confirm the maximum supported capacity and file system before ordering in volume.
Which card for which device?
| Device | Recommended | Why |
|---|---|---|
| Full HD camera, basic camcorder | U1 / V10 | Low sustained bitrate |
| 4K action camera, drone, mirrorless camera | U3 / V30 | Sustained 30 MB/s covers most 4K bitrates |
| High-bitrate 4K/6K/8K, burst shooting | V60 or V90, UHS-II | Prevents dropped frames and long buffer clearing |
| Phone, tablet, handheld console, Raspberry Pi | A2 (A1 on older hosts) | Random IOPS decide app responsiveness |
| Dashcam, security camera, loop recording | High-endurance U3 / V30 | Continuous writes wear out standard cards |
| Industrial and embedded equipment | Industrial or high-endurance grade, wide temperature | Reliability, temperature range and fixed BOM matter more than speed |
Endurance, NAND type and fake cards
Speed class says nothing about how long a card lasts. Continuous-recording devices can rewrite a card thousands of times, so endurance depends on the NAND type (MLC lasts longer than TLC, which lasts longer than QLC), on firmware wear-levelling and on how much spare area the card reserves. For dashcams, surveillance and industrial logging, ask for the rated endurance in recording hours or total bytes written.
Counterfeit and fake-capacity cards remain common in marketplaces. They report a large capacity but silently overwrite data once the real flash is full. Always run a full-capacity write-and-verify test on samples; our OEM sourcing checklist explains how.
Checklist for resellers and device makers
- Match the speed class to the device manual (U3/V30, V60, V90), not the headline MB/s.
- Choose the A-class for app-running devices, and confirm the host supports A2.
- Confirm the bus (UHS-I, UHS-II, SD Express) and the maximum capacity the device supports.
- For continuous recording, specify endurance and NAND type.
- For volume orders, lock the controller and NAND combination and test samples at full capacity.
UNITRENTH supplies SD and TF (microSD) cards in U3, V30–V90 and A1/A2 classes, with private-label packaging available for resellers and OEM programs.
Frequently asked questions
Is U3 the same as V30?
Both guarantee a minimum sustained write of 30 MB/s, so a U3 card meets V30 and vice versa for most purposes. Video Speed Class adds higher tiers (V60, V90) for professional video.
Does an A2 card work in a phone that only supports A1?
Yes. The card works normally but performs at roughly A1 level, because A2 speed depends on host support for command queuing and cache.
Can I use a UHS-II card in a UHS-I camera?
Yes. UHS-II cards are backward compatible and run at UHS-I speeds, up to 104 MB/s bus speed, in a UHS-I slot.
Why does my 64GB card not work in an older device?
Cards above 32GB are SDXC and use the exFAT file system. Devices that only support SDHC cannot read them; check the maximum capacity your device supports.
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