The five markings are five different jobs
The SD Association groups the speed markings as Speed Class (C), UHS Speed Class (U), Video Speed Class (V), Application Performance Class (A) and SD Express Speed Class (E).
| Marking | What it promises | Common values |
|---|---|---|
| Speed Class (C) | Minimum sequential write | C2, C4, C6, C10 |
| UHS Speed Class (U) | Minimum sequential write on UHS hardware | U1, U3 |
| Video Speed Class (V) | Minimum sustained write for recording | V6, V10, V30, V60, V90 |
| Application Performance Class (A) | Random I/O plus sustained sequential | A1, A2 |
| SD Express Speed Class (E) | Minimum read/write on SD Express | E150, E300, E450, E600 |
The C, U, V and E marks are minimums under conditions defined by the standard, mainly useful for devices that record video. The SD Association's own description is that these classes assure minimum sequential access performance. That is a floor, not a ceiling.
Same number, different class
C10, U1 and V10 all mean 10 MB/s. U3 and V30 both mean 30 MB/s. It's tempting to treat them as interchangeable, and the SD Association says not to. Its guidance is to match the host's class symbol, or pick a higher class of the same type. A host needing U1 can use U1 or U3. The Association's own example of a mismatch is a host that wants Class 10 paired with a U1 card, or a host that wants U1 paired with a V10 card. The expected write speed isn't assured there, even though the numbers match.
The practical rule is to read the device manual. If a camera asks for V30, buy V30 or higher.
Each class also has its own bus requirement:
- V6 and V10 work with High Speed or UHS products.
- V30 works with UHS products.
- V60 and V90 need UHS-II or UHS-III.
The Roman numerals describe the pipe, not the card
UHS-I, UHS-II and UHS-III are bus-interface generations. The SD Association lists their theoretical ceilings as:
- UHS-I: 104 MB/s
- UHS-II: 312 MB/s
- UHS-III: 624 MB/s
These are limits on the interface, not guarantees from a particular card. A fast card in a UHS-I reader is still held to the UHS-I ceiling, unless the vendor uses proprietary extensions with compatible hardware. For PC users, the reader and its connection to the computer are part of the equation. A budget reader can quietly throttle an expensive card.
A1 and A2 measure something else entirely
Application Performance Class is about random 4 KB operations, not big-file throughput. The SD Association defines the tiers like this:
- A1: at least 1,500 random-read IOPS, 500 random-write IOPS and 10 MB/s sustained sequential.
- A2: at least 4,000 random-read IOPS, 2,000 random-write IOPS and 10 MB/s sustained sequential.
The A2 catch is that its numbers depend on the host. The SD Association says A2 uses Command Queuing and Cache, and that A2 performance is available only with an A2-supported host and card. That quote is from the Association's own page, which the research packet captured. Independent explainers say the same. One guide notes that in a device without that support, an A2 card performs closer to A1. In other words, A2 isn't a guaranteed upgrade. It is only a better card if the device can use it.
A1 and A2 matter most when software runs from the card, such as on a phone with adoptable storage or a handheld. One buying guide lists Steam Deck among the devices where A-class matters more than V-class. For a Windows handheld or a Steam Deck, the A rating is the more relevant one, with a decent sequential rating for large installs.
Video recording: sustained writes are the whole game
A camera's buffer can hide a slow card only briefly. Once the buffer fills, the camera has to deal with the backlog, and depending on the device that can mean dropped frames, reduced performance or an error. The SD Association describes video recording as needing a constant minimum write speed to avoid frame drops.
So a 190 MB/s read claim says nothing about whether a card can sustain a 4K recording mode. The SD Association draws the distinction itself:
- Speed classes assure a minimum under defined conditions.
- Speed ratings, meaning the "up to" figures vendors print, represent maximum speed and aren't standardized by the SD specifications.
Fragmentation also plays a part. The SD Association notes that speed-class writes assume a completely un-fragmented area. A card with free space can still fail to record at class speed if it has been heavily written and deleted. Its fix is to move the data elsewhere and reformat the card.
SD Express and the Switch 2 example
SD Express brought the E150, E300, E450 and E600 classes. The SD Association says they're based on the same access methods as Video Speed Class, run over NVMe and PCIe. They add multi-stream recording, power management and thermal management. For multi-stream use, the Association's example sums the streams. Two 50 MB/s streams plus two 100 MB/s streams add up to 300 MB/s, which is the SD Express Speed Class 300 level.
Nintendo's support page gives a current example of the host deciding the outcome:
- Switch 2 is only compatible with microSD Express cards.
- A conventional microSD card previously used in a Switch won't let a Switch 2 save or load digital games or save data. Switch screenshots and videos on it can still be loaded.
- The first use of a microSD Express card needs an internet system update. Cards up to 2 TB are supported.
- In an original Switch, a microSD Express card reads and writes at the same speed as an ordinary microSD card.
Nintendo's page doesn't name a required E class. The requirement is microSD Express itself, so don't assume a particular E number is mandatory.
A short buying checklist
- Find the host's requirement. Check the camera, drone, dash cam or console manual for the class symbol it names.
- Match the type, and go up within it. If the manual asks for V30, V30 or higher is fine. Don't substitute U3 and assume it's equal.
- For apps and handhelds, look at A1/A2. Only expect A2 speeds if the device supports A2 features.
- For PC ingest, check the reader. The card's "up to" read figure is relevant only if the reader and port can keep up.
- Check the card is genuine. Fake or falsified-capacity cards make the markings meaningless. Test new cards before relying on them.
Bottom line
None of these markings is a bad number. They answer different questions, and the card that wins on the package may not be the one your device needs. Match the host's required class first. Treat the Roman numeral as an interface limit and the big "up to" figure as marketing for a different task. Then check the card is real.