SD card speed classes: C10, U3 and V30 are minimum-write guarantees, not the whole speed story

The symbols on an SD or microSD card answer different questions. C10, U1, U3 and V-class markings describe assured minimum sequential performance under the SD Association's defined conditions. UHS-I, UHS-II and SD Express describe host-card interfaces. A large advertised “up to” number is something else again.

Updated 26 August 2026 · Class relationships checked against the SD Association

The useful numbers behind the common class symbols

These are class floors, not a promise that every transfer will occur at exactly that rate. Flash behavior, workload, fragmentation, controller behavior, host support and test conditions matter. The SD Association describes speed classes as ways to assure minimum sequential card-access performance under defined conditions, which is why the markings are particularly useful for recording workloads that cannot tolerate the write rate dropping below a requirement.

C10, U1 and V10 can all show “10” without being interchangeable labels

Three class systems include a 10 MB/s minimum: Class 10, UHS Speed Class 1 and Video Speed Class 10. That numerical overlap tempts people to treat the symbols as synonyms. The SD Association specifically warns against assuming that different class-symbol systems are interchangeable for host requirements, even where the nominal minimum number is the same.

If a device manual calls for a particular class family, match that family. For example, a host specifying V10 is making a Video Speed Class requirement. A card showing only C10 is not automatically the same compliance statement. Use the symbol required by the device maker rather than reverse-engineering compatibility from one number.

V30 means a 30 MB/s class floor, not “30 MB/s maximum”

The number after V is the minimum sequential write-performance class expressed in megabytes per second. V30 corresponds to 30 MB/s, V60 to 60 MB/s and V90 to 90 MB/s under the standard's conditions. A V30 card may advertise a much higher peak read speed or even a higher peak write speed. The V30 symbol is still telling you about the assured class floor, not the card's best-case headline number.

This difference matters for video. A benchmark burst can look impressive while a recording workload needs sustained writes over time. The class mark gives the host and buyer a standardized floor to match against a recording requirement.

Bus interface and speed class are separate layers

UHS-I, UHS-II, UHS-III and SD Express are interface families. They describe how the host and card can communicate, not the same thing as the C, U, V or E performance-class floor.

A card can therefore carry both an interface mark and one or more speed-class marks. Read them as separate statements. “UHS-II V60” means more than simply “60 MB/s card”: V60 describes the class floor, while UHS-II describes the compatible bus family needed for that mode.

The host can be the bottleneck

A high-spec card does not upgrade the controller inside a camera, console, laptop or card reader. The achieved transfer mode is a property of the card-host combination. If a reader supports only a slower interface, buying a card with a faster interface may still be useful for another device, but it cannot force that reader beyond its own capabilities.

The same logic applies when recording. The card must meet the device's recording requirement, but the host also must support the class/interface combination. The SD Association's guidance is to match the speed-class symbol requested by the host and use the same or a higher class within that class system.

Advertised read speed is not a substitute for a recording class

Retail packaging often gives an “up to” read speed because fast reads matter when copying files off a card. That figure does not by itself establish a sustained recording floor. When your problem is “will this camera maintain the required recording write rate?”, the relevant evidence is the device requirement plus the card's applicable speed-class marking and manufacturer specification.

For file-copy estimates, also keep bits and bytes straight. A 100 MB/s storage transfer rate is not the same unit as a 100 Mb/s network link. See MB/s versus Mb/s and the file-transfer time guide.

What about A1 and A2?

Application Performance Class markings A1 and A2 answer a different question. They are intended to characterize minimum random and sequential performance for running and storing applications under the SD standard's conditions. They are not replacements for V30 or another video recording class.

A microSD card can carry an application class and a video class because those marks describe different workloads. If a handheld device cares about app responsiveness, A1/A2 may matter. If a camera specifies a recording class, follow the recording-class requirement separately.

Capacity labels are separate again

SDHC, SDXC and SDUC are capacity/file-system families, not speed guarantees. The SD Association currently defines SDHC as more than 2 GB through 32 GB, SDXC as more than 32 GB through 2 TB, and SDUC as more than 2 TB through 128 TB. A card being SDXC therefore does not tell you whether it is V30, V60 or any other speed class.

This is the recurring pattern with storage labels: capacity, interface and performance are different dimensions. Treat each marking as a separate field rather than compressing them into one idea of “fast card.”

A practical card-selection workflow

1. Read the device manual or manufacturer's support page before the card listing.
2. Write down the required capacity family and maximum supported capacity.
3. Write down the exact speed-class family and minimum class requested by the host.
4. Check the host's bus-interface support, especially before paying extra for UHS-II or SD Express performance.
5. Choose the same or a higher class within the class system the host specifies.
6. Treat “up to” read/write claims as separate performance claims, not replacements for the required class symbol.
7. For critical recording, follow the camera/device maker's tested-card or compatibility guidance when it publishes one.

Worked examples

A camera manual requires V30

Start with a card marked V30 or higher within Video Speed Class, then confirm the capacity and bus interface are supported by the camera. Do not choose a card only because it says “up to 170 MB/s” on the package; that headline does not replace the V30 requirement.

A reader is UHS-I but the card is UHS-II

The card cannot make the reader become UHS-II. The pair will operate according to the mutually supported path. If fast offloads are the reason you bought the UHS-II card, the reader must support the corresponding interface too.

A microSD card says A2 and V30

Those two symbols can coexist because they characterize different workloads. A2 concerns application-performance requirements; V30 concerns the video speed-class floor. Neither makes the other redundant.

Primary sources

• SD Association — Speed Class Standards for Video Recording, for the current class families and minimum sequential performance table.
• SD Association — Speed Class, for host/card class matching, bus-mode constraints and class definitions.
• SD Association — Bus Speed, for the distinction between UHS/SD Express interfaces and speed classes.
• SD Association — Capacity, for SD, SDHC, SDXC and SDUC capacity ranges.
• SD Association — SD Memory Card Choices, for Application Performance Class and consumer-facing symbol categories.

Bottom line

An SD card can carry several honest-looking speed numbers that describe different things. Use C, U, V and E marks as standardized minimum-performance classes; use UHS-I/UHS-II/SD Express as interface information; and treat advertised peak speeds as separate claims. The safest purchase is the one that matches the exact class and interface requirements of the host device, not the card with the largest number on the package.