An SD or microSD card can carry several speed symbols at once: perhaps a 10 inside a broken circle, a 3 inside a U, and a mark such as V30. It is easy to assume that these numbers are competing estimates of the card’s maximum speed.
They are not. Most of these marks are speed classes: standardized ways to state a minimum level of write performance under defined conditions. Different class systems were introduced for different generations and uses, so one card can legitimately qualify for several of them.
Understanding that distinction makes the labels much easier to read. It also helps when a camera, drone, recorder, or other device says that it requires a particular class.
Start with the difference between a speed class and a maximum speed
A memory card has to perform two related but different jobs.
For a file copy, you may care about how quickly the transfer can finish. Packaging may advertise a high read or write speed for this purpose. Such a figure can describe peak or maximum performance under particular conditions, and the result you see depends on the card, reader, device, interface, workload, and other factors.
Continuous recording creates a different requirement. A camera writing video cannot simply tolerate a long pause whenever the card becomes slower. Data keeps arriving, so the card needs to sustain an adequate write rate.
SD speed classes are intended to communicate this kind of minimum performance. A higher advertised maximum speed therefore does not make a required speed-class mark irrelevant. The two figures answer different questions.
The original Speed Class uses C2, C4, C6, and C10
The older Speed Class system is commonly represented by a number inside a C-shaped symbol. Its class number corresponds to a minimum sequential write speed in megabytes per second under the standard’s defined conditions.
The classes are:
- C2: 2 MB/s;
- C4: 4 MB/s;
- C6: 6 MB/s;
- C10: 10 MB/s.
This explains why C10 appears on many cards. It does not mean the card can write at only 10 MB/s. It means that the card meets that class’s minimum requirement; its actual performance can be higher.
That distinction matters when reading a label. A card advertised with a much larger maximum transfer figure can still display C10 because C10 is a minimum-performance classification, not a statement of its top speed.
U1 and U3 are UHS Speed Classes
The UHS Speed Class system uses a number inside a U-shaped symbol. UHS stands for Ultra High Speed.
There are two familiar marks:
- U1 corresponds to a minimum sequential write speed of 10 MB/s;
- U3 corresponds to a minimum sequential write speed of 30 MB/s.
This creates an apparent duplication: C10 and U1 both correspond to 10 MB/s minimum sequential write performance. They belong to different speed-class systems, however, so a card can carry both marks.
Do not confuse the U1 or U3 speed-class mark with labels such as UHS-I or UHS-II. Those describe generations of the card’s bus interface—the connection used to move data between the card and the host device. A speed class describes assured performance under its class conditions; an interface describes how the card and device communicate and the transfer modes they can support.
A fast card also needs a compatible host interface to make full use of its interface capabilities. Putting a card designed for a newer, faster interface into a device with a more limited interface does not make the device’s card slot faster.
V6 through V90 are Video Speed Classes
The Video Speed Class system uses a V followed by a number. The number again corresponds to a minimum sequential write speed in megabytes per second:
- V6: 6 MB/s;
- V10: 10 MB/s;
- V30: 30 MB/s;
- V60: 60 MB/s;
- V90: 90 MB/s.
The system was designed around sustained recording requirements, including demanding video workloads. This is why camera specifications often mention a V rating.
Notice another overlap. U3 and V30 both correspond to 30 MB/s minimum sequential write performance in their respective class systems, just as C10, U1, and V10 all use 10 MB/s thresholds. Seeing several of these symbols together is therefore normal rather than contradictory.
The important practical rule is to follow the recording device’s stated requirement. Video resolution alone is not enough to choose a class. Two cameras recording at the same nominal resolution can use different bit rates, formats, frame rates, or recording features and therefore place different demands on storage.
A1 and A2 describe a different kind of workload
Some SD and microSD cards also carry A1 or A2, which belong to the Application Performance Class system.
These marks are not simply another set of video-speed labels. They were created for workloads such as running applications from a memory card, where many small, scattered reads and writes can matter as well as sequential transfer speed. The classes therefore include requirements for random input/output operations in addition to sustained sequential performance.
This is a useful example of why one large speed number cannot describe every storage workload. Reading one large video file sequentially is different from repeatedly accessing many small pieces of application data.
An A2 card also depends on suitable host support to take advantage of features associated with the A2 specification. The A2 mark should therefore not be treated as a promise that every device will make an A2 card noticeably faster than an A1 card.
Why one card can have so many symbols
Imagine a card marked C10, U3, and V30.
Those symbols do not mean the card has three unrelated real-world write speeds. They say that the card qualifies under several standardized class systems. C10 represents the older 10 MB/s class, while U3 and V30 each represent a 30 MB/s minimum sequential-write class within their respective systems.
The card may also have a separate figure such as an advertised read speed. That number serves a different purpose again and should not be substituted automatically for a required speed class.
A useful mental model is to treat the label as a collection of qualifications rather than a single speedometer. Each standardized mark tells you that the card meets a particular set of requirements.
Match the card to the device, not just the largest number
When buying a card for a device, start with the device manufacturer’s storage requirements. Look for the required card capacity type, supported interface, and speed class rather than choosing solely from the largest number printed on the package.
If a camera requires V30, for example, look for a card carrying V30 or a suitable higher Video Speed Class that the device supports. Do not assume that an unrelated high maximum-read figure proves the same thing.
Also remember that the host device can be the limiting part of the system. A card reader, camera, or computer slot that supports a slower interface may not achieve the card’s highest possible transfer rate. Conversely, buying a card with a much higher class than the device requires may provide little practical benefit for that particular recording mode.
Capacity compatibility matters separately from speed. A device’s documentation may limit which SD capacity families or card sizes it supports, so a card can be fast enough yet still be unsuitable for another reason.
The symbols are easier to read when you ask what each one guarantees
SD card labels look crowded because several generations of performance classifications can coexist on the same product. The marks become clearer once you stop treating them as competing maximum-speed claims.
C2 through C10, U1 and U3, and V6 through V90 are speed classes that communicate minimum sequential write performance within their respective systems. A1 and A2 address application-oriented performance, while UHS interface labels describe the connection between the card and host rather than the same kind of speed guarantee.
For everyday buying decisions, the safest approach is simple: check what the device requires, match the relevant standardized mark, and treat advertised maximum transfer speeds as additional information rather than a replacement for that requirement.