The symbols on an SD card describe two different things: a minimum sustained-write class and, sometimes, the card’s bus interface. C2–C10, U1/U3, and V6–V90 are speed-class marks; UHS-I, UHS-II, and UHS-III identify interface generations. To choose a card that records reliably, match the class required by your device’s exact recording mode, then check the supported interface, card format, and capacity. A higher number or newer interface is not automatically compatible or useful.
What the symbols on an SD card actually mean
SD cards can carry several marks at once because the marks belong to separate standards. A speed class specifies a minimum sequential write performance under defined test conditions. The UHS label next to it identifies the electrical bus used to communicate with the host device. Neither mark is a guarantee of the card’s maximum read speed, and neither tells you by itself whether a particular camera mode will work.
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| Marking family | Symbols | Named minimum performance | What it describes |
|---|---|---|---|
| SD Speed Class | C2, C4, C6, C10 | 2, 4, 6, 10 MB/s | The original C-shaped speed-class system. |
| UHS Speed Class | U1, U3 | 10, 30 MB/s | A minimum-write class used with UHS-capable card families. |
| Video Speed Class | V6, V10, V30, V60, V90 | 6, 10, 30, 60, 90 MB/s | A class system intended for modern video-capture workloads. |
| SD Express Speed Class | E150, E300, E450, E600 | 150, 300, 450, 600 MB/s | A separate class family for SD Express cards; it is not a replacement name for C, U, or V marks. |
The number in each family corresponds to that family’s named minimum megabytes-per-second value. These are assured floors for the relevant class method, not a promise that every operation will run at that rate. Actual behavior depends on the card, the host device, the interface and the workload.
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Camera recording is a sustained-write task. The device must keep accepting a continuous stream of data, even when the card’s flash memory is busy with housekeeping. Speed classes were created to give a minimum sequential-access guarantee under standard conditions, including multiple-stream access where the standard defines it. The SD Association summarizes this principle as: “All speed class types assures minimum sequential card access performance including multiple stream access (in SD Express Speed Classes) under defined conditions specified by the standard.”
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A package may advertise a much higher read or burst-write figure than the class mark. That figure can be useful when copying files, but it does not replace the minimum class required by a camera. Conversely, a card that meets a class floor may perform differently in random access, on a different host, or after its cache is exhausted.
UHS Speed Class versus UHS bus interface
U1 and U3 are class symbols
The U-shaped symbol containing “1” or “3” is a UHS Speed Class mark. U1 names a 10 MB/s minimum and U3 names a 30 MB/s minimum under the UHS class method. It answers the question, “What minimum sustained write class does this card claim?”
UHS-I, UHS-II and UHS-III are interfaces
UHS-I, UHS-II and UHS-III describe bus generations—the connection between card and host. The SD Association lists maximum interface figures of 104 MB/s for UHS-I, 324 MB/s for UHS-II and 624 MB/s for UHS-III. Those are capabilities of the interface, not guaranteed transfer rates for every card. Both the card and the host must support a generation to obtain its benefits.
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Therefore, “UHS-II” does not mean “U3.” A card can carry a UHS-II interface label and have a class marking that still needs to be checked against the camera’s recording requirement. Read both marks independently: one concerns the bus, the other sustained-write performance.
Why the host matters
A faster card cannot make a host device use an interface it does not implement. A camera with a UHS-I slot may continue to operate at its UHS-I limit when a UHS-II card is inserted, and a camera may also reject a recording mode if the card class is insufficient even when the bus is fast enough. The device manual is the authority for the combination it supports.
Video Speed Class: V6 through V90
Video Speed Class was introduced in February 2016 to describe sustained video workloads across newer flash technologies. Its marks are V6, V10, V30, V60 and V90, corresponding to 6, 10, 30, 60 and 90 MB/s minimums under the V-class method.
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A V mark is not simply a redesigned U mark with the same number. The class systems use different access methods and requirements. A card can satisfy one method without satisfying another at the same nominal rate. The SD Association gives the concrete example of an SDXC card that meets V30 requirements while meeting only C10 under the older Speed Class method. When a manual specifies V30, look for V30 rather than assuming that any 30 MB/s label is equivalent.
Broad video-use guidance—and its limits
The SD Association associates classes with common video ranges as a general guide:
| Typical association | Classes named by the SD Association | How to interpret it |
|---|---|---|
| HD through Full HD | Class 10, U1 or V10 | A broad starting point for lower-data-rate recording. |
| Full HD through 4K | U3 or V30 | Often used for higher-bitrate or 4K modes. |
| 4K UHD through 8K | V60 | Intended for substantially heavier sustained streams. |
| 8K UHD | V90 | For modes whose sustained rate falls within the camera’s V90 requirement. |
These associations are not a universal resolution-to-card rule. A camera’s bitrate, codec, frame rate, bit depth and selected recording mode determine the data stream. Two cameras that both say “4K” can specify different cards. Use the recording-mode table in the exact camera manual or the card warning shown on the device rather than buying solely from the resolution name.
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How to choose a card for a camera or recorder
- Identify the exact mode. Write down the resolution, frame rate, codec and quality setting you intend to use. If the device lists separate requirements for different modes, use the requirement for the heaviest mode you plan to record.
- Find the required class mark. Look for a minimum such as C10, U3, V30, V60 or V90 in the manual. Treat that mark as mandatory for reliable recording, not as an optional performance target.
- Check the bus interface. Confirm whether the slot supports UHS-I, UHS-II or UHS-III. A card with a newer interface may operate without its extra bus capability on a slower host, but it will not override a host’s class or compatibility limits.
- Verify card family, form factor and capacity. The manual may restrict the accepted SD-family format, physical size or maximum capacity. A speed mark alone does not establish that the card’s format and capacity are accepted.
- Compare useful headroom. A class above the minimum can provide margin for a demanding mode or a future device, but it can cost more without improving performance in a host that cannot use it. Do not choose by the largest advertised read number when sustained video writing is the task.
- Format and test in the device. After inserting a compatible card, use the camera’s own formatting command when the manual recommends it, then make a test recording in the intended mode. Confirm that the recording starts, continues without a card-speed warning and transfers correctly before an important shoot.
How to read common combinations on a card
C10 with UHS-I
C10 says the card meets the 10 MB/s SD Speed Class floor. UHS-I identifies the bus generation. This combination may be appropriate where the manual asks for Class 10 or a comparable lower-rate mode, but the camera’s own specification decides.
U3 with V30
U3 and V30 both indicate a 30 MB/s nominal minimum in their respective class systems. Seeing both marks can be useful, yet they are still separate claims made under separate methods. If a manual explicitly calls for V30, select a card carrying V30 rather than inferring equivalence from U3 alone.
UHS-II with V60 or V90
Here the interface generation and the video class address different constraints. V60 or V90 describes the sustained-write floor; UHS-II describes the bus. A device must support the interface and specify the class for the mode. A V90 mark by itself does not prove that a UHS-II host will obtain the interface’s maximum figure.
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Common questions and mistakes
Does V30 mean the card always writes at 30 MB/s?
No. V30 assures the class minimum under defined conditions. It is not a statement that every operation, file copy or burst reaches exactly 30 MB/s, and it is not a peak read-speed claim.
Are C10, U1 and V10 interchangeable?
They share a nominal 10 MB/s figure, but their class protocols differ. Use the symbol named by the host’s manual. The shared number is not enough to establish interchangeability.
Does UHS-II mean U3?
No. UHS-II is a bus-interface label; U3 is a speed-class label. Check both independently against the device specification.
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Should you always buy the highest number?
No. Without the host’s requirements there is no universally correct highest class. Extra headroom can be sensible when the host supports it, but an unsupported interface or unnecessary class can add cost without changing recording performance.
Do you need a V90 card?
Only if the exact recording mode requires V90 or your workflow has a documented reason to use that class. The SD Association’s broad guidance associates V90 with 8K UHD, but a camera-specific manual remains decisive. Do not select V90 merely because it is the largest V number.
What to do when recording fails
- “Card too slow” warning: Recheck the mode’s required class, not just the printed read speed. Move to the specified class or a supported higher class.
- Camera sees the card but limits modes: Confirm that the card carries the required class mark and that its format and capacity are allowed. Some devices expose only modes that meet their verified card requirements.
- New card performs like an older one: Compare the host’s bus generation with the card’s interface. A newer interface cannot raise a slower host’s bus limit.
- Intermittent stopped recordings: Test with a card that meets the manual’s sustained-write class, format it in the device if instructed, and avoid judging suitability from a short burst or a computer benchmark alone.
- Conflicting marks: When C, U and V symbols appear together, follow the exact symbol named in the recording-mode table. Do not substitute based only on equal numbers.
The practical rule
Start with the recording mode, not the card shelf. Select the minimum sustained-write class the device specifies, verify the supported UHS interface and card family, then pay for additional headroom only when the host can use it. C, U and V numbers are minimum class guarantees under their own methods; UHS-I, UHS-II and UHS-III are bus capabilities. Keeping those two ideas separate prevents most SD-card buying and recording errors.
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