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The symbols on a microSD card describe different performance characteristics, not one overall speed. C, U and V marks specify minimum sequential performance, which matters when a device writes a continuous stream such as video. A1 and A2 describe application-oriented random and sequential access. UHS-I, UHS-II and UHS-III identify the bus interface between the card and host device.
Choose a card by matching the device manual’s required capacity, interface and speed-class family. A higher number or a newer interface cannot make an incompatible host faster, and none of these marks is a guarantee of the card’s maximum read speed.
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Read the label as several separate specifications
A single card can carry multiple marks because each one answers a different question. For example, a card labeled U3 V30 A2 UHS-I has a 30 MB/s minimum sequential tier under both the U3 and V30 systems, an application-performance designation, and a UHS-I interface. Those statements do not reveal its maximum read speed, nor do they prove that every device can use all of the card’s capabilities.
| Mark family | What it measures | Defined figures | What it does not tell you |
|---|---|---|---|
| C (Speed Class) | Minimum sequential performance | C2: 2 MB/s; C4: 4 MB/s; C6: 6 MB/s; C10: 10 MB/s | Maximum read speed or app performance |
| U (UHS Speed Class) | Minimum sequential performance when the supported UHS combination is in use | U1: 10 MB/s; U3: 30 MB/s | The bus generation’s ceiling or random-access performance |
| V (Video Speed Class) | Minimum sequential performance for video-oriented recording requirements | V6: 6 MB/s; V10: 10 MB/s; V30: 30 MB/s; V60: 60 MB/s; V90: 90 MB/s | The correct setting for every camera mode; consult the camera manual |
| A (Application Performance Class) | Application-style random and sequential access | A1 and A2 use separate combinations of requirements | A video-write guarantee or a replacement for host compatibility |
| UHS-I, UHS-II, UHS-III | Bus-interface generation between host and card | Interface maximums listed by the SD Association: 104, 324 and 624 MB/s respectively | A promise that a particular card or host will reach that maximum |
The SD Association’s Speed Class guidance defines C, U and V figures as minimum sequential access performance under specified standard conditions. They are rating thresholds, not ordinary laboratory measurements of one product.
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- Transfer files fast with up to 150MB/s[2] read speeds and SanDisk MobileMate USB micro 3.0 microSD card reader[6].
- Load apps faster with A1-rated performance[3].
C, U and V: the sequential-performance ratings
C Speed Class
The original Speed Class symbol is a capital C containing a number: C2, C4, C6 or C10. The number is the minimum sequential performance tier in megabytes per second (MB/s) under the standard’s conditions. C10 therefore represents a 10 MB/s minimum sequential tier.
Sequential performance is the relevant behavior when data is written or read as a continuous stream. Video recording is the clearest example: the camera must keep writing a sustained stream rather than handling a series of small, unrelated requests.
UHS Speed Class: U1 and U3
UHS Speed Class uses a U-shaped symbol containing 1 or 3. U1 indicates a 10 MB/s minimum sequential tier and U3 indicates 30 MB/s. The rating applies when the card and host are operating with a supported UHS interface and class combination, as explained in the Association’s Speed Class material.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesU1 is numerically equivalent to the 10 MB/s tier represented by C10, and U3 is numerically equivalent to the 30 MB/s tier represented by V30. That numerical similarity does not make every mark interchangeable for a device requirement. If a manual specifically calls for a particular class family, follow that wording or the manufacturer’s compatibility list.
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- EXPAND YOUR STORAGE. Insert your card to add massive storage up to 1.5TB[1] to your Android smartphones and tablets, digital cameras, and laptops.
- SPACE FOR MORE. With expansive capacities up to 1.5TB[1], capture and store hours of Full HD video[4], movies, music, games, photos, and podcasts.
- MOVE FILES FAST. Use your card with the SANDISK QuickFlow microSD UHS-I Card USB-A Reader[6] to achieve up to 195MB/s[2] read speeds [128GB-1.5TB models] and offload your content fast.
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Video Speed Class: V6 through V90
Video Speed Class places a V before the minimum sequential tier: V6, V10, V30, V60 or V90. The SD Association’s current video guidance lists V30 at 30 MB/s, V60 at 60 MB/s and V90 at 90 MB/s; the number in each mark denotes the class’s minimum sequential tier. See the Speed Class Standards for Video Recording page for the Association’s current definitions.
Do not choose a video class from resolution alone. A camera’s frame rate, bit rate, capture mode and file format can change the sustained-write requirement. A manual might specify one class for a lower-rate mode and a higher class for a demanding mode at the same nominal resolution. The camera or recorder’s own requirement controls.
A1 and A2: application performance, not another video tier
A1 and A2 are Application Performance Class marks. They address the access pattern produced when an operating system runs applications from the card: a mixture of random reads, random writes and sequential transfers. They are therefore different from C, U and V, which primarily communicate a minimum sequential tier.
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The Association defines A1 and A2 through separate combinations of random and sequential performance requirements rather than by a single MB/s number. Its Application Performance Class guide explains those requirements. An A2 mark is not a guarantee that a camera will sustain a particular video mode, and a card without an A mark is not automatically unsuitable for ordinary file storage.
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- MAXIMIZE WITH MASSIVE CAPACITY. Record longer and store more with up to 2TB[1] of storage.
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Consider A1 or A2 only when the host actually supports installing or running applications from removable storage. If the device uses the card only for photos, video or document files, the application’s access pattern is not the deciding specification. Capacity, compatibility and the required sequential class remain more important.
UHS-I, UHS-II and UHS-III: the bus interface
UHS in a bus label means Ultra High Speed interface generation; it is not the same thing as U1 or U3. The SD Association FAQ lists maximum interface rates of 104 MB/s for UHS-I, 324 MB/s for UHS-II and 624 MB/s for UHS-III. These are interface ceilings defined for compatible card-and-host combinations, not guaranteed transfer rates for every product. The figures and the compatibility explanation appear in the Association’s FAQ.
Both ends of the connection matter. A UHS-II card in a UHS-I host cannot make that host communicate at the UHS-II ceiling; the system operates according to the compatible interface it supports. Conversely, a host with a faster bus still needs a card whose own specifications and class meet the recording or application requirement.
Bus speed and class speed answer different questions. A high interface ceiling does not itself prove that a card meets a required sequential class, while a card with the required class cannot add a missing bus capability to the device.
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- Save time with card offload speeds of up to 190MB/s powered by SanDisk QuickFlow Technology (Up to 190MB/s read speeds, engineered with proprietary technology to reach speeds beyond UHS-I 104MB/s, requires compatible devices capable of reaching such speeds. Based on internal testing; performance may be lower depending upon host device interface, usage conditions and other factors. 1MB=1,000,000 bytes. SanDisk QuickFlow Technology is only available for 64GB, 128GB, 256GB, 400GB, 512GB, and 1TB capacities. 1GB=1,000,000,000 bytes and 1TB=1,000,000,000,000 bytes. Actual user storage less.)
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- Up to 90MB/s write speeds for fast shooting (Based on internal testing; performance may be lower depending upon host device interface, usage conditions and other factors. 1MB=1,000,000 bytes.)
- 4K and 5K UHD-ready with UHS Speed Class 3 (U3) and Video Speed Class 30 (V30) (Compatible device required. Full HD (1920x1080), 4K UHD (3840 x 2160), and 5K UHD (5120 X 2880) support may vary based upon host device, file attributes and other factors. See HD page on SanDisk site. UHS Speed Class 3 (U3) designates a performance option designed to support real-time video recording with UHS-enabled host devices. Video Speed Class 30 (V30), sustained video capture rate of 30MB/s, designates a performance option designed to support real-time video recording with UHS-enabled host devices. See the SD Association’s official website.)
Why a 30 MB/s class is not a 30 MB/s promise in every situation
The SD Association describes the C, U and V figures as minimum sequential-access performance under the conditions specified by the standard. They are not statements that the card always reads or writes at exactly that rate. Real transfers vary with the host controller, firmware, file size, workload, temperature, free space and other system conditions.
Class marks also do not state the card’s maximum read speed. Manufacturers may advertise a much higher peak read or write figure, but that figure is a separate product specification and should be compared under the manufacturer’s stated test conditions. Do not substitute an advertised maximum for the class requirement a device manual gives you.
How to select a microSD card for a particular device
- Open the device manual or support page. Record the accepted card type, capacity range, required interface, and the exact speed-class mark or family. A device can impose a capacity limit even when the card’s physical size fits.
- Identify the workload. Continuous recording points you toward the required C, U or V minimum sequential tier. Running applications from the card makes A1/A2 relevant. Simple file storage may need only the class and capacity stated by the host.
- Meet or exceed the required grade in the required family. The SD Association says a host that calls for a class can use the same or a higher grade in that class family. Do not assume that equal numbers from different families are automatically interchangeable; a U3 requirement and a V30 label both show a 30 MB/s tier, but the manual’s wording still governs.
- Check the interface on both sides. Match the card’s UHS-I, UHS-II or UHS-III capability to what the host supports. A faster card is usable only at the host’s supported interface level, and a faster host cannot raise a card’s own limits.
- For video, check the exact recording mode. Look for frame rate, bit-rate, codec or multi-file recording requirements in the camera or recorder documentation. Use the class named for that mode rather than selecting solely by “HD” or “4K” branding.
- Consider A1/A2 only for app workloads. If the device does not run applications from removable storage, an A mark should not outweigh the host’s required video class, bus interface or capacity.
- Compare advertised maximum speeds separately. Once compatibility and minimum class are satisfied, maximum read/write figures can help estimate file-transfer time. Treat them as manufacturer specifications, not as replacement class guarantees.
Worked examples of reading common labels
U3 V30 A2 UHS-I
U3 identifies a 30 MB/s minimum sequential tier in the UHS Speed Class system. V30 identifies the same 30 MB/s tier in the Video Speed Class system. A2 signals application-oriented performance requirements, and UHS-I identifies the bus generation. None of those marks states the card’s peak read rate.
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C10 and U1 each represent a 10 MB/s minimum sequential tier, but they belong to different class systems. If a device manual names one family specifically, use a card that satisfies that stated requirement rather than treating the matching number as universal equivalence.
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V60 on a camera that lists V30
V60 is a higher video-class tier than V30, but you still need to verify that the card’s capacity and interface are accepted by the camera. A higher class does not correct an unsupported capacity or bus, and the camera’s manual remains the authority for the recording mode.
Comparing two cards without being misled by the symbols
Use the following order when two cards appear similar:
| Comparison question | Why it matters |
|---|---|
| Do both meet the required C, U or V family and minimum tier? | The host’s sustained-write requirement is the first compatibility gate. |
| Do both support the interface the device can use? | UHS-I, UHS-II and UHS-III describe different bus generations; the host limits the usable mode. |
| Does the workload run applications from the card? | If so, compare A1/A2 and the device’s app-storage support; otherwise those marks may be irrelevant. |
| Are capacity and card type supported? | A card can meet a speed class yet fail the host’s capacity or format compatibility requirement. |
| What are the advertised maximum read/write figures? | These can affect transfer time after compatibility is established, but they are separate from minimum class ratings. |
Common mistakes and how to avoid them
- Reading UHS-I as U1. The Roman-numeral-style interface label and the U-shaped class mark are separate specifications.
- Assuming the highest number is always best. A V90 or UHS-III label is useful only if the host supports it and the workload benefits from it. It does not bypass a capacity or interface restriction.
- Using resolution as the only video rule. Frame rate and recording mode can change the required sustained performance, so check the manual’s exact entry.
- Confusing minimum sequential speed with peak read speed. A class threshold is a floor under standard conditions; a product’s advertised maximum is a different figure.
- Buying A2 for a device that never runs apps. Application marks address a specific access pattern and do not replace a video or host-compatibility requirement.
- Assuming a card label overrides the host. The device determines the interface it can negotiate and the capacities it can address.
Transferring files to a computer
A separate USB microSD reader is relevant only when the computer lacks an appropriate card slot or has a slot built to an earlier standard. The reader must also support the interface and card type you intend to use; otherwise it can become the limiting component during transfers. The SD Association discusses reader and host behavior in its FAQ.
Why Video Speed Class was introduced
Video Speed Class was designed to give device and card makers a sustained-write class system for increasingly demanding video formats. In a February 25, 2016 announcement, then SD Association president Brian Kumagai said:
“With Video Speed Class, SD device and memory card manufacturers can offer the highest-quality video recording to consumers and professionals that keep pace with the awesome video resolutions not only offered today, but anticipated tomorrow.”
The statement comes from the Association’s February 25, 2016 announcement. It describes the standard’s intended role at launch, not a recommendation for a particular current card or camera.
The practical rule
Start with the manual, then read the card label in layers: C, U or V for minimum sequential performance; A1 or A2 for app-oriented access; and UHS-I, UHS-II or UHS-III for the bus interface. Match the required family, grade, capacity and interface before considering peak transfer figures. That process prevents the most common mistake: treating several different symbols as if they were one universal speed score.
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