The short answer
Class 10, U1, U3 and V30 are labels from the SD Association that describe the slowest speed at which a card keeps writing. They are floors, not top speeds. Class 10 means at least 10 megabytes per second of sustained writing, U1 means 10 megabytes per second, U3 means 30 megabytes per second, and V30 means 30 megabytes per second. A 1080P dash cam usually works with Class 10 or U1. A 4K dash cam is best paired with U3 or V30. Because these are public standards, you can check them on any card, and any supplier can print them in a specification. This note explains the symbols and how to apply them.
The three marking systems
Cards carry up to three speed markings because the standard grew over time. The oldest, the Speed Class, shows a number inside a circle: Class 10 is the highest. The UHS Speed Class shows a number inside a U: U1 and U3. The Video Speed Class shows a V followed by a number: V6, V10, V30, V60, V90. All three measure the same idea, minimum sustained write speed, but in different ways and for different workloads. The table collects the common values.
| Marking | Minimum sustained write | Typical dash cam use |
|---|---|---|
| Class 10 | 10 MB/s | 1080P single and dual-channel |
| U1 | 10 MB/s | 1080P and some 1440P |
| V10 | 10 MB/s | 1080P to 1440P |
| U3 | 30 MB/s | 4K single-channel, dual-channel 4K |
| V30 | 30 MB/s | 4K single-channel, dual-channel 4K |
Higher video classes such as V60 and V90 exist for professional cameras and give no benefit in a dash cam. A card can carry several markings at once, for example U3 and V30 together, and that is the normal pattern for a card meant for 4K.
Megabytes versus megabits
A frequent confusion is the unit. Speed classes use megabytes per second. Camera bitrates use megabits per second. One byte is eight bits, so a 20 megabit stream is 2.5 megabytes per second. Against that, the 10 megabytes per second floor of Class 10 looks generous, and indeed a 1080P or 4K stream at typical dash cam bitrates sits well below even Class 10. So why ask for U3 on a 4K camera? The reasons are margin and consistency. The card must keep up while also handling file system work, writing two channels, and processing event locks. A card with more headroom is less likely to stall at the moment of an impact, when the camera is busiest.
Why the class is not the whole story
Speed class describes writing speed, not durability. A fast card that is not rated for continuous recording can still wear out early, as the high-endurance note explains. The two properties are independent: a card can be fast but fragile, or slow but durable. For a dash cam the right card meets the speed floor and carries an endurance rating. Another point is that the class describes a sustained minimum measured under defined conditions; the burst speed on the packaging, often a large number, is a read speed and tells you little about video writing. A buyer who reads only the largest number on the box may pick the wrong card.
Matching the class to the camera
The camera's resolution and codec decide the bitrate, and the bitrate decides the floor. Use the camera's spec sheet and the manual, which often names a recommended class. If the manual says U3, use U3, even though a lower class may seem to work in a short test. Cameras tested briefly with a slow card may look normal and then fail on long drives, when the card's internal housekeeping slows it down. The 4K dash cam lines are specified with U3 or V30 for this reason, and the storage note shows how bitrate and capacity combine.
What happens with a card that is too slow
A card below the camera's need produces symptoms that look like camera faults: the recording stops with a card error, files have missing seconds, the screen shows a write error, or the camera restarts. A seller sees these as returns marked "defective". Before returning a camera, check the card class and test with a card of the right class. A quick test is to record a long clip in a busy scene and play it back in full; stalls and gaps point to the card. The corrupted files note lists other causes to rule out.
Reading the symbols on a real card
In practice the buyer meets these classes on the card body and in the supplier's listing. On the card, look for the circle, the U and the V symbols printed near the capacity. In a listing, the class usually appears in the title or the specification table; if it appears nowhere, ask, because absence of a class statement is itself information. Also separate two other numbers often shown on packaging: read speed, given as a large figure, and the class markings, which are about writing. A listing that quotes only a big read speed and no class tells you how fast the card empties onto a computer, not whether it can keep up with a camera. When comparing two cards at the same capacity and class, the decision should rest on endurance and temperature range, not on read speed.
Putting it in a specification
When you write a card requirement, include the class by name and by resolution. A typical line for a 1080P program reads: microSD, 64GB, Class 10 or U1, high-endurance. A typical line for a 4K program reads: microSD, 128GB, U3 and V30, high-endurance. Add that the marking must be printed on the card, because a card with no marking tells you nothing. For a pre-bundled program, ask the supplier to attach a photo of the printed card marking to the sample report. The dash cam SD cards page lists the lines we bundle, and the selection checklist gives the full list of questions.
Where the desk fits in
The desk reads the card marking on each sample, matches it to the camera's resolution and records it in the program sheet; final specifications are confirmed in writing for each program. For more on the process, see our procurement process, and to add a card line to a quotation, use the quote form.