Memory card speed classes are minimum sustained write guarantees, not peak transfer numbers, which is why a card that benchmarks at 200 MB/s can still stop a 4K recording four minutes in. The marks on the label — U1, U3, V30, V60, V90 — describe the slowest continuous write the media will hold under load. If the camera’s bitrate exceeds that floor, the recording ends rather than slowing down.
Buying rules are short. V30 covers most 4K recording at common bitrates; V60 and V90 cover high-frame-rate 4K, 8K, and most RAW video modes. Anything below V30 belongs in a stills camera. The rest of this page explains what each mark actually guarantees and how a card that is merely slightly too slow costs you footage.
Speed Classes Measure the Slowest Write, Not the Fastest Read
Card packaging leads with the largest number it can print: 170 MB/s, 300 MB/s, sometimes 985 MB/s. Those figures are sequential read speeds, the rate at which the card hands data to a computer. Recording is the opposite direction. A camera writes continuously for minutes at a time, and the number that decides whether that works is sustained write speed, which is typically one-third to one-sixth of the advertised read speed.
The SD Association defines three overlapping grading systems, all expressed in megabytes per second:
- Speed Class (C): C2, C4, C6, C10 — the original marking, still printed on many cards. C10 guarantees 10 MB/s.
- UHS Speed Class (U): U1 and U3, guaranteeing 10 MB/s and 30 MB/s. These apply only when the card runs in a UHS bus mode.
- Video Speed Class (V): V6, V10, V30, V60, V90 — the current standard, and the only one designed around video recording. A V-class rating guarantees the stated write speed even when the card is fragmented, which is the state a camera card spends most of its life in.
That last point is the reason V-class numbers exist at all. A card full of interleaved clips is harder to write to than an empty one, because the controller has to place each write across scattered free blocks. A card that writes 80 MB/s fresh out of the package can fall well under 30 MB/s when it is 90 percent full. The V-class test is specified to hold the guaranteed rate in exactly that condition.
The guarantee is a floor certified on new media, not a promise about the card’s worst day. Flash wears out, and a card that has exhausted its spare blocks will eventually drop below its own class rating. That is one reason a card that worked for two years suddenly begins failing long recordings.
What Each Label Actually Guarantees
The marks are cumulative rather than exclusive — a V30 card is normally also stamped U3 and C10 — but they are not interchangeable, because the bus requirement differs.
| Mark on the card | Guaranteed minimum sustained write | Bus typically needed | Typical use |
|---|---|---|---|
| C10 | 10 MB/s | Default or High Speed | Stills, 1080p |
| U1 | 10 MB/s | UHS-I | 1080p, light 4K |
| U3 | 30 MB/s | UHS-I | 4K at moderate bitrates |
| V6 | 6 MB/s | Any | 720p, 1080p |
| V10 | 10 MB/s | UHS-I | 1080p, 4K at low bitrates |
| V30 | 30 MB/s | UHS-I | 4K 30p/60p, 100 Mbps Log |
| V60 | 60 MB/s | UHS-II | High-bitrate 4K 120p, 4K RAW |
| V90 | 90 MB/s | UHS-II | 8K, 6K RAW, ProRes |
The arithmetic that matters is simple: megabits per second divided by eight gives megabytes per second. A 400 Mbps recording mode writes 50 MB/s, so it needs V60, and a V30 card will fail it no matter how good the camera is. A 100 Mbps mode writes 12.5 MB/s and is comfortable on a V30 card with headroom for the camera’s write overhead.
A1 and A2 marks also appear on many cards, rating random input/output operations per second for running apps from the card in a phone or handheld. They say nothing about video recording; the V mark is what a camera cares about.
What Actually Happens When the Class Is Too Low
A card that cannot sustain the required write rate produces four distinct failures, and only one is obvious.
The recording stops and the clip is short or missing. Most cameras detect the underrun, close the file, and return to standby. Because the camera keeps working afterward, the loss is easy to miss until import, especially on a helmet-mounted action camera with no screen in view.
The clip exists but plays back with dropped frames. In high-bitrate Log or All-Intra modes, the encoder keeps producing frames while the card absorbs fewer than it should. The result is footage that opens normally and stutters through motion.
Burst shooting locks up. A slow card drains the buffer slowly, so a two-second RAW burst is followed by ten or fifteen seconds of the camera refusing to fire. Photographers usually blame autofocus or the body; the card is the bottleneck.
An interrupted recording leaves an unplayable file. Power loss and card underruns destroy the file index rather than the video data. The clip usually contains every frame it captured but lacks the moov atom a player needs to navigate it, so it shows a normal file size in Windows and refuses to open.
Heat makes all four more likely, and it explains a pattern that puzzles shooters: a card that passed a bench test indoors fails after twenty minutes in direct sun. Compact and action cameras have no active cooling, and the card sits beside the sensor and battery. Sustained write speed falls when the controller throttles, so a card that held V60 on the desk may not hold V30 in the field. Throttling-related recording stops are covered in our page on action camera thermal throttling.
Which Class Your Camera Actually Needs
The recording mode, not the camera’s marketing tier, sets the requirement. Resolve the mode’s bitrate, divide by eight, and choose a class above the result, not equal to it — cameras add write overhead on top of the stream, and a card should not run at 100 percent of its rating for an hour.
| Recording mode | Typical bitrate | Continuous write | Minimum class | Notes |
|---|---|---|---|---|
| 1080p 60 H.264 | 50 Mbps | 6 MB/s | U1 or V10 | U3 gives useful headroom |
| 4K 30p H.264 | 100 Mbps | 12.5 MB/s | V30 | Most current mirrorless defaults |
| 4K 60p HEVC | 200 Mbps | 25 MB/s | V30 | Close to the V30 floor |
| 4K 120p or All-Intra | 400–600 Mbps | 50–75 MB/s | V60 or V90 | UHS-II body required |
| 6K or 8K 30p | 600–800 Mbps | 75–100 MB/s | V90 | Often CFexpress instead |
| ProRes 422 HQ 4K | about 880 Mbps | 110 MB/s | V90 or CFexpress | SD cannot sustain this |
| RAW still bursts, 45 MP and up | Burst-dependent | 40–90 MB/s | V60 or UHS-II | Buffer clearing speed |
Two entries deserve emphasis. First, 4K is not a speed requirement — 4K at 100 Mbps and 4K at 800 Mbps differ by a factor of eight, which is why advice to “buy V30 for 4K” is correct for one camera and wrong for the next. Second, RAW video formats such as ProRes and Blackmagic RAW exceed what any SD card can write reliably; those modes live on CFexpress or a dedicated SSD path. Professional media is its own topic, covered in our guide to CFexpress card recovery.
Faster Cards That Still Fail: Bus, Reader, and Counterfeit Traps
Buying above the required class does not automatically solve the problem, because three other links in the chain can cap the effective speed.
Bus mismatch. A UHS-II card in a UHS-I slot falls back to UHS-I signaling, and the extra row of contacts goes unused. A V90 card in such a body behaves like a well-made UHS-I card, which is usually fine, but you paid for speed the camera cannot reach. The reverse never happens: a UHS-I card cannot be made faster by a UHS-II body.
Reader bottleneck. A USB 2.0 reader caps imports near 40 MB/s, which affects copy time rather than recording but pushes people to buy faster cards for the wrong reason.
Counterfeit capacity. The most damaging trap is a card that reports more capacity than the NAND inside can hold. Such a card writes quickly until it passes its real capacity, then silently overwrites the oldest data — which is why files that “already copied fine” vanish later. Fakes are common in high-capacity, high-speed listings at implausible prices. The verification method is set out in our article on fake memory card risks.
One advantage of card media has nothing to do with speed: SD and microSD cards have no TRIM command. When data is deleted or quick-formatted, the bytes stay in place until something writes over them, which is why deleted footage is often still recoverable — the same mechanism that explains why deleted files can be recovered at all.
Buying and Testing Rules That Prevent Mid-Recording Failures
- Size the class for your worst-case mode, not your most common one. If you shoot 4K 120p twice a year, those two shoots are the ones that matter.
- Prefer two 128 GB cards to one 512 GB card. Capacity is a single point of failure, and two cards spread write wear.
- Buy from authorized retailers and verify capacity on arrival. A tool such as h2testw on Windows or f3 on macOS writes until the card is full and reads everything back, catching both counterfeit capacity and failing cells. Our card health testing guide covers the procedure.
- Format in the camera. Camera formatting matches the write pattern the camera expects and is effectively a quick format that rebuilds the index.
- Retire a card at its first error. A card that stops a recording once will do it again, and the next failure may land on a shot you cannot repeat.
If a card has already caused a loss, the first move is to stop writing to it. If you are on Windows and the card is still detected by a computer, a purpose-built recovery tool such as XRecovery (xappset.com) can scan SD, microSD, and other memory cards for deleted or formatted photos and videos. As with any recovery tool, stop writing to the card first and always save recovered files to a different drive. When the card has been overwritten, no software recovers it; when the card is physically damaged, the work belongs to a data recovery laboratory. Our how to recover camera data walkthrough covers what to do in each case.
Frequently Asked Questions
Is U3 the same as V30?
Both guarantee 30 MB/s of sustained write, but they are tested differently. U3 applies only in a UHS bus mode and is verified on a freshly formatted card, while V30 must hold 30 MB/s on a fragmented card, which is closer to real shooting conditions.
Do I need a V90 card for 4K video?
Only for demanding variants. Ordinary 4K 30p at 100 Mbps writes about 12.5 MB/s, so a V30 card is more than adequate. High-frame-rate 4K, All-Intra 4K, 8K, and 6K RAW video push 50 MB/s and higher, and those modes justify V60 or V90.
Why did my V30 card stop a 4K recording?
Three causes dominate: the recording mode exceeds the class (4K 120p All-Intra can need 75 MB/s), the card overheated and throttled below its rating, or the card is a counterfeit with less real capacity than it reports.
Does a slow card corrupt files, or only stop recording?
It can do both. An underrun that interrupts the write leaves the clip without a proper index, so the file appears intact but will not play. The video frames are usually still present; the file needs repair rather than recovery.
Can I use a V60 card in a camera that only supports UHS-I?
Yes. The card runs at UHS-I speeds, so you will not get the rated 60 MB/s, but nothing breaks. Class ratings are backward compatible; the bus, not the card, sets the ceiling.