Buying an SD card by its storage size is easy; buying it by its speed rating is where most mistakes happen. A 512 GB card with a slow write speed will choke a 4K recording mid-clip, while a smaller, faster card sails through. Speed classes exist precisely to answer one question: can this card sustain the data rate your camera produces?
Why Sustained Write Speed Matters
Cameras write video as a continuous stream. If the card cannot accept data as fast as the camera sends it, the buffer fills and the recording stops — the dreaded "recording interrupted" error. For still photography, a slow card means longer waits between burst shots and sluggish previews. Card makers advertise maximum read speeds — "up to 170 MB/s" — but reads are for offloading files to a computer, while writes are what the recording depends on. The headline number tells you almost nothing about recording performance; only the speed class guarantees what matters.
How SD Speed Classes Work
Speed classes are a promise of minimum sustained write speed — the slowest the card is allowed to get, not the fastest it can burst to. Three overlapping systems appear on card labels, and they all describe the same thing in different dialects:
- Speed Class (C2–C10): the original system; C10 guarantees 10 MB/s.
- UHS Speed Class (U1, U3): U1 guarantees 10 MB/s, U3 guarantees 30 MB/s.
- Video Speed Class (V6–V90): the one built for video; the number is the guaranteed MB/s.
The Video Speed Class is the simplest to read, because the rating is the answer:
| Class | Minimum sustained write | Typical use |
|---|---|---|
| V6 | 6 MB/s | 720p video, casual photos |
| V10 | 10 MB/s | 1080p video |
| V30 | 30 MB/s | 4K at 60–100 Mbps |
| V60 | 60 MB/s | 4K at high frame rates, 200 Mbps codecs |
| V90 | 90 MB/s | 8K, RAW video, demanding cinema codecs |
Converting Bitrate to Card Speed
Camera specs quote bitrate in megabits per second (Mbps), but cards quote speed in megabytes per second (MB/s). There are eight bits in a byte, so the conversion is one division:
MB/s = Mbps ÷ 8 — then add a 50% safety margin, because sustained write must exceed, not merely equal, the data rate.
A camera shooting 100 Mbps produces 12.5 MB/s of data. With a 50% margin, the card must sustain 18.8 MB/s — so the minimum purchase is a V30 card. A 200 Mbps codec needs 25 MB/s sustained, 37.5 MB/s with margin: that is V60 territory, and V60 cards require a UHS-II bus.
Match the Card to the Job
Speed is only one half of a safe purchase; the other half is fit. Work out your camera's bitrate first, apply the margin rule, then pick from this map:
| What you shoot | Data rate | Minimum card |
|---|---|---|
| 1080p60 action camera | 25 Mbps → 3.1 MB/s | V6 |
| 4K30 drone or camera | 100 Mbps → 12.5 MB/s | V30 (UHS-I) |
| 4K60 sports | 200 Mbps → 25 MB/s | V60 (UHS-II) |
| 8K or RAW video | 400+ Mbps → 50+ MB/s | V90 (UHS-II) |
| Burst stills only | Varies | V30 is a safe default |
A Worked Example: One Hour of 4K30
Take a typical 4K30 recording at 100 Mbps. The sustained data rate is 12.5 MB/s, and with the 50% margin the requirement is 18.8 MB/s — comfortably inside V30's 30 MB/s guarantee. Over an hour, the footage occupies about 43.9 GB. A 64 GB card holds roughly one hour; a 128 GB card holds a little under three. Upgrade the codec to 200 Mbps and both numbers double — 37.5 MB/s needed, about 87.9 GB per hour — which pushes the requirement up to V60 on a UHS-II card.
UHS-I vs UHS-II: Why the Bus Matters
Speed class is only half the story. V60 and V90 are UHS-II classes: the card carries a second row of contacts and only reaches its rated speed in a UHS-II reader or camera slot. Put a V90 card in a UHS-I camera and it falls back to UHS-I speeds, which is money wasted on performance you cannot use. For V6–V30 on the UHS-I bus the match is one-to-one; for anything above V30, check that both card and device speak UHS-II before paying the premium.
Practical Buying Tips
- Buy the speed class for the camera you will own next, not the one you own now — codecs only get hungrier.
- Dashcams and security cameras deserve endurance-rated cards; constant overwrites kill ordinary flash faster.
- The A2 rating (for running apps on phones) is separate from V-ratings — they answer different questions, so do not shop by A2 for video.
- Test every new card before an important trip: counterfeit cards lie on their labels, and a write test catches them early.
Try it now: SD Card Speed Requirement Calculator — enter your bitrate and recording length to find your minimum V-class and card size.
Frequently Asked Questions
What SD card speed do I need for 4K video?
A 4K30 clip at 100 Mbps needs 12.5 MB/s sustained, or 18.8 MB/s with a 50% safety margin — so V30 is the minimum on a UHS-I card. For 4K60 at 200 Mbps, you need 37.5 MB/s with margin, which means V60 on a UHS-II card.
What is the difference between U3 and V30?
Both guarantee 30 MB/s minimum sustained write; U3 is the UHS Speed Class marking and V30 is the Video Speed Class marking. V30 adds video-specific guarantees around frame-drop behavior, so for video work it is the label to look for.
Is a faster SD card always better?
A faster card never hurts — it also speeds up offloading files to your computer — but it costs more, and above V30 you need a UHS-II device to see the benefit. Match the card to the bitrate with the 50% margin rule, then spend the savings on capacity.