SD Card Speed Requirement Calculator

Why your card's speed matters more than its size

When you are shopping for a memory card, the first number you look at is usually the capacity — 128 GB, 256 GB, and so on. But for video work, capacity is the easy part. Speed is what decides whether your footage survives the shoot. If a card cannot write data fast enough, the camera's buffer fills up and recording simply stops — often with a dreaded recording error, or worse, a corrupted file. Photographers hit the same wall during long bursts: the camera locks up while it drains the buffer. That is why matching the card's sustained write speed to your camera's video bitrate matters far more than buying the biggest card you can afford.

How the speed-class math works

The math is refreshingly simple. Your camera's video bitrate is quoted in megabits per second (Mbps). Divide it by 8 to get megabytes per second (MB/s) — that is the sustained write speed your card must deliver, non-stop, for the whole take. Then multiply by 1.5x as a safety margin, because real-world cards slow down as they heat up and fill. The video speed classes printed on the card tell you the guaranteed minimum sustained write speed: V6 = 6 MB/s, V10 = 10 MB/s, V30 = 30 MB/s, V60 = 60 MB/s, and V90 = 90 MB/s. Pick the first class at or above your required number. One more thing: V60 and V90 cards need a UHS-II bus (the extra row of pins) to reach those speeds, so your camera must support UHS-II — an expensive V90 card in a UHS-I-only camera is wasted money.

Worked example

Say you are shooting 4K at 30 frames per second with a 100 Mbps codec, and you plan to record for a full hour. Divide 100 by 8 and you get 12.5 MB/s of sustained write traffic. Multiply by the 1.5x headroom and you need 18.75 MB/s. The first class that covers it is V30 (30 MB/s), which runs on the common UHS-I bus — a cheap and widely available card. Storage is the other half of the planning: 100 Mbps works out to about 43.9 GB per hour of footage, so a 64 GB card comfortably holds your one-hour shoot with room to spare.

Practical tips

Buy cards from reputable sellers only. Counterfeit cards are everywhere, and they lie about their speed ratings — a card labeled V30 that cannot sustain 30 MB/s will ruin a shoot. Always format the card in the camera, not on your PC: the camera writes the exact file system and allocation size it expects. Keep 10-20% of the card free, since nearly-full cards write slower and fragment more. There is no downside to a faster card than you need — a V90 card on a V30 workload is perfectly fine, and it also speeds up burst photography and file transfers to your computer. Finally, check your camera maker's approved-media list before buying, especially for high-bitrate modes, because some cameras are picky about specific card models.