What size SD card for an action camera?

The one calculation you need

Video file size is bitrate multiplied by time. Nothing else. The confusion comes from the units, because cameras publish bitrate in megabits and cards are sold in gigabytes.

Divide the bitrate in megabits per second by eight to get megabytes per second. Multiply by 60 for a minute, by 3600 for an hour, and divide by 1000 to land in gigabytes. That is the whole method, and it means you never have to trust a card manufacturer's "holds up to 20 hours of video!" claim, which is always quoted at a resolution nobody shoots.

Two caveats on that table. Card capacities are quoted in decimal gigabytes and formatted capacity is always a little lower, so treat every figure as an optimistic ceiling. And higher-resolution modes carry higher bitrates — a 5.3K or 8K setting is the top row of nobody's calculation.

Speed is a different specification, and it is not optional

Capacity decides how much you can record. The speed class decides whether the recording survives at all. A card that cannot sustain the write rate stops the clip by itself, usually with an unhelpful error, and usually in the middle of something you cannot repeat.

V30 is the floor for 4K recording on every camera we rank, and all three cards above publish U3, V30, A2 and Class 10. The SanDisk Extreme publishes up to 190MB/s read and 130MB/s write; the Samsung PRO Plus publishes up to 180MB/s read and 130MB/s write; the Lexar Professional 1066x publishes up to 160MB/s read and 120MB/s write at 128GB and above. Those read figures are about offloading speed, not recording — the write figure and the V rating are what keep a clip alive. SD card speed classes compared unpacks what each symbol on the card actually guarantees.

Why we recommend two 128GB cards over one 256GB

This is the part most size guides skip, and on water it is the whole argument.

A card is a single point of failure. Cards fail rarely and completely. One 512GB card holding a week of a trip is a week of a trip you can lose at once; two or three smaller cards spread that risk so a failure costs one day.

Cards get lost, and they are small. A microSD card dropped on a dock or in the bottom of a kayak is gone. Spreading a trip across several cards means a lost one is annoying rather than catastrophic.

Offloading is slow. Copying 256GB of footage off a card in a cabin with a laptop is an evening. Smaller cards let you offload one while you shoot on the next.

A full card on the water has no good answer. Changing a card in a boat is how cards end up in the lake. Size the card to the whole session, then stop.

The exception is a format with fixed storage. The Insta360 GO 3S, for example, ships with fixed 64GB or 128GB and no card slot, so the capacity you buy is the capacity you keep — on those bodies, buy up front.

Sizing by what you actually shoot

A day paddle, mostly short clips: 128GB is comfortable at any 4K setting, and you will come home with spare room.

A full day of riding with the camera running on a helmet: run the arithmetic at your camera's published bitrate before you leave. A high-bitrate 4K60 mode will fill 128GB in under two hours, and continuous recording also raises the thermal question — stop fogging and overheating covers that.

A time lapse running for hours: far less data than it feels like, because a lapse writes a fraction of the frames. Battery is the constraint there, not the card — time lapse vs TimeWarp has the intervals.

A 360 camera: size up. A 360 camera captures a whole sphere at a high bitrate and then you keep the original file as well as the reframed export, so the card and the laptop both need room.

A multi-day trip with no laptop: this is the only case where a single large card is the right answer, because you have nowhere to offload. Even then, carry a second card rather than one bigger one.

Mistakes that cost footage

Buying on capacity alone. A 512GB card with no V rating printed on it is a card that will stop a 4K recording. The rating matters more than the size, every time.

Buying a suspiciously cheap high-capacity card. Counterfeit cards report a capacity they do not have, and the failure shows up when the card passes the size it really is — which is to say, in the middle of your trip. Buy from a seller you can hold responsible.

Formatting in a computer rather than the camera. Format in the camera, every time. It writes the file system the camera expects, and it is also the quickest way to clear a card that has started behaving strangely.

Never formatting at all. Offload, verify the copy, then format in the camera before the next trip. A card used for a year without a format accumulates fragments and odd directories, and the recording that fails will be the one you cared about.

Leaving the only copy on the card. A card is not a backup, and a card in a camera on a boat is the least safe place your footage will ever live. Copy it the same evening — how to edit on a phone covers getting clips off without a computer.