How much video fits on an SD card?

The number that matters is bitrate

Resolution alone does not determine file size. Two cameras can both record 4K while producing radically different amounts of data because their codecs, frame rates, bit depths and compression settings differ. The reliable input is the recording mode’s bitrate, normally shown in megabits per second (Mb/s or Mbps).

To convert bitrate into storage, multiply the card’s usable gigabytes by eight to turn bytes into bits, then divide by the bitrate. Apertally reserves a small percentage because formatted capacity, file-system overhead and sensible working headroom mean the number printed on a card is never all available to your clip.

Capacity and speed solve different problems

A larger card records for longer. A faster card sustains demanding recording modes without the camera stopping or refusing the setting. Buying 512 GB does not help if its guaranteed write speed is below what the codec requires; buying V90 does not extend record time if the capacity stays the same.

Video Speed Class labels are minimum sustained write guarantees: V30 means 30 MB/s, V60 means 60 MB/s and V90 means 90 MB/s. Camera bitrates are usually quoted in megabits, so divide by eight before comparing them with a card’s megabytes-per-second rating, then leave margin.

A field-ready card plan

For paid work, one enormous card is not always the safest answer. Several known-good cards reduce the amount of material exposed to a single loss or failure, while a simple naming and backup routine prevents accidental formatting.

• Check the exact recording mode in the camera manual, not a generic 4K label.
• Use the calculator with the highest bitrate you expect to select.
• Add at least one spare card and format cards in-camera before the job.
• Back up to two places before reusing a card when the footage matters.