How the calculation works
Usable bytes = capacity × 1,000,000,000 × (1 − reserved %); photos = usable bytes ÷ photo size; minutes of video = usable bytes × 8 ÷ bitrate ÷ 60
A worked example
A 128 GB card with 3 % reserved leaves 124.16 GB usable: 4,966 photos at 25 MB each, or 165 minutes of 4K video at 100 Mbps.
Bitrate, not resolution, fills the card
Video size depends on the bitrate the camera records at: Mbps means megabits per second, and eight bits make a byte. 100 Mbps writes 12.5 MB every second, 45 GB an hour. Check the bitrate in your camera's video settings rather than going by resolution alone.
Speed class matters as much as capacity
A card must keep up with the bitrate. V30 cards sustain at least 30 MB/s of writing, enough for most 4K at 100–200 Mbps; V60 and V90 cards are for high-bitrate 4K, 8K and RAW video. Your camera's manual names the class it needs.
Common questions
Why does my camera show fewer shots remaining?
Cameras estimate conservatively and file sizes vary with each scene: a detailed landscape makes a bigger file than a plain sky. Use your camera's own average file size for the best estimate.
What is Mbps?
Megabits per second, the rate a camera writes video at. Divide by 8 for megabytes per second: 100 Mbps is 12.5 MB/s.
Method: arithmetic from the label values you enter. Reviewed .