How this calculator works
Subtract the safety margin from the usable storage, then divide the remaining capacity by the total data rate of the video and audio streams.
The formula
(capacity bytes × (1 − reserve)) ÷ ((video bitrate + audio bitrate) ÷ 8)Bitrates are normalized to bits per second, durations to seconds, and storage to bytes before calculation.
Worked examples
- A 128 GB card (10% safety margin) at 25 Mbps video and 192 Kbps audio holds roughly 10 hours and 12 minutes.
- A 64 GB card (5% safety margin) at 100 Mbps 4K holds roughly 1 hour and 21 minutes.
- A 256 GB drive (0% safety margin) at 50 Mbps holds roughly 11 hours and 22 minutes.
Frequently asked questions
Why does my camera show less card space than the label?
Storage manufacturers define 1 GB as 1,000,000,000 bytes, while cameras and computers often format filesystems and reserve space for metadata.
Should I reserve safety space?
Yes. Many cameras stop recording before reaching zero, and formatting structures take a fraction of the card.