How it works
The formula, without the black box.
days = usable card gigabits ÷ recorded gigabits per day
Nominal GB is converted to gigabits, reduced by the unavailable-capacity allowance, then divided by bitrate × recorded seconds per day.
Inputs that deserve a second look
- Card capacity
- Nominal decimal GB, subject to the camera’s supported maximum.
- Bitrate
- Average bitrate of the stream actually written to the card.
- Activity
- Useful only when event recording truly skips non-event video.
- Unavailable capacity
- A combined planning allowance for formatting, filesystem, indexes, and space not used for video.
Common planning mistakes
- Buying a card larger than the camera supports.
- Using a non-endurance card for continuous overwrite.
- Assuming motion activity from one scene applies to another.
- Treating edge storage as the only copy of critical footage.
Questions installers ask
How long will 256 GB hold at 4 Mb/s?
With continuous recording and 10% unavailable capacity, the estimate is about 5.33 days.
Does overwrite mode change the calculation?
It changes what happens when full, not how quickly the card fills. In loop recording, the result approximates the rolling window.
Does card endurance appear in the result?
No. Capacity and endurance are different. Verify the card type, write endurance, temperature rating, and camera compatibility.
Sources and boundaries
These links support the method or terminology. They do not turn a planning estimate into a site guarantee.
SD Association capacity information ↗
Primary reference for SD capacity families; camera compatibility remains device-specific.