Camera storage calculator
Camera footage is a stream, not a pile of files. It runs at a constant bitrate until the card is full, then it starts writing over the oldest clip. Everything about storage planning comes down to one question: how many days of history do you actually have before that happens?
Set the numbers below to your own system. The bitrate box fills in automatically from the resolution and codec you pick, and you can override it if your camera app reports a real figure, which is always better than an estimate.
H.264 costs roughly 85 per cent more space for the same picture.
Prefilled from the two boxes above. Type over it with the number your camera app reports, if it gives you one.
| One camera, one day | |
|---|---|
| Whole system, one day | |
| Smallest microSD that covers one camera | |
| Days a 2 TB recorder drive holds | |
| Days an 8 TB recorder drive holds |
The arithmetic, with nothing hidden in it: GB = Mbps × 3600 × hours × days × cameras ÷ 8 ÷ 1024. Drive sizes are converted the way the operating system counts them, so a drive sold as 2 TB is treated as 1863 GB of usable space and a card sold as 128 GB as about 119 GB once formatted.
Reading the result
The figure that matters is the total, but the per-camera-per-day number is the one to remember. Once you know that a 4K H.265 camera writes roughly 4 GB a day recording continuously, you can size any system in your head.
Two things throw the estimate off, both in the same direction. Scene complexity raises the real bitrate above the nominal one: a camera watching a hedge in the wind writes far more than a camera watching a garage door. And most cameras raise bitrate at night, because infrared noise is expensive to compress. Give yourself a fifth more than the calculator says if the view is busy.
Continuous or motion only
Motion recording cuts the numbers hard, and it is the right default for most homes. The catch is what counts as motion. A camera set to record on any pixel change will trigger on rain and headlights and give you continuous recording with extra steps. A camera set to person detection only will miss the moment before someone walks into frame, which is often the useful part. Pre-roll buffering of a few seconds is worth more than a bigger card.
Cards, and why they die
A surveillance camera writes continuously, which is the worst thing you can do to a consumer microSD card. Cards sold for phones are built for occasional writes, and they fail after months in a camera, usually silently. Look for cards rated for continuous video recording, and treat any card in a camera as consumable rather than permanent.
The usable capacity in this calculator is set at 93 per cent of the printed number. That is the space left after formatting on a typical card, and it is why a 128 GB card never gives you 128 GB.