How it works
A stream of 1 megabit per second, recorded for a whole day, fills 86,400 megabits: 10.8 GB. So:
Storage = bitrate (Mbps) × 10.8 × (hours ÷ 24) × days × cameras, plus headroom for the file system and for days when cameras are busier than usual.
The presets are rough figures at 15–25 frames per second. Your camera's settings page shows its real bitrate, and night scenes, rain and busy streets push it up. Recording on motion is like recording fewer hours: a quiet car park might add up to two or three hours a day.
Why does my disk show less than it says on the box?
Disks are sold in decimal terabytes; computers usually count in binary ones (TiB), about 9% smaller. A 4 TB disk shows as about 3.6 TB.
More on bitrates, retention and ways to need less disk: How much storage do security cameras need?