Security cameras · retention
CCTV retention days calculator.
Enter your disk size and cameras to see how many days of footage fit before the oldest is overwritten.
Inputs
Days of footage
Enter the numbers above.
How many days of CCTV footage fit on a disk?
Divide the usable disk space by the amount written each day. Disk makers count a terabyte as 10^12 bytes. The usable share lets you take off space lost to RAID parity, the operating system or a reserve you want to keep free. Each day the system writes cameras x bitrate x 86,400 seconds / 8 bytes, multiplied by the share of the day it records.
Worked example. Two 8 TB disks at 90 percent usable give 14.4 TB. Eight cameras at 4 Mbps recording continuously write 345.6 GB a day, so the disk holds 14,400 / 345.6 = 41.7 days. Recording only 30 percent of the time stretches that to about 139 days.
Most recorders overwrite the oldest footage when the disk fills, so this is the length of the rolling window. Locked or exported clips take space from it.
Frequently asked questions
How many days will a 4 TB disk record 4 cameras?
At 4 Mbps each and 100 percent usable, continuous recording writes 172.8 GB a day, so 4 TB lasts 23.1 days.
What usable share should I use?
Use 100 percent for a single disk with nothing reserved. For RAID take off the parity disks, and many people keep 10 percent free; 90 percent is a rule of thumb, not a standard.
What happens when the disk is full?
Most recorders overwrite the oldest footage so the window rolls forward, but check your recorder setting because some stop recording instead.
How do I get a longer retention?
Lower the bitrate or frame rate, use H.265, record on motion only, or add disk. The CCTV storage calculator works from days to terabytes.
Sources
Checked
- Wikipedia, "Gigabyte", opened 8 October 2026. IEC 80000-13: a gigabyte (GB) is 10^9 bytes, a gibibyte (GiB) is 2^30 bytes
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Scope
Planning arithmetic, not a guarantee of how long footage is kept. Local recording and retention rules vary. Usable share and bitrate are inputs. Real footage per day varies with scene activity.