CCTV storage calculator
How many days of footage will you keep before the recorder starts overwriting? Pick a hard drive size and your camera count and the number drops out below.
Retention
13days
Based on 4K H.265, 24/7 continuous recording (~77 GB per camera per day). Motion-only recording will stretch this considerably.
How the maths works
A single 4K camera recording 24/7 in H.265 chews through roughly 77 GB a day. That works out to about 12.9 camera-days per terabyte. So three cameras on a 1 TB drive lasts a touch over four days before the recorder starts overwriting the oldest footage. Add cameras and retention drops; bump the drive size and retention climbs in a straight line.
The shorthand we use on a site survey:
Days ≈ (TB × 12.9) ÷ cameras
Most domestic installs sit comfortably at 7 to 14 days of retention. Anything beyond a fortnight is overkill for a typical home: if something happens, you tend to know within a day or two and pull the footage long before it rolls. Where longer retention matters is commercial sites, properties with no neighbours overlooking, or anywhere a quiet incident might not be spotted for a week.
Common scenarios
A few of the combinations we install most often, so you can see what falls out at typical drive sizes.
| Cameras | Hard drive | Days retention |
|---|---|---|
| 3 | 4 TB | 17 days |
| 3 | 8 TB | 34 days |
| 4 | 4 TB | 13 days |
| 4 | 8 TB | 26 days |
| 6 | 8 TB | 17 days |
| 6 | 16 TB | 34 days |
| 8 | 16 TB | 26 days |
| 8 | 24 TB | 39 days |
Things that change the number
The figure above assumes a fixed setup: 4K resolution, H.265 compression, and continuous 24/7 recording. Real installs vary on a few axes:
- Motion-only recording. Recording only when something moves can cut storage use by half or more on quiet sites. The trade-off is that you lose the constant timeline, which sometimes matters when piecing together a sequence.
- Frame rate. Dropping from 25 fps to 15 fps cuts bitrate roughly in half. Most domestic CCTV is recorded at 15 fps because the footage is still smooth enough to identify faces and number plates.
- Scene activity. H.265 compresses static scenes very efficiently. A back garden that barely moves uses far less storage than a busy frontage facing a main road.
- Drive headroom. NVR drives are never run to 100% full. Plan for 80–90% usable to keep the recorder happy long-term.
If you want a system spec'd around a specific retention target (a fortnight is the usual ask), we'll size the drive accordingly during the site survey. The NVR recorder we fit takes drives up to 24 TB without fuss.