CCTV Hard Disk Size: How to Work It Out

The short answer

Storage depends on three things: how many cameras you have, the bitrate each camera records at, and how many days of footage you want to keep. Multiply them together and you have your answer.

The one number to remember: a camera recording continuously at 1 Mbps uses about 10.8 GB per day. Everything else is multiplication.

The formula

Storage (GB) = number of cameras × bitrate in Mbps × 10.8 × days of recording.

Where does 10.8 come from? One megabit per second for 86,400 seconds a day is 86,400 megabits, which is 10,800 megabytes, or 10.8 GB. Divide the final answer by 1,000 to get terabytes.

Typical bitrates to start from

Bitrate is set on the recorder or camera, and it varies with resolution, frame rate, compression and how busy the scene is. These are realistic starting points for H.265 recording at 15–20 frames per second. Your installer should confirm the actual figures from your recorder once the system is running.

Worked examples

• A home with 4 × 2MP cameras, 15 days: 4 × 2 × 10.8 × 15 = 1,296 GB. A 2 TB disk is enough.
• An office with 8 × 4MP cameras, 30 days: 8 × 4 × 10.8 × 30 = 10,368 GB, about 10.4 TB. Plan for 12 TB.
• A factory with 24 × 4MP cameras, 30 days: 24 × 4 × 10.8 × 30 = 31,104 GB, about 31 TB. That usually means an NVR with several disk bays, for example 4 × 8 TB or 3 × 12 TB.
• An apartment complex with 32 × 2MP cameras, 30 days: 32 × 2 × 10.8 × 30 = 20,736 GB, about 21 TB.

Why the disk shows less than its label

Disk makers count 1 TB as 1,000,000,000,000 bytes. Recorders and computers count in binary units, so a 12 TB disk shows up as roughly 10.9 TB. Keep a margin of about 10–15% over your calculated figure, and do not plan to fill the disk to the last gigabyte.

Recorders overwrite the oldest footage when the disk is full. That is normal. The question is only whether the oldest footage on the disk is old enough for your needs.

Ways to fit more days on the same disk

• Record in H.265 instead of H.264 — it roughly halves storage for the same picture.
• Use the recorder's smart codec (such as Hikvision's H.265+), which saves more on quiet scenes like corridors and stores at night.
• Lower the frame rate. 15 fps is plenty for most security cameras; 25 fps is rarely needed outside cash counters.
• Use motion or event recording on cameras that watch empty areas, but keep continuous recording on gates, entrances and cash areas.
• Use variable bitrate (VBR) with a sensible cap so the camera only uses high bitrate when the scene is busy.

Mistakes we often see

• Using a normal desktop hard disk. Surveillance disks are built to write 24 hours a day; desktop disks are not.
• Calculating for 2MP cameras, then upgrading to 4MP or 8MP cameras later without adding storage.
• Forgetting that night scenes with grain or rain can push bitrate up sharply.
• Buying an NVR with one disk bay when the site needs 20 TB or more. Check the NVR's bay count and maximum disk size first.

Get the numbers right first time

If your building has 100 or more people, Karnataka's public safety rules expect 30 days of storage, so size for that from the start. During a site survey we count the cameras, agree how many days you need to keep, and size the disks around the recorder we propose. The estimator on our CCTV page also links retention days to terabytes so you can try different numbers.

Quick answers