How much disk does a CCTV system need?

The only formula you need

Disk in terabytes = channels x bitrate in Mbps x 86,400 seconds x retention days / 8 / 1,000,000. In practice: one camera at 4 Mbps for 30 days is about 1.3 TB. Four cameras at 4 Mbps for 30 days is about 5.2 TB. Every figure below comes from that one line, so you can redo it for your own numbers.

Motion recording changes the number a lot

The table assumes continuous recording. If the recorder records only on motion, the disk requirement drops with the duty cycle, not with the clock: a corridor with two hours of real movement a day needs roughly a twelfth of the disk. That is why two identical sites can need 1 TB and 12 TB. Decide the recording mode before sizing the disk, and keep continuous recording on the views where the absence of motion is itself the evidence.

Why H.265 does not halve your disk

H.265 is usually quoted as saving 40 to 50 percent against H.264 at the same quality. On a busy scene with foliage, traffic and changing light, the real saving is smaller, often 20 to 35 percent, because the encoder spends bits on the parts of the frame that keep moving. On a quiet corridor the saving can be larger. Treat 30 percent as a planning number and measure the first week of a real installation before you commit a year of retention to it.

• Compare bitrate at the same resolution and frame rate, not at the same quality setting.
• Smart or plus codecs cut static scenes hard and moving scenes barely.
• Raising frame rate from 15 to 25 fps costs more than most people expect.

Where the disk actually goes

Recorders usually take two to four drives and mirror or span them. Spanning gives one large pool and no redundancy: one failed drive takes part of the archive with it. Mirroring costs half the capacity and survives a single drive failure. For a shop, spanning is normal; for a site where footage may become evidence, mirroring or an off-site copy is worth the cost.

Surveillance drives are built for continuous write cycles. Desktop drives work, then fail early under 24/7 recording. The price difference per terabyte is small compared with losing the archive.

Size the disk for the retention you will be asked for

Retention requirements usually come from an insurer, a landlord or a policy, not from the installer. Ask what period must be available before you choose the disk, then add 30 percent headroom for bitrate drift, firmware changes and the extra cameras that always arrive.