The basic storage estimate

For a first estimate, multiply each camera's average bitrate by the number of recording seconds, divide by eight to convert bits to bytes, then multiply by the retention period and number of cameras.

Storage in decimal gigabytes ≈ bitrate in megabits per second × 1,000,000 ÷ 8 × 86,400 × retention days × camera count ÷ 1,000,000,000.

For example, one camera averaging 2 Mbps for 30 days needs about 648 GB before overhead: 2 × 1,000,000 ÷ 8 × 86,400 × 30. Four cameras at the same average bitrate need about 2,592 GB, or 2.6 TB decimal, before overhead.

This is an estimate, not a guarantee. The real bitrate varies with resolution, frame rate, codec, scene movement, lighting, and camera configuration. A busy scene can use more storage than a quiet one.

Account for recording settings

Use the camera or recorder's expected average bitrate when available. If it lists a range or only a maximum, plan conservatively and test actual recordings. Continuous recording uses more storage than motion-based schedules. Motion recording can lower the average, but detection zones, sensitivity, and pre-event recording affect how much footage is retained.

Modern codecs may reduce bitrate at similar perceived quality, but cameras and recorders must support the same codec. Avoid choosing a drive based only on a marketing claim that omits the number of streams, retention period, or recording schedule.

Convert the estimate into a drive plan

Add a reasonable margin for bitrate variation, filesystem and recorder overhead, and future needs. Check the NVR manufacturer's maximum supported drive capacity, drive count, and recommended surveillance-drive requirements. If the system uses RAID, usable capacity is less than the sum of the drive labels; calculate it using the recorder's specific RAID level and configuration.

Drive manufacturers usually label capacity in decimal terabytes, while operating systems may display binary tebibytes. The displayed number can therefore look smaller without indicating missing space.

Validate before relying on the system

Install the planned cameras and settings, record representative busy and quiet periods, and check the recorder's actual storage consumption. Confirm that old footage is overwritten at the expected retention point and that important clips can be exported. Revisit the estimate after changing resolution, frame rate, codec, or recording schedule.

For a system that must retain evidence for a business or legal requirement, confirm the policy with the responsible organization and local rules. Do not promise a retention period until the installed recorder has been tested with the final settings.