CCTV Storage Calculator Guide: Retention, Bitrate and Hard-Drive Capacity
Essential Security Supplies
How much CCTV storage does a New Zealand site need for 14, 30 or 90 days of footage? Camera count alone will not answer that question. Retention depends on the recorded bitrate of each stream, hours recorded per day, recording mode and the usable capacity of the NVR. This guide gives installers a transparent calculation, worked examples and a commissioning method to check whether the planned retention is actually achieved.
Need a quick estimate? Use our online CCTV storage calculator to estimate hard-drive capacity from camera count, bitrate, recording hours and retention period, then use the guidance below to validate the result against the final recorder configuration.
Start with a retention and recording brief
Agree the retention period with the site owner before selecting a hard drive. Record which cameras must capture continuously, whether audio is enabled, what happens outside trading hours and whether particular entrances or incident areas need a different period. A request for “30 days” should specify 30 days for every camera at its approved image quality, including busy periods. Do not assume a universal New Zealand retention requirement for every CCTV installation. Confirm the organisation's purpose, privacy policy, contractual obligations and any applicable sector requirements.
Separate routine rolling footage from clips that must be preserved for an investigation. Exported evidence needs its own secure destination and process; it should not depend on the NVR retaining a particular event beyond its normal overwrite date. Document who can retrieve footage and how access is controlled.
How to calculate CCTV hard-drive capacity
Use the measured average recording bitrate for each camera or camera group. With decimal units, the calculation is:
Storage in TB = number of cameras × average bitrate in Mb/s × 3,600 × recording hours per day × retention days ÷ 8,000,000.
For a continuously recording camera, this simplifies to approximately 0.0108 TB per day for each 1 Mb/s, or 10.8 GB per day. Sum groups with different bitrates separately. A planning margin should then be added for changing scenes, audio, metadata, system use and capacity unavailable to recordings. A 20% allowance is an illustrative design assumption, not a manufacturer's guarantee; increase or adjust it using recorder measurements and the project's risk tolerance.
| Continuous recording example | Raw video for 30 days | With illustrative 20% allowance |
|---|---|---|
| 4 cameras at 2 Mb/s each | 2.59 TB | 3.11 TB |
| 8 cameras at 4 Mb/s each | 10.37 TB | 12.44 TB |
| 16 cameras at 6 Mb/s each | 31.10 TB | 37.32 TB |
These figures use decimal TB and assume all streams write continuously for 24 hours a day. They are examples, not promises about any camera model. A recorder may display capacity in binary units, and formatting and system reservations reduce the space available for footage. Check its reported usable recording space after installation. Seagate's surveillance storage calculator is useful as a second estimate, provided its inputs match the final camera settings.
A mixed-camera worked example
Suppose four cameras average 2 Mb/s and four average 5 Mb/s. Their combined average is 28 Mb/s. At 24 hours a day for 30 days, 28 × 0.0108 × 30 = 9.07 TB of raw video. An illustrative 20% allowance gives 10.89 TB of required usable capacity. A nominal 10 TB drive would not meet that planning figure. Check the NVR's drive count, maximum supported capacity per bay and any reserved or redundant space before choosing an arrangement.
Why resolution does not tell you the bitrate
Two 4 MP cameras can produce very different amounts of data. Compression codec, frame rate, image quality, maximum bitrate setting, scene movement, foliage, rain, low-light noise and supplementary streams all matter. H.265 can save capacity compared with H.264 under suitable conditions, but a fixed percentage saving should not be assumed across every scene or recorder. Verify that the camera, recorder, live-view clients and export workflow support the selected codec. Axis explains how compression and bitrate affect storage in its surveillance video compression guidance.
Variable bitrate can rise when a scene becomes more complex. A quiet daytime sample is therefore a weak basis for a 30-day guarantee at a busy entrance or a noisy night scene. If a recorder offers a bitrate cap, inspect recorded detail at that cap before treating it as a storage solution. Storage savings that remove usable identification detail defeat the design brief. Measure the main recording stream rather than relying on the low-bandwidth substream used for remote viewing.
Continuous, scheduled and event recording
Continuous recording is the simplest case: use 24 hours per day in the equation. For a genuine fixed schedule, use the scheduled hours, while accounting for weekends, public holidays and any extended operation. Event recording needs more care. A claim that motion recording saves 70% is only a guess until the camera and detection rules have been observed on the actual site.
For event mode, estimate an effective duty cycle from a representative pilot: recorded hours divided by total hours observed. Multiply the continuous storage estimate by that fraction, then include pre-event and post-event recording, regular false triggers and periods of sustained activity. Size critical coverage conservatively. If footage must exist even when detection misses an event, continuous recording may be the appropriate design. Record the chosen mode for each channel in the handover document.
Choose drives and recorder capacity together
The calculation produces a usable recording capacity target. It does not select a drive by itself. Check the NVR's number of bays, supported drive capacities, aggregate incoming recording bandwidth, firmware and drive compatibility list. Confirm whether its capacity display includes space reserved for the operating system or other functions. If redundancy is configured, calculate usable space after the chosen scheme rather than simply adding the printed drive capacities. RAID reduces some drive-failure exposure but is not a substitute for exporting important footage.
Browse surveillance hard drives, including the Seagate SkyHawk 4 TB ST4000VX013 and Seagate SkyHawk AI 8 TB ST8000VE001. These are capacity examples, not blanket compatibility recommendations. The exact drive model, recording technology and recorder support should be checked before specifying a system. A Dahua 24-channel NVR option illustrates why available drive choices and recorder configuration need checking together; confirm its current specification against the manufacturer's documentation for the exact hardware revision.
Commission and verify the actual retention
- Set the intended resolution, frame rate, codec, quality and recording schedule on every channel. Record these values.
- Check the NVR's incoming bandwidth and the reported usable disk capacity after formatting or any redundancy setup.
- Observe each channel's recorded average and peaks across representative day and night periods, including busy conditions. Recalculate the total using those figures.
- After the system has run, compare the oldest playable timestamp for each camera with the required retention. Do not infer success merely because the disk is installed.
- Test playback and export, then document the results, alerts, overwrite behaviour and the procedure for replacing a failed drive.
Recheck after adding cameras, changing compression settings, enabling audio, altering detection rules or installing firmware that changes recording behaviour. A storage estimate is only as reliable as the configuration and scene assumptions behind it.
Frequently asked questions
How many days will a 4 TB CCTV hard drive record?
There is no single answer. At an aggregate measured 8 Mb/s with continuous recording, raw video uses about 86.4 GB per day. A nominal 4 TB is therefore roughly 46 days before formatting, reserved space, variable bitrate and any design margin. Use the recorder's actual usable capacity and verify its oldest playable footage.
Does H.265 always double retention?
No. The result depends on the image quality target, scene, encoder and playback compatibility. Measure the resulting stream at the final settings and compare recorded detail before relying on an assumed saving.
Can motion recording guarantee 30 days?
Only if event frequency and recording behaviour remain within the tested assumptions. Busy scenes, weather, lighting changes and false triggers can increase usage. For critical cameras, choose a conservative duty cycle or continuous recording and test the oldest footage.
Should I buy a drive with exactly the calculated capacity?
Allow for capacity unavailable to footage and reasonable variation in bitrate. Confirm the recorder's supported drives, bays and usable space. If a requirement is strict, build in a documented margin and validate it on the installed system.
Get a storage specification checked
Send Essential Security Supplies the camera schedule, intended recording settings, target retention, recorder model and any redundancy requirement. We can help review the storage calculation and identify suitable recorder and surveillance-drive options for your New Zealand project. Contact Essential Security Supplies for specification help.
