CCTV Hard Drive Calculator: Terabytes for Your Cameras, Days and Recording Hours

CCTV hard drive calculator

Enter how many cameras you run, the bitrate each one streams, the hours a day they record and the number of days you want to keep. The calculator returns three figures: the storage that footage needs, what it writes per day and what it writes in a year. It does not pick a drive for you. The yearly figure is the one to carry to a data sheet, because that is what a drive's workload rating is measured against.

The starting bitrate is WD's own reference stream: one stream per camera at 3.2Mbps, which WD describes as 1080p, H.265 at 25 fps. WD adds that real results vary with resolution, format, frame rate and software, so treat the default as a placeholder and replace it with the figure your recorder shows for each camera.

How the calculator turns cameras into terabytes

The arithmetic is ours, and it is short. A bitrate is megabits per second, so the calculator works in four steps:

• Multiply cameras by bitrate to get the total megabits per second reaching the drive.
• Divide by 8 to turn megabits into megabytes, then multiply by a million to get bytes per second.
• Multiply by the seconds recorded: hours per day, times 3,600, times the days you keep.
• Divide by one trillion to get terabytes.

That last step uses decimal terabytes because drive makers do. WD defines one terabyte as one trillion bytes, so the result compares directly with the capacity printed on a drive label and with the capacities in our price tables.

Worked through for one camera at WD's 3.2Mbps reference: 3.2 divided by 8 is 0.4 megabytes, or 400,000 bytes, every second. A day has 86,400 seconds, so a camera recording all day writes 34.56 GB. One terabyte therefore holds a little under 29 days of that camera, and 30 days needs about 1.04TB.

Worked examples at WD's 3.2Mbps reference

Every figure in that table scales in a straight line. Double the bitrate and every number doubles; halve the days kept and storage halves while the yearly figure stays the same. That last point matters: retention decides capacity, but only cameras, bitrate and hours decide how hard the drive works.

Three everyday cases, all at the 3.2Mbps reference and by the same arithmetic. Two cameras at home recording all day for 30 days need just over 2TB, so a 2TB drive falls slightly short. Four cameras in a shop recording 12 hours a day for 31 days need about 2.14TB. A 16-camera recorder running all day and keeping 14 days needs about 7.7TB, but writes about 202TB a year.

NVR hard drive calculator: setting the yearly figure against a rating

Capacity is one question; workload is the other. Surveillance drives carry a workload rating, and WD defines it as terabytes transferred, annualised as TB transferred times 8,760 divided by the recorded power-on hours. The calculator's yearly figure counts the recording writes on the same yearly basis; playback and exports add reads on top, so leave headroom. A drive rated below your yearly figure is being asked to write more than its maker states.

The WD and Seagate surveillance sheets give three rungs, and on WD's standard line the rung follows the part number rather than the capacity:

• 180TB a year: WD Purple with AllFrame 4K, the 1TB to 8TB configurations on WD's April 2019 sheet, and Seagate SkyHawk, which Seagate designs for always-on workloads of 180TB a year.
• 360TB a year: the WD Purple models with AllFrame AI, WD121PURZ, WD101PURZ and WD82PURZ. WD81PURZ, listed next to them on the same sheet, sits on the 180TB rung.
• 550TB a year: WD Purple Pro, and Seagate SkyHawk AI, which Seagate calls more than three times a standard surveillance drive.

At the 3.2Mbps reference, each camera recording all day adds about 12.6TB a year. By our arithmetic that keeps up to 14 such cameras on one drive inside 180TB a year, up to 28 inside 360TB and up to 43 inside 550TB. If the cameras record 12 hours a day instead, those limits roughly double, to 28, 57 and 87.

Those limits assume every stream lands on one drive. If your recorder spreads recordings across several drives, check in its manual how it divides them before you apply the figure per drive. NVR hard drive lists the 550TB-class drives in stock.

Camera limits and workload limits are separate ratings

WD says each WD Purple drive is optimised to support up to 64 cameras. That figure and the workload rating answer different questions, and a system can be inside one and outside the other.

At WD's own 3.2Mbps reference, 64 cameras recording all day to one drive would write about 807TB a year by our arithmetic, well above even 550TB. So a camera count inside the maker's limit does not settle the question on its own: run the yearly figure as well.

How bitrate control changes the storage you need

The weak point in any storage estimate is the bitrate, and cameras control it in different ways. Axis, the camera maker, sets out the options in its paper on bitrate control:

• Variable bitrate (VBR): Axis says the main drawback is that storage needs may be unpredictable. Motion or other events can raise the stream size and bitrate significantly, so a VBR system must keep fairly large storage margins. If storage, network and the video management system all had unlimited capacity, VBR would give the best video quality.
• Maximum bitrate (MBR): a cap that keeps the bitrate below a set value regardless of scene complexity. Axis warns that most storage calculation tools base their calculations on the low MBR limit, which can leave too little storage for high-quality recording.
• Constant bitrate (CBR): Axis calls it not optimal for storage, because the stream may contain padding data that wastes space without improving the picture.
• Average bitrate (ABR): you set the retention time and the storage allocated to each camera, and the camera adjusts compression to stay inside that budget, which Axis says makes storage predictions reliable.

For this calculator, that means two things. A VBR camera's average bitrate understates its busy hours, so enter the highest bitrate your recorder reports and keep a margin above the result. A camera on an ABR budget already has a storage figure: Axis lists retention time, storage or target bitrate, and an optional maximum bitrate as the inputs, and its calculator converts between storage and target bitrate.

Codec is a second check. WD's reference stream is H.265, while Axis's paper says H.264 is the most popular video encoding standard and that H.265 has not yet been widely deployed in surveillance, citing complexity, limited low-light performance and licensing. Confirm which codec each camera actually uses before you lean on the reference figure.

Camera makers also sell their own compression. Axis states that its Zipstream cuts bandwidth and storage by 50% or more; that is Axis's claim for its own cameras, not a saving you can apply to any camera. For an estimate built around a particular scene, Axis points to its free AXIS Site Designer, which it says works out storage and bandwidth from camera configurations, recording schedules and the monitored scene, and which Axis calls the safest and most accurate source of storage estimates for its cameras.

Motion-triggered recording lowers the real figure, and a recorder that mirrors drives or runs RAID needs more raw capacity than the total shown here. A short routine for sizing a replacement:

• Find each camera's bitrate and codec in the recorder's stream settings.
• Note which cameras record all day and which record on motion only, and enter them as separate runs if they differ a lot.
• Decide how many days you must keep, then run the calculator.
• Set the yearly figure against the workload rating on the sheet of the drive you plan to buy.
• Check the largest drive your recorder accepts and, for Hikvision, its compatibility list.

Hikvision hard drive calculator: using it for a Hikvision recorder

The arithmetic is the same for any recorder brand, so the calculator works for a Hikvision DVR or NVR if you enter the bitrate the recorder shows for each camera. We have not read Hikvision's own storage calculator documentation, so we do not reproduce its defaults or claim our figures match it.

Which drive to fit is a separate question that Hikvision answers in its own drive list, which runs table by table for each recorder series. Hikvision hard drive matches the part numbers in it to drives on Amazon.co.uk.

Surveillance drives on Amazon.co.uk

In all, the table draws on 53 listings in stock. The table shows 6 listings, from £25.76/TB, in pounds sterling inc VAT. This short table mixes the workload tiers; CCTV hard drive has the full list, and DVR hard drive the standard tier on its own.