How Much CCTV Storage Do You Really Need? Formula, Examples and Table

In Depth

In Depth

How many days a CCTV recorder keeps depends on one number most people never check: the cameras' bitrate. This guide gives a simple formula you can work out on a phone calculator, worked examples, a drive-size table and the reasons real retention is usually a little shorter.

How CCTV recording storage works

Each camera sends a continuous stream of compressed video to a recorder: an NVR (network video recorder) for IP cameras or a DVR for analogue cameras. The recorder writes the streams onto one or more hard drives. When the drives are full, most recorders overwrite the oldest footage first, so the drive holds a rolling window of the most recent X days. X is what you are really choosing when you choose a drive.

Two numbers decide X: how much data the cameras produce (their bitrate) and how much space you can actually use on the drives.

Resolution vs bitrate

Resolution is how many pixels each picture has (2 MP is 1920 × 1080; 8 MP is 3840 × 2160, or 4K). Bitrate is how much data the camera sends every second, shown in kbps (kilobits per second) or Mbps (megabits per second; 1 Mbps = 1,000 kbps). Recorder menus often write "Kbps" and may use 1,024 instead of 1,000, which makes no practical difference to an estimate.

Storage depends on bitrate, not resolution. Higher-resolution cameras are usually set to higher bitrates, but two 4 MP cameras can produce very different amounts of data depending on their codec, frame rate, quality setting and what they are looking at.

Tip The bitrate is in the camera's or recorder's video or encoding settings. That figure, or better still the average the recorder reports over a few days, beats any typical value from the internet.

For help choosing the cameras themselves, see CCTV cameras explained.

What makes the numbers go up or down

• Number of cameras: storage grows in direct proportion. Twice the cameras at the same bitrate need twice the space, and adding cameras later shortens retention for every camera.
• Bitrate per camera: the biggest lever. A camera at 4 Mbps uses twice the space of one at 2 Mbps.
• Frame rate: more frames per second usually means a higher bitrate. Many business systems record at 10 to 15 fps rather than 25 or 30. That still shows what happened and uses less space.
• Recording hours and mode: recording 12 hours a day uses roughly half the space of recording 24 hours.

Continuous vs motion (event) recording

Continuous recording saves everything. It uses the most space, but you never miss the moments around an incident.

Motion or event recording saves only when the camera or recorder detects movement or another event, plus a few seconds before and after. It can greatly extend retention for quiet areas such as a store room at night, but it is less predictable:

• A busy shop floor or hotel lobby has movement most of the day, so the saving is small.
• Trees, headlights, rain, insects near infrared lights and moving shadows trigger false recordings.
• If detection misses something, it is not recorded at all.

A sensible approach for many businesses is continuous recording on key cameras (entrances, tills, reception) and event recording on low-traffic areas.

Compression: H.264, H.265 and "smart" codecs

Raw video is enormous, so cameras compress it with a codec. The two you will see are H.264 and the newer H.265 (also called HEVC), both international standards published by the ITU. When H.265 was approved, the ITU said it needs about half the bit rate of H.264 for the same quality. In CCTV the real saving depends on the camera and the scene, so Extrion's calculator treats "about half" as a starting estimate only.

Many manufacturers add their own "smart" settings on top, under names such as H.265+ (Hikvision), Smart H.265 (Dahua) or Zipstream (Axis). These compress an unchanging background more heavily and spend data on moving objects. Axis, for example, says Zipstream can cut bandwidth and storage by 50% or more. Treat such figures as vendor claims under favourable conditions: savings are largest on quiet, static scenes and smallest on busy ones. Before relying on one, check that the camera, recorder and viewing apps all support it.

The formula, step by step

All you need is the total bitrate of all your cameras in Mbps. The tutorial above shows the same steps as calculator screens.

Step 1: data per second. 1 Mbps = 1,000,000 bits per second. There are 8 bits in a byte, so that is 125,000 bytes per second.

Step 2: data per day. A day has 86,400 seconds. 125,000 × 86,400 = 10,800,000,000 bytes, or 10.8 GB per day for every 1 Mbps recorded continuously.

GB per day ≈ total Mbps × 10.8 (continuous recording)

Step 3: storage for your retention period. Divide by 1,000 to get terabytes, then scale for the hours you record and the days you want:

TB ≈ total Mbps × 0.0108 × (hours per day ÷ 24) × days

To go the other way and find how many days a drive will hold:

Days ≈ usable storage in GB ÷ GB per day

These are planning estimates in decimal units (1 TB = 1,000 GB), the units printed on drive labels.

Worked example 1: how big a drive? Eight cameras at 4 Mbps each, recording 24 hours a day, kept for 30 days.

• Total bitrate: 8 × 4 = 32 Mbps
• Per day: 32 × 10.8 = 345.6 GB
• 30 days: 345.6 × 30 = 10,368 GB, about 10.4 TB
• Add 20% headroom: about 12.4 TB, for example two 8 TB drives in a two-bay recorder

Worked example 2: how many days will my drive hold? Four cameras at 2 Mbps each on a 4 TB drive, recording continuously.

• Total bitrate: 8 Mbps, so 8 × 10.8 = 86.4 GB per day
• 4,000 GB ÷ 86.4 = about 46 days in theory
• Using 80% of the drive as the planning figure: about 37 days

Worked example 3: part-day recording. Four cameras at 4 Mbps, recording 12 hours a day for 14 days: 16 × 0.0108 × (12 ÷ 24) × 14 = about 1.21 TB.

How 1, 2, 4, 6, 8 TB and larger compare

Estimated days of continuous, 24-hour recording. The first figure uses the full labelled capacity; the figure in brackets is the planning figure at 80% of capacity, allowing for overheads, bitrate swings and spare space.

Illustrative estimates from the formula above. Your real bitrates will differ.

Double the drive and you double the days; double the total bitrate and you halve them. Recorders with several drive bays let you add capacity later instead of replacing a drive.

Why real retention differs from the estimate

• Variable bitrate (VBR). Many cameras send less data when the scene is still and more when it is busy, up to a maximum you set. Constant bitrate (CBR) is easier to predict but can waste space on quiet scenes and lose quality on busy ones. With VBR, plan from the average bitrate over a few full days, not the maximum setting or a quiet-hour reading.
• Scene complexity. A busy street, moving leaves or a full reception produce more data than an empty corridor.
• Night-time noise. In low light, cameras boost the signal or switch to infrared. The grainy "noise" this adds is recorded as detail, so night footage often uses more data than day footage.
• Overwrite settings. If overwrite is switched off, many recorders stop recording when the drive is full. Check it is set the way you intend.
• TB vs TiB. Drive makers use decimal units: one terabyte is one trillion bytes. Many recorders and computers count in binary units, where 1 TiB is 1,099,511,627,776 bytes. So an 8 TB drive appears as about 7.28 "TB". No space is missing; it is a different unit.
• Formatting and system space. The file system and indexes take a small slice of the drive.
• RAID. Recorders with several drives can protect footage with RAID, which uses capacity for redundancy. RAID 1 (mirroring) keeps half the total; RAID 5 loses one drive's worth and RAID 6 two. Four 4 TB drives in RAID 5 give about 12 TB before formatting.

Warning If footage is copied to cloud storage or an off-site recorder, the internet connection must carry the full bitrate continuously. Eight cameras at 2 Mbps need about 16 Mbps of steady upload speed, which many business connections cannot sustain. Our backup and disaster recovery guide covers off-site copies in general.

Usable storage, drives and spare capacity

Plan on using about 80% of the labelled capacity. Extrion's calculator applies the same idea in reverse by adding 20% headroom. The margin absorbs busy days, noisy nights, formatting and a camera or two added later.

Use surveillance-class hard drives. They are designed for continuous writing from many camera streams. Check the drive's datasheet for its workload rating and supported number of streams, and confirm the drive is on your recorder's compatibility list and within its maximum supported size per bay.

Protect the recorder with a UPS. Repeated power cuts interrupt recording and are hard on drives that write all day.

Common mistakes

• Sizing the drive from resolution alone and ignoring the actual bitrate.
• Assuming motion recording will save 80% on cameras that face busy areas.
• Comparing an "8 TB" label with a "7.28 TB" screen reading and thinking the drive is faulty.
• Using a desktop hard drive in a recorder that writes 24 hours a day.
• Turning on "smart" codecs without checking that the recorder and apps support them.
• Never checking the date of the oldest recording after installation.

How to choose a retention target

Start from a business question: how long might it take before you know you need the footage?

• Shops: shrinkage is often found at a weekly or monthly stock-take, so 30 days is a common starting point.
• Hotels and guesthouses: guests may report a problem after they have gone home. Around 30 days is a common choice.
• Offices: incidents are usually noticed within days. Many offices choose 14 to 30 days.
• Homes: 7 to 14 days is often enough to cover a holiday away.

These are common practices, not legal requirements. We could not find a single fixed retention period for CCTV in Tanzania. Tanzania's Personal Data Protection Act, 2022 requires personal data to be kept no longer than necessary for its purpose, and recognisable images of people can be personal data. The Act's application in Zanzibar is limited to Union matters. Check the position for your organisation with the Personal Data Protection Commission or a lawyer. Your insurer, bank or licensing authority may also set requirements.

For comparison, the UK regulator says its law sets no minimum or maximum retention period and warns against keeping footage longer just because storage allows it.

Tip When something happens, export the clip straight away. Do not rely on it still being on the drive next month. If the recorder says recordings are missing, see CCTV recording not available.

Plays from YouTube when you press play.

When to call Extrion

If you want help sizing storage for a new or existing system, or your recorder keeps fewer days than it should, Extrion can review your cameras' bitrates, drives and retention needs. See our CCTV installation service, request a quote or contact us.

This guide is general information. Procedures, product features and firmware change, so check your own equipment's documentation before you start.