How many days of footage will your system keep? That depends almost entirely on the size of the hard drives relative to how much video the cameras produce. The sums are not difficult once you know which three numbers to use.
The method
Storage needed is the data rate of each camera, multiplied by the number of cameras, multiplied by the time you want to keep.
Storage = bitrate x number of cameras x seconds of retention, converted to terabytes
Here are the pieces.
- Bitrate. The amount of data a camera produces each second, usually quoted in megabits per second (Mbps). It is set in the camera's configuration and varies with resolution, frame rate, compression and image complexity.
- Number of cameras. Straightforward, but count the cameras you plan to add as well.
- Days of retention. How far back you want to be able to look.
Converting units
Bitrates are in bits. Hard drives are in bytes. There are eight bits in a byte.
- Megabits per second divided by 8 gives megabytes per second.
- Multiply by 86,400 (seconds in a day) to get megabytes per day.
- Divide by 1,000 for gigabytes, and by 1,000 again for terabytes.
Drive makers use decimal terabytes, so the 1,000 conversion is appropriate here.
A worked example
The numbers below are assumptions, chosen to show the method. They are not typical figures for any product, and your real values will differ.
Assumptions
- 8 cameras.
- Each camera records continuously at an assumed 4 Mbps.
- Footage is kept for an assumed 30 days.
Working
- 4 Mbps / 8 = 0.5 megabytes per second per camera.
- 0.5 x 86,400 = 43,200 megabytes, or about 43.2 GB per camera per day.
- 8 cameras x 43.2 GB = about 346 GB per day.
- 30 days x 346 GB = about 10,370 GB, or roughly 10.4 TB.
So under these assumptions, you would want a little over 10 TB of usable storage, and in practice you would round up to the next available drive configuration. Leave headroom, because bitrates fluctuate and drives do not format to their full labelled capacity.
If the same cameras ran at an assumed 2 Mbps instead, the requirement roughly halves. If you kept footage for 14 days rather than 30, it falls by more than half. The method scales in a straight line, which makes it easy to try variations.
What changes the numbers
| Factor | Effect on storage |
|---|---|
| Resolution | Higher resolution generally means a higher bitrate |
| Frame rate | More frames per second means more data |
| Compression | A more efficient codec can reduce bitrate. See H.265 vs H.264 |
| Recording mode | Event-only recording uses less than continuous recording |
| Scene activity | Busy scenes compress less efficiently than still ones |
| Night mode | Image noise in low light can raise bitrate |
| Quality setting | A higher quality setting uses more data |
Continuous versus event recording
Continuous recording saves everything. Event-based recording saves footage when something triggers it, such as detected motion or an AI event on compatible systems. Event recording can reduce storage use, but it depends on how reliably the trigger works and how busy the scene is. Many sites use a mix: continuous on key cameras, events on others.
Variable and constant bitrate
Many cameras can be set to a constant bitrate, which gives a predictable figure, or a variable bitrate, which saves space when little is happening. Variable settings make estimates less exact. For planning, use the maximum bitrate and treat the result as a safe upper limit.
Do not forget
- Spare capacity. Plan for more cameras later. Adding cameras reduces the days available.
- Drive suitability. Use surveillance-rated drives for continuous recording.
- Number of bays. The recorder limits the total. See the Dahua NVR guide.
- Redundancy. If footage matters a great deal, ask about drive configurations that tolerate a failure. It is a trade-off with cost and capacity.
Rules of thumb for planning
- Always plan from the real configured bitrate of the installed cameras, not from a brochure figure.
- Add a margin of safety on the result, then round up to a real drive size.
- Recalculate whenever you add cameras, raise the resolution or change the retention period.
- Check the recorder's reported capacity and days of recording after the first week. Most recorders can show how far back the footage currently goes, which is a quick sense check on your sums.
Use the calculator
You do not need to do this by hand. The storage calculator lets you enter cameras, bitrate and days and see the result. If you want a sense of how much bandwidth the cameras use on your network, the bandwidth tool is a companion.
How to choose your retention period
There is no correct number. It depends on how soon you would notice an incident. A shop checked daily might be comfortable with two weeks. A holiday property that is empty for a month may want longer. Business owners may have their own record-keeping needs and should confirm any specific obligations themselves. This is general information, not legal advice, and rules differ by state and territory.
If you want a recorder sized around your cameras and retention needs, you can get a quote. Unfamiliar terms are in the glossary.
Frequently asked questions
How many days of footage should I keep?
It depends on how soon you would discover an incident. Two to four weeks is a common starting conversation, but there is no universal answer.
What happens when the drive fills up?
Most recorders overwrite the oldest footage first, so storage works as a rolling window. Check this setting on your system.
Does adding AI detection increase storage?
Detection itself does not necessarily change the bitrate. It can change what is recorded if you switch to event-based recording on compatible systems.
Can I add another hard drive later?
Only if the recorder has a free bay and supports the extra capacity. Ask about spare bays before you buy.
Is the example above what my system will need?
No. The example uses assumed figures to show the method. Use your own cameras' actual bitrates.