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How long can my DVR record ?

Last Updated 13 days ago

One of the most common questions when buying a DVR is:

“How many days can my DVR record?”

Unfortunately, there is no exact answer.

The recording time depends not only on the size of the hard disk, but also on what the cameras are recording.

Why Can't We Give an Exact Number?

Let's use digital photographs as an example.

Imagine we have two photographs:

Both are 1280 × 960
Both use the same High compression setting
One is 1.28 MB
The other is only 88 KB (0.088 MB)

A picture of flowers contains lots of details, colours and patterns.
1280 x 960 - 1.28MB

A picture of a plain wall contains very little detail.
1280 x 960 - 88KB (0.088MB)

The first photograph is about 14.5 times larger than the second one.

Why?

Because the pictures are different.

A picture of flowers contains lots of details, colours and patterns.

A picture of a plain wall contains very little detail.

Even though both pictures have exactly the same resolution and compression setting, one needs much more space to store than the other.

This is also why a digital camera can only estimate how many photographs you can store on a memory card. It cannot tell you the exact number.

Video Recording Works in a Similar Way

Modern DVRs commonly use H.264 or H.265 compression to reduce the amount of storage required.

Video compression is actually quite clever.

If your camera is looking at an empty room and nothing is moving, there isn't much new information to store.

But if the camera is looking at a busy road with cars and people constantly moving, there is much more information changing from one moment to the next.

Therefore, the amount of storage used can be very different.

For example:

Camera A:
Looking at an empty room → less storage used

Camera B:
Looking at a busy road → more storage used

Both cameras could have exactly the same:

Resolution
Frame rate
Compression
Quality setting

Yet Camera B can use considerably more storage.

This is because movement and the amount of detail in the picture affect how much data needs to be stored.

So How Long Can My DVR Record?

This is why it is difficult to simply say:

“A 2 TB hard disk can record 30 days.”

It might be 30 days in one location, but only 20 days in another location.

It could even be 40 days somewhere else.

It depends on what the cameras are recording.

The Best Way to Find Out

If you already have the DVR installed, the easiest way to find out is to let it run normally for one week.

A week is useful because it includes both weekdays and weekends. The amount of activity may be very different between them.

For example:

Your DVR has 4 TB of storage.

After running for one week, you check the DVR and find that it used 700 GB.

You can then estimate:

4 TB ÷ 700 GB ≈ 5.7 weeks

So you can expect roughly 40 days of recording.

This is only an estimate. If the following weeks are busier, the DVR may keep fewer days. If they are quieter, it may keep more days.

This method is usually much more useful than relying on a theoretical calculation because it is based on your actual cameras and your actual location.

Fixed Bitrate

DVRs allow you to use a Fixed Bitrate setting.

This means the DVR is set to use approximately the same amount of data every second, regardless of what the camera is recording.

For example, if the Fixed Bitrate is set to 1,580 kbps, the DVR will continuously record at approximately 1,580 kbps.

This makes the recording time relatively easy to calculate because the amount of storage used is predictable.

However, there is a disadvantage.

A quiet scene may not require the full 1,580 kbps to maintain good picture quality. However, the DVR will still continue using approximately 1,580 kbps.

The extra data uses storage even though the video may not actually need it.

As a result, the hard disk fills up faster and the DVR retains fewer days of recording.

In simple terms:

With Fixed Bitrate, the DVR uses the set bitrate continuously. Setting it higher uses more storage and results in a shorter recording period.

Variable Bitrate

Our DVRs are set to Variable Bitrate by default.

With Variable Bitrate, the DVR does not use the same amount of data all the time.

Instead, it automatically adjusts the bitrate according to what is happening in the video.

For example, if the camera is recording a quiet scene, it may use less than 1,580 kbps.

When there is more movement or more detail in the scene, it can use more data to maintain better picture quality.

Variable Bitrate also has a Maximum Bitrate setting.

For example, if the Maximum Bitrate is set to 1,580 kbps, the DVR can use up to 1,580 kbps when necessary, but it does not necessarily use that amount all the time.

This means that when the scene is simple, the DVR can use less storage. When the scene becomes more complicated, it can use more storage.

As a result, Variable Bitrate generally makes better use of the available storage.

The trade-off is that the exact recording period cannot be calculated simply from the Maximum Bitrate.

If you calculate the recording time using the Maximum Bitrate of 1,580 kbps, the result represents the minimum recording period.

The actual recording period can be longer because the DVR will normally use less than 1,580 kbps for at least some of the recording time.

For this reason, we set our DVRs to Variable Bitrate by default. It allows the DVR to use storage more efficiently while still allowing more data to be used when the video requires it.

Fixed Bitrate vs Variable Bitrate

Fixed Bitrate	Variable Bitrate
How it works	Uses approximately the same bitrate continuously	Bitrate changes according to the video
Example	Always around 1,580 kbps	Up to 1,580 kbps when necessary
Quiet scene	Still uses around 1,580 kbps	Uses less data
Busy scene	Still uses around 1,580 kbps	Can use more data
Storage usage	More predictable	Varies
Recording duration	Easier to calculate	Cannot be calculated exactly in advance
Storage efficiency	Can waste storage	Uses storage more efficiently
Our default	—	Variable Bitrate

What About Older DVRs?

Older DVRs used older compression methods such as MPEG-2, MJPEG and wavelet-based compression.

These methods generally required more storage than modern H.264 and H.265 compression.

However, their storage requirements could sometimes be easier to estimate.

Modern H.264 and H.265 systems are much more efficient and can store much more video in the same amount of hard-disk space.

The trade-off is that the actual recording time can vary depending on what the cameras are recording.

In Simple Terms

Think of your DVR's hard disk like the fuel tank of a car.

The hard disk has a fixed amount of storage, just like a car has a fixed-size fuel tank.

But how long the fuel lasts depends on how quickly the car uses it.

If you are driving on an empty road at a steady speed, the car may use less fuel.

If you are driving in heavy traffic with lots of stopping and starting, it will use more fuel.

A DVR works in a similar way.

If the cameras are recording quiet scenes with little movement, storage is used more slowly.

If the cameras are recording busy scenes with lots of movement and detail, storage is used more quickly.

Therefore, two DVRs with the same size hard disk may not necessarily give you the same number of days of recording.

The bigger the hard disk, the more recording it can store — but exactly how many days depends on how much data the cameras are using.

If you want to know your actual recording time:

Let the DVR run for one week → check how much storage it used → use that figure to estimate the total recording period.

This takes into account the actual activity at your cameras and will give you a much more realistic estimate for your particular installation.