Dashcam Storage Calculator

About this calculator

A dashcam records at a bit rate and a card holds a number of bytes, so the recording time is one division — once the units are reconciled. That is where most answers go wrong. A bit rate is in megabits and a card is in gigabytes, and there are eight bits in a byte, so a 10 Mbps camera fills 4.5 GB an hour rather than 36.

The second reconciliation is the card itself. A "128 GB" card is sold on the decimal gigabyte, and the file system takes a share, so what the camera can actually write to is several per cent below the label.

How to use this calculator

1. Find the bit rate in the camera's manual or on its settings screen, and enter it in Mbps.
2. Say how many cameras write to this card — a front-and-rear pair is two.
3. Enter the card size from the label.
4. Optionally say how long you drive each day, to see how far back the footage reaches.

The formula

Bytes per second = Bit rate in Mbps x 1,000,000 / 8
Usable bytes     = Card GB x 1,000,000,000 x Usable %
Recording time   = Usable bytes / Bytes per second
GB per hour      = Bytes per second x 3,600 / 1,000,000,000

A megabit per second is 125 kilobytes per second, which is where the factor of eight comes from and where most published figures lose their way.

Worked example

A front-and-rear pair at 10 Mbps each, on a 128 GB card:

Combined rate = 20 Mbps         = 2.5 MB/s
Per hour      = 2.5 x 3,600     = 9 GB
Usable        = 128 x 0.93      = 119 GB
Time          = 119 / 9         = 13.2 hours

At an hour and a half of driving a day, that is under nine days before the oldest footage is written over.

Notes and limits

• Most dashcams vary the bit rate with the scene, so a quoted rate is usually a ceiling and real footage runs a little under it. This estimate is therefore slightly conservative.
• Loop recording overwrites the oldest footage once the card is full. This is how far back the card reaches, not how much it will ever store.
• Locked incident files are normally kept in a reserved area and are not overwritten, which reduces the space left for the loop. A card full of locked events records very little.
• Dashcams write continuously and wear cards out. A high-endurance card lasts far longer in this duty than an ordinary one, whatever the capacity.
• A 128 GB card showing as about 119 GB on a computer is not faulty — that is the same space counted in binary gigabytes.

What this calculation assumes

• The bit rate is the figure you entered. Most cameras vary it with the scene, so this is slightly conservative.
• Usable space defaults to 93% of the labelled size, which is a starting point rather than a specification, and the result names it.
• Locked incident files are not accounted for and reduce the space left for the loop.