CCTV calculator
Pixel density calculator
How many pixels a camera puts on each metre of a target at a given distance, which DORI tier that meets, and what it means for a face or a number plate.
Result
The formula
Pixel density is the camera's horizontal pixel count divided by the width of the scene at the target's distance:
density = W / (2 · d · tan(α / 2))where W is the number of horizontal pixels, α the horizontal angle of view and d the distance. It falls off in a straight line with distance: twice as far, half the pixels on every metre.
What the numbers buy
| Density | DORI tier | Good enough to |
|---|---|---|
| 25 px/m (8 px/ft) | Detect | Tell that someone is there |
| 62.5 px/m (19 px/ft) | Observe | Describe clothing and what they are doing |
| 125 px/m (38 px/ft) | Recognise | Say it is someone you know |
| 250 px/m (76 px/ft) | Identify | Identify a stranger beyond reasonable doubt |
At identify density a 16 cm wide face is 40 pixels across, the usual floor for a face a person or software can match. Reading a number plate takes about the same density on a 52 cm European plate; smaller American plates need more. See Reading number plates and the Johnson criteria for where these thresholds come from.
Other calculators
DORI distance calculator
How far a camera can detect, observe, recognise and identify a person, from its resolution and lens.
Lens and field-of-view calculator
Angle of view and scene size from a focal length and sensor, or the focal length a scene needs.
CCTV storage calculator
Bit rate and disk space for a system, from cameras, resolution, codec, frame rate and retention.
Camera database
Every figure these calculators need, already worked out for over fifteen thousand real camera models.