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Lighting Beam Pool Calculator

A beam angle only means something once you know how far the light is from what it is lighting. The same fixture covers a metre or five depending on where it hangs, and the two ways of getting a bigger pool — a wider lens or a longer throw — do opposite things to brightness. That trade is the whole art of rigging, and it is exact arithmetic underneath.

What this generator does

Uses the beam angle and throw distance to work out the width covered on the surface, and reports how bright the light is relative to the same fixture at one metre.

How to use this tool

  1. Enter the throw distance and the fixture's beam angle.
  2. Read the pool width — that is the coverage on the surface.
  3. Note the relative brightness: it falls with the square of the distance.
  4. Compare a wide lens close in against a narrow one further back.

Understanding the controls

Throw distance
How far the fixture is from the surface it is lighting, in metres. Doubling this doubles the pool and quarters the brightness.
Beam angle
The fixture's beam angle in degrees. Common profile lenses run from about 10 degrees for a tight special to 50 for a wash.

Common use cases

  • Working out coverage before rigging a fixture
  • Choosing between a wider lens and a longer throw
  • Understanding why a light twice as far away needs four times the output
  • Planning a wash so pools overlap rather than gap
  • Teaching the inverse square law with a real consequence

How this generator works

Half the pool width over the throw distance is the tangent of half the beam angle — straightforward trigonometry on the triangle from the fixture to the edges of the pool. The check works it both ways: the angle must give the width, and the width must give back the angle, which catches an error either side of the conversion.

Randomness and fairness

Nothing here is random. The pool follows entirely from the angle and distance you enter.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

  • Beam angle rather than field angle. Manufacturers quote both, and the field angle — where intensity has fallen to a tenth — is noticeably wider.
  • Assumes the light is square on to the surface. Any tilt stretches the pool into an ellipse.
  • Relative brightness is inverse-square only, with no allowance for lens efficiency, atmosphere or surface reflectance.
  • Says nothing about colour, gobo projection or how pools blend where they overlap.

Privacy and your data

The trigonometry runs in your browser and nothing you enter is transmitted or stored.

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