How is beam spread calculated?

For a circular cone aimed squarely at a flat surface, footprint diameter equals two times the distance times the tangent of half the beam angle.

Does a wider beam mean a brighter area?

Not by itself. A wider beam spreads light across more area. Center intensity, lumens, optics, surface reflectance, and mounting direction also affect what you see.

Can I calculate beam angle from BR, PAR, MR, or GU10?

No. Those codes describe an envelope or base, not one guaranteed beam angle. Use the beam angle published for the exact lamp.

Spot and flood reference

How wide will the beam be?

Enter the beam angle printed in the exact lamp specification and the distance to the lit surface. The result is a geometric footprint, not a brightness prediction.

How beam angle changes the lighting effect →Published August 27, 2026 · Calculation runs locally

Two inputs

Estimate the beam footprint

Use the published beam angle for the exact lamp, not the BR, PAR, MR, or GU10 code alone.

Beam angle°

Distance to the surfaceft

UnitFeetMeters

Estimated diameter5.20 ft

At 8.00 ft with a 36.0° beam

Radius: 2.60 ft
Circular area: 21.2 ft²

The calculation

A circular-cone estimate

The calculator uses diameter = 2 × distance × tan(beam angle ÷ 2). It assumes that the lamp's centerline meets a flat surface at 90 degrees.

The Illuminating Engineering Society defines beam angle using directions where intensity falls to 50% of the maximum. That boundary is a photometric convention, not a hard edge you will see on a wall or floor.

IES beam-angle definition ↗

Before using the result

What the number leaves out

An angled lamp makes an oval rather than a circular footprint.
Center-beam intensity and distribution vary even when two lamps list the same angle.
Trim, lenses, dirt, walls, and reflective surfaces can alter the visible pool of light.
Use a photometric file or manufacturer layout data when illuminance matters.

Worked reference

Beam diameter at an 8 ft throw

These values use the same cone formula and are rounded to the nearest tenth of a foot.

Beam angle	Typical description	Estimated diameter
15°	Narrow spot	2.1 ft
25°	Spot	3.5 ft
36°	Flood	5.2 ft
40°	Flood	5.8 ft
60°	Wide flood	9.2 ft

Keep the specification together

Beam is only one replacement field.

Compare reflector shapes →Build the complete replacement brief →Check an exact model record →