Beam Coverage Calculator
Estimate nominal beam diameter and circular footprint from distance and full beam angle.
Calculation inputs and results
Enable JavaScript for the interactive calculator. For a circular beam aimed perpendicular to a flat plane: diameter D = 2 × distance d × tan(θ ÷ 2); footprint area = π × (D ÷ 2)². Use the full beam angle θ in degrees.
This is nominal beam geometry, not a guaranteed illuminated area or lux level. Light can extend beyond the beam boundary. Do not use the footprint area alone to set fixture spacing or fixture count.
Beam calculation method & limits
Diameter D = 2d × tan(θ/2). Circular footprint area = π(D/2)². The calculator converts degrees to radians for the tangent and uses unrounded values for area. Switching distance units uses 1 ft = 0.3048 m; area is shown in the matching square unit.
The model assumes a circular, rotationally symmetric beam directed perpendicular to a flat target plane. Tilted luminaires, asymmetric distributions, near-field optics and obstructions need a photometric calculation.
The CIE beam-angle definition uses the full angle between directions at 50% of centre-beam luminous intensity. This is an intensity convention, not an illuminance threshold on the target plane. Check the angle convention in the manufacturer’s data and use IES/LDT files to assess illuminance, uniformity and overlap.