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Beam Angle & Spot Size Calculator (Stage, Track and Landscape Lighting)

Calculate the diameter of the lit spot from beam angle and throw distance (and field angle at 10%), the beam angle needed to cover a given area from a given distance, centre illuminance from candela, and a table of spot sizes by distance for common beam angles.

Beam angle (50% intensity)

Field angle (10% intensity, 0 = estimate ×1.6)

Throw distance

Angle from vertical / off-axis to the surface

Desired spot diameter (to find the beam angle)

Centre-beam candlepower (optional)

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How it works

Spot diameter = 2 × throw × tan(beam ÷ 2); beam needed = 2 × atan(diameter ÷ 2 ÷ throw); centre E = CBCP ÷ throw² (metres → lux); field angle ≈ 1.6 × beam angle if unlisted

A luminaire's beam angle is the cone within which intensity stays above half its centre value, and the field angle — usually listed for stage fixtures, about 1.6 times the beam angle for reflector lamps — marks the 10% edge where the light visibly ends. Simple trigonometry turns either into a pool size at a given throw, which is how lighting designers choose an ellipsoidal lens tube, a track head or a landscape spot: a 25° unit at 20 ft makes an 8.9 ft pool. Light striking a surface at an angle stretches into an ellipse with a dim far edge, and centre brightness follows the inverse square law from the centre-beam candlepower, so long throws need narrow, bright fixtures and short throws wide, gentle ones.

Worked example

A 25° beam thrown 20 ft lights an 8.9 ft circle (14 ft to the 10% field edge); aimed 30° off-axis onto a wall it stretches to about 12 ft. To light exactly 8 ft from 20 ft you need a 23° beam — a 25° or 26° lens is the nearest stock choice — and a 20,000 cd unit puts about 50 fc at the centre.

Frequently asked questions

Why does my spot look bigger than calculated?

Because the eye sees the field angle (10%) and even the spill beyond it, not the 50% beam. Use the field angle for "how far does the light reach" and the beam angle for "where is it usefully bright".

How do zoom ellipsoidals map to this?

A 25–50° zoom covers the whole calculator range at a given throw; shutter cuts trim the pool afterwards. Fixed lens tubes (19°, 26°, 36°, 50°) are chosen from the required angle rounded up.

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