ShapeBeam angle without the guesswork
A beam-angle number becomes useful only when it meets a mounting distance, a surface, and the fixture’s real photometric distribution.
Shape · Optics
Beam spread calculator
See how beam angle and throw distance change the approximate diameter and footprint of a pool of light.
- Question
- How wide is a 36° beam at 8 feet?
- Runs
- Locally in this browser
- Reviewed
- 2026-08-06
Live result
36°
Approximate beam diameter5.2 ft
At 8 feet from the target surface.
Radius2.6 ft
Geometric area21.23 ft²
Metric reference1.58 m
This cone describes geometry, not illuminance. Check the actual fixture photometry and field angle.
From number to decision
Measure.
Read. Verify.
At an 8 ft throw, a 36° beam is approximately 5.2 ft across. The edge will not be perfectly sharp in a real fixture.
Measure
- Measure from the luminous opening or fixture face to the target surface.
- Read the stated beam angle from the exact product specification.
- Keep the fixture normal to the surface for this circular-footprint approximation.
Read
- The calculator treats the beam as a symmetrical geometric cone.
- Doubling throw distance doubles diameter; angle changes diameter nonlinearly.
- Area is the circular footprint implied by the geometric diameter.
Verify
- Beam angle versus field angle definitions
- Candela distribution and illuminance at the target
- Aiming, shielding, spill, texture, and overlapping sources
The visible method
diameter = 2 × distance × tan(beam angle ÷ 2)
LimitThe geometric cone does not describe field angle, candela distribution, aiming losses, spill light, or surface reflectance.
Results are computed from the fields you enter. Glow & Gauge does not infer a product, electricity rate, room reflectance, or professional approval.
Read the site-wide methodStarting references
Keep reading
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