The Beam Angle Calculator finds the relationship between a light's beam angle, throw distance, and the spot width it casts on a surface — useful for stage lighting, architectural lighting, and photography setups. Enter any two of the three values to calculate the missing one. Select your unit system (meters or feet) to also see the coverage area of the illuminated circle.
Beam Angle
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Throw Distance
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Spot Width
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Coverage Area
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Enter any two known values (beam angle, throw distance, or spot width) and the calculator will determine the third value using trigonometry. All calculations assume a symmetrical circular beam pattern.
Beam angle is the angle between the directions in which the luminous intensity is 50% of the maximum. It determines how wide or narrow the light spread will be from a fixture.
Beam angle is measured in degrees from the center axis to the point where light intensity drops to 50% of peak intensity. Most lighting manufacturers provide beam angle specifications for their fixtures.
Beam angle measures to 50% intensity while field angle measures to 10% intensity. Field angle is always larger and represents the total light spread including spill light.
Yes, this calculator works for all light sources including LED, halogen, incandescent, and discharge lamps. The beam angle formula applies universally regardless of light source technology.
You can use any unit (feet, meters, inches) for throw distance and spot width, but both values must use the same unit. The calculator provides a unit selector for convenience.
Beam angle determines coverage area, light intensity, and uniformity. Narrow beams (10-30°) provide focused lighting, while wide beams (60-120°) offer broader coverage with lower intensity.
Coverage area is calculated as π × (spot width ÷ 2)². The spot width is determined by the beam angle and throw distance using the formula: spot width = 2 × distance × tan(beam angle ÷ 2).