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Beam Angle & Light Spread Calculator

Calculate beam diameter from beam angle and throw distance, determine the beam angle needed for a target width, or calculate the required throw distance. Use professional options to review aimed-beam footprints and estimate center-beam illuminance when CBCP data is available.

Saleh Deco Lighting Tool

Beam Angle & Light Spread Calculator

Calculate beam diameter from distance and beam angle, determine the beam angle required for a target width, or calculate the throw distance needed. Professional options add aimed-beam footprint and optional center-beam illuminance context.

Symmetrical beam geometryDirectional beams 1°–120°Metric & imperial units
Calculation state loaded

Calculate Beam Width

Enter the beam angle and throw distance along the beam axis.

Full beam angle. This tool is intended for directional symmetrical beams from 1° to 120°.
Common beam angles
Distance from fixture to target center along the beam axis.
Professional calculation options
0° = perpendicular to the target plane. Used to estimate the stretched target footprint.
Enter candela/CBCP, not lumens, to estimate center-beam illuminance.
Aimed-beam model
The core beam angle/diameter calculation remains based on a symmetrical conical beam. Aiming angle estimates the elliptical footprint where that cone intersects a flat target plane. If the cone becomes too oblique to create a bounded ellipse, the tool warns instead of inventing a footprint.

Calculation basis

Beam diameter = 2 × throw distance × tan(beam angle ÷ 2). Inverse modes solve the same geometry for angle or distance.

Fixture 24° Beam diameter 1.28 m
Beam Diameter
1.28 m
at 3.00 m throw distance
24° · Narrow Beam
24°Beam angle
3.00 mThrow distance
1.28 mPerpendicular beam diameter
1.28 m²Circular geometric beam area
0°Aiming angle
1.28 × 1.28 mTarget footprint major × minor
—Approx. center-beam lux
Geometric result readyThe beam is perpendicular to the target plane. Actual light distribution still depends on the luminaire photometrics and optics.

Practical optic comparison

Common beam comparison

Approximate beam diameters at the current throw distance.

1.28 m24° beam
1.95 m36° beam
3.46 m60° beam

Typical applications

Practical guidance only. Manufacturer classifications and suitable applications vary with distance, output and optics.
TreesColumnsSculpturesArchitectural accents

How to Use the Beam Angle & Light Spread Calculator

Use the three modes to solve the same beam geometry from different known values, then compare common optics and continue to layout planning when appropriate.

1Choose what you need to calculate

Calculate beam width when beam angle and throw are known, calculate beam angle when target diameter and throw are known, or calculate throw distance when angle and target diameter are known.

2Enter the beam geometry

Throw distance is measured from the fixture to the target center along the beam axis. The core calculation assumes a symmetrical conical beam and a circular cross-section perpendicular to that axis.

3Compare common beam options

In beam-angle mode, the exact geometric requirement is compared with common optics. The tool distinguishes the numerically closest common optic from the first common optic wide enough to cover the target.

4Use professional aiming context when needed

An aimed symmetrical beam creates a stretched footprint on a flat target plane. Enter an aiming angle to estimate the major and minor footprint dimensions. Enter CBCP in candela only if you want an approximate center-beam illuminance estimate.

5Continue to Spotlight Layout

Transfer the selected or calculated beam angle into the Spotlight Layout Calculator when you need to review fixture spacing and room arrangement. Throw distance is not automatically treated as ceiling height.

Calculation method

Beam diameter = 2D × tan(θ/2). Beam angle = 2 × atan(diameter ÷ 2D). Throw distance = diameter ÷ [2 × tan(θ/2)].

For an aimed symmetrical beam intersecting a flat target, the professional footprint estimate uses the conic-section geometry of the beam cone and target plane. A bounded ellipse exists only while the beam does not become tangent to or pass parallel to the target plane.

Optional center-beam illuminance uses CBCP ÷ distance² and applies a cosine correction for target incidence angle.

What still requires photometric verification?
  • Real beam profile, field angle and spill light outside the nominal beam
  • Asymmetric, oval, wall-washer or multi-axis optics
  • Uniformity across the illuminated surface
  • Actual CBCP/intensity distribution away from the beam center
  • Near-field behavior of large or extended luminaires
  • IES/LDT photometric distribution and final aiming in the real project

What Does Beam Angle Mean?

Beam angle describes the spread of a directional light beam. Narrow beams concentrate light on smaller targets, while wider beams cover broader surfaces. The resulting beam diameter also depends on throw distance, so beam angle should always be considered together with fixture position and the size of the area being illuminated.

Read the Beam Angle Lighting Guide →

How Accurate Is the Beam Angle Calculator?

The calculator uses geometric beam relationships to estimate beam diameter, required beam angle and throw distance for symmetrical directional beams. Professional options can also estimate the footprint of an aimed beam and approximate center-beam illuminance when valid CBCP data is available.

Actual results depend on the luminaire's photometric distribution, optics, aiming position and target surface. Beam classifications vary by manufacturer, and asymmetric, oval, wall-washer or very wide distributions require product-specific photometric data. Use the calculator for preliminary planning and comparison rather than as a substitute for IES/LDT-based photometric design.

Need Help Selecting the Right Beam or Fixture?

Use your calculated beam angle and target coverage to compare suitable directional lighting, or contact Saleh Deco with your mounting distance, target dimensions and intended application for product-selection support.

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