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Fixture Spacing Estimator

Maximum centre-to-centre spacing from mounting height, luminaire distribution, and the illuminance target.

Inputs

Mounting

Spacing-to-mounting-height ratio for the distribution. Area and street lights throw much further than downlights.

Ceiling height for interiors; pole height above grade for exterior lighting.

Switch to metres for metric drawings.

Fixture and targets

Delivered lumens, from the photometric file or the product label.

Share of fixture lumens that reaches the work plane.

Light lost to dirt and ageing. Exterior installs sit lower than interiors.

Target illuminance at the work plane.

Result

Maximum spacing to use
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Maximum spacing (metric)
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Mounting height
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Spacing to height ratio
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Spacing to mounting height
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Spacing allowed by illuminance
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Spacing allowed by distribution
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Fixtures per 1,000 ft²
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What governs
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What to check

  • Mounting height measured above the work plane, not above the finished floor
  • Distribution type from the photometric file, not from the catalogue photograph
  • Wall and end of row offsets, which are usually half the spacing
This calculator is a first-pass screen using spacing-to-mounting-height rules and a uniform illuminance approximation. Verify the final positions with the manufacturer IES file: spacing rules ignore wall offsets, obstructions, and the uniformity requirement of the task.

Saved calculations

Keep the assumptions with the result.

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