Lighting Spacing Calculator
Work out the maximum spacing between ceiling luminaires and how many fittings a room needs for reasonably even lighting, using the spacing method (spacing-to-height ratio × mounting height).
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the mounting height Hm — the height of the luminaires above the working plane (bench/desk level), not above the floor.
- Set the spacing-to-height ratio SHR from the fitting's photometric data (default 1.5) and the room length and width in metres.
- Read the recommended maximum spacing, the row × column layout, the total number of luminaires, and the half-spacing offset to keep from the walls.
How it works
Maximum even spacing S = SHR × Hm. Dividing the room length by S and rounding up gives the number of columns; dividing the width by S and rounding up gives the rows; multiplying them gives the luminaire count. The outer rows and columns are set half a spacing (S/2) in from the walls so illuminance near the perimeter matches the centre.
Worked example
Worked example. A room 10 m × 8 m has luminaires mounted 2.5 m above the working plane with an SHR of 1.5. Spacing S = 1.5 × 2.5 = 3.75 m. Columns = ceil(10 / 3.75) = 3, rows = ceil(8 / 3.75) = 3, giving 9 luminaires set 1.875 m (S/2) from the walls.
Common mistakes
- Measuring the mounting height from the floor instead of from the working plane — the spacing method uses the height above the working plane.
- Placing the outer fittings hard against the walls; they should sit about half a spacing (S/2) in, or the edges go dim.
- Treating this as a lux calculation — spacing only fixes uniformity, not the light level, which needs the lumen method.
Frequently asked questions
Does this tell me if the room is bright enough?
No. The spacing method only ensures even coverage. To confirm you meet a target illuminance (lux), use the lumen method, which also needs each luminaire's lumen output, the utilisation factor for the room, and the maintenance factor.
Where do I get the spacing-to-height ratio?
It is a photometric property published in the luminaire's data sheet (sometimes as SHR or spacing criterion), typically around 1.25–1.75. Using the actual fitting's value gives a more reliable spacing than the 1.5 default.
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