SettingsSettings for this calculationUS
The longer dimension of the room.
Measured wall to wall at the working plane.
The shorter dimension.
Measured wall to wall at the working plane.
The height the luminaires hang or sit at, measured from the floor.
For recessed fittings, the ceiling height; for suspended ones, the ceiling height less the suspension.
The surface the light is designed for — a desk, a bench, or the floor itself.
Desks are usually about 0.75 to 0.85 m (2 ft 6 in to 2 ft 9 in), benches about 0.9 m (3 ft); circulation areas are designed at floor level. The formulas use the luminaires' height above this plane, not above the floor.
The maintained average the task needs, from BS EN 12464-1 or the IES recommendation.
Offices and general workshops are commonly designed at 300 to 500 lx (28 to 46 fc), fine work higher. The box follows the unit switch.
The light output of one luminaire, from its data sheet — the luminaire's lumens, not the bare lamp's.
Lumens are the same in every unit system. For LED luminaires the data sheet gives the luminaire output directly.
The share of the lumens that reaches the working plane, read from the luminaire's table at this room index and these surface reflectances.
The page gives the room index to read it with. Dark surfaces cut it hard: the same luminaire might reach 0.6 in a pale open hall and 0.35 in a narrow dark bay.
The share of the initial output left at the end of the maintenance cycle, for dirt and lumen depreciation.
About 0.8 is common for clean interiors with LED luminaires and regular cleaning; dirtier spaces and longer cycles are lower.
From the luminaire's photometric data: the widest pitch, as a multiple of the height above the working plane, that keeps the light even.
Generally around 1.25 to 1.5 for a direct distribution and higher for a wide-beam batten.
Luminaires needed
52 luminaires
The lumen method's count is rounded up; the grid is the fewest luminaires in rows that keep every pitch within the spacing-to-height limit, and the outside rows sit half a pitch from the walls.
- Height of the luminaires above the working plane
- 16.5 ft
- Room index K
- 2.39
- Luminaires by the lumen method, before rounding up
- 51.65
- Rows along the length
- 13
- Luminaires in each row, across the width
- 4
- Luminaires in that grid
- 52
- Pitch along the length
- 7.54 ft
- Pitch across the width
- 16.5 ft
- Largest pitch the SHR allows
- 20.62 ft
- Average illuminance the grid gives
- 46.31 fc
They open the calculator with your figures already in it
Lumen Method Lighting Calculator (Room Index, Luminaire Count and Spacing): 52 luminaires — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- The lumen method (SLL Code for Lighting, CIBSE; IES Lighting Handbook): N = E × A ÷ (Φ × UF × MF); room index K = L × W ÷ (Hm × (L + W)), Hm the height of the luminaires above the working plane
- Maximum spacing = the luminaire's spacing-to-height ratio (SHR, from its photometric data) × Hm
- BS EN 12464-1, Lighting of work places — Indoor work places: maintained illuminance on the task area
Inputs used
- Room Length
- 98 ft
- Room Width
- 66 ft
- Height of the Luminaires Above the Floor
- 19.5 ft
- Height of the Working Plane Above the Floor
- 3 ft
- Target Average Illuminance
- 46 fc
- Luminaire Output (lumens)
- 12000
- Utilisation Factor (UF)
- 0.6
- Maintenance Factor (MF)
- 0.8
- Spacing-to-Height Ratio (SHR)
- 1.25
Intermediate steps
- Height of the luminaires above the working plane
- 16.5 ft
- Room index K
- 2.39
- Luminaires by the lumen method, before rounding up
- 51.65
- Rows along the length
- 13
- Luminaires in each row, across the width
- 4
- Luminaires in that grid
- 52
- Pitch along the length
- 7.54 ft
- Pitch across the width
- 16.5 ft
- Largest pitch the SHR allows
- 20.62 ft
- Average illuminance the grid gives
- 46.31 fc
Confidence note: The lumen method's count is rounded up; the grid is the fewest luminaires in rows that keep every pitch within the spacing-to-height limit, and the outside rows sit half a pitch from the walls.
What this calculation does not cover
- The utilisation factor comes from the luminaire's own table at this room index and the room's reflectances; the default is a placeholder, and a dark room can halve it.
- The lumen method gives an average. Uniformity, glare and the light on walls and ceilings need a point-by-point calculation in lighting software, which is also where obstructions and the real ceiling grid come in.
- The grid is the arithmetic ideal; ceiling tiles, joists, sprinkler heads and ducts move luminaires on site, and a luminaire moved has left the layout the calculation described.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of a bigger job
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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations3
- The lumen method (SLL Code for Lighting, CIBSE; IES Lighting Handbook): N = E × A ÷ (Φ × UF × MF); room index K = L × W ÷ (Hm × (L + W)), Hm the height of the luminaires above the working plane
- Maximum spacing = the luminaire's spacing-to-height ratio (SHR, from its photometric data) × Hm
- BS EN 12464-1, Lighting of work places — Indoor work places: maintained illuminance on the task area
Which documents these citations point at
Standards referenced: BS EN 12464-1 (European Committee for Standardization, as published in the UK by BSI, European (EN)).
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