How the Lumen Method Sizes a Lighting Scheme
The lumen method is the classic way to size a general lighting scheme to an
average maintained illuminance. The number of luminaires is
N = E × A / (Φ × UF × MF) — the target lux
E times the floor area A, divided by the luminaire output Φ, the
utilisation factor UF and the maintenance factor
MF. The utilisation factor depends on the room index
k = (L·W) / (Hm·(L+W)) — how the room's proportions and the mounting height above the
working plane, together with the ceiling, wall and floor reflectances, determine how much of the
light actually reaches the working plane. This tool computes all of that, lays the fittings out
on a sensible grid and checks the spacing-to-height ratio for uniformity.
BS EN 12464-1 Lux Levels
Pick the space and the calculator loads the maintained illuminance and glare
(UGR) target from a built-in subset of BS EN 12464-1
— 500 lx for offices, 300 lx for circulation-heavy areas, 750 lx for fine
industrial work, and so on — or type your own target. The result shows the average illuminance
achieved, the power density in W/m² and the luminaire efficacy, so you can sanity-check a scheme
or a manufacturer's quote in seconds. It reuses the same standards our
DIALux lighting design service works to.
From a Quick Check to a Full DIALux Design
The lumen method gives you the average — it does not give uniformity (Uo),
point-by-point illuminance, glare (UGR) at the observer, emergency lighting to BS 5266, or a
3D render. For that we build the room in DIALux evo with the real photometric
files and produce a submission-ready pack. To get a head start, this tool can
export a DIALux STF file (beta) — open it in DIALux evo with
Import STF… to rebuild the room and luminaire grid, then finish the photometric
calculation there. Need the distribution side too? Size the final circuits with our
BS 7671 cable sizing calculator.
When you need the full lighting design done and checked, see our
DIALux lighting design service.
How does the lumen method work?
It sizes the number of luminaires needed for an average illuminance across a room: N = (E × A) / (Φ × UF × MF), where E is the target lux, A the floor area, Φ the lumen output per luminaire, UF the utilisation factor and MF the maintenance factor. The utilisation factor accounts for how much of the light actually reaches the working plane given the room shape and surface reflectances; the maintenance factor accounts for the light lost to dirt and lamp ageing over the maintenance interval.
What lux level should I design to?
BS EN 12464-1 sets maintained illuminance by task, not by room name — 500 lux for general office work and writing, 300 lux for a corridor-adjacent circulation task, 200 lux for a warehouse aisle, and considerably more for fine detail work. It is a maintained figure, meaning the level must still be met at the end of the maintenance cycle, which is exactly what the maintenance factor is protecting. Check the specific task listed in the standard rather than assuming a value for the space.
Why does the calculator ask for the room reflectances?
Because they change how much light gets to the working plane. Light that leaves a luminaire upwards or sideways only becomes useful after bouncing off the ceiling and walls, so a room with a dark ceiling needs materially more luminaires than the same room with a light one. The utilisation factor is a function of the room index — the room's proportions relative to the mounting height — and those reflectances.
The tool says PASS but flags spacing. What does that mean?
The lumen method gives you the average illuminance; it says nothing about how evenly it is spread. The tool screens the spacing-to-height ratio against a typical maximum for the distribution class, so a layout that meets the lux target with too few fittings spaced too far apart is flagged for review. That is an indicative screen only — real uniformity comes from a point-by-point calculation with the actual photometry.
Can I use this instead of DIALux?
For a first pass and a sanity check, yes. For a design that gets issued, no. The lumen method gives an average over a bare rectangular room. It does not give point-by-point illuminance, uniformity, glare at the observer position (UGR), the effect of furniture and obstructions, daylight, emergency lighting to BS 5266, or a render. The tool can export an STF file to give a DIALux evo model a head start, but the photometric calculation belongs there.
How accurate is the utilisation factor it uses?
It is generic, and it is the biggest source of error in the result. The tool interpolates a utilisation factor from indicative tables by distribution class, room index and reflectance — reasonable for a first pass, but the real figure comes from the specific luminaire's photometric file and can differ significantly. If you have the manufacturer's UF table, enter it as an override; that is the accurate path and the tool will use it in preference.