The method

The lumen method

The total light a room needs is not simply target level × area. A large fraction of what a fixture emits never reaches the working plane — it is absorbed by walls, ceilings and the fitting itself — and fixtures dim as they age and collect dust. Both losses have to be paid for in advance:

N = (E × A) ÷ (F × UF × MF)
N = number of fixtures · E = target illuminance in lux · A = floor area in m² · F = lumens per fixture · UF = utilisation factor · MF = maintenance factor

The room index and the utilisation factor

The utilisation factor is the share of emitted light that actually lands on the working plane. It depends on the room's proportions, captured by the room index:

RI = (L × W) ÷ (Hm × (L + W)) — where Hm is the mounting height above the working plane, not above the floor.

A squat, wide room has a high room index and uses light efficiently. A tall, narrow one wastes far more on its walls. This site interpolates the utilisation factor from a standard table running from 0.40 at RI 0.6 up to 0.82 at RI 5.0, assuming light-coloured surfaces and a recessed downlight. Dark walls will push the real figure lower — if your room is painted a deep colour, add 10–20%.

The maintenance factor is fixed at 0.8, a normal assumption for a clean domestic interior with LED fittings.

The iso-lux contours on every plan

The dashed rings drawn around each fitting are not decoration and they are not the beam angle. Each fitting is treated as a Lambertian source of F lumens, so its intensity falls as the cosine of the angle off nadir and the illuminance it puts on the working plane falls as the fourth power of that cosine:

E(r) = F ÷ (π × Hm²) × cos⁴θ, where cos θ = Hm ÷ √(Hm² + r²)
so directly beneath the fitting E₀ = F ÷ (π × Hm²), and the contour at a fraction f of E₀ sits at r = Hm × √(f^−1/2 − 1).

The three rings are drawn at f = 0.75, 0.50 and 0.25, which puts them at 0.39, 0.64 and 1.00 times the mounting height. An 800 lumen downlight 8 ft above the floor is therefore worth about 43 lux beneath itself and 11 lux eight feet away.

They are the direct component of one fitting only. The lumen method above sizes the total, including every ray that arrives after bouncing off a wall or ceiling — which is exactly why the installed count is higher than any single ring suggests, and why the rings have to overlap before a room holds its target. Reading the contours as the finished light level would understate the room by the whole interreflected component. The point of drawing them is that a mean illuminance figure hides the one thing a reader can act on: whether the pools of light meet.

Where the fixtures go

Fixture count without a layout is useless, so every page solves the grid too. The site picks the rows × columns whose proportions best match the room's own shape while staying close to the required count — an even grid of six beats a lopsided five. Fixtures are then spaced evenly, with the outer row set half a spacing in from the end walls, which is what stops the perimeter going dark.

Spacing is sanity-checked against the spacing criterion, roughly 1.3 × mounting height for a typical downlight. Go wider and you get visible scallops of light and shade on the floor; pages flag it when a layout crosses that line.

Where the target levels come from

Target illuminance figures are published recommendations, quoted as ranges and drawn from freely-available secondary sources. The authoritative standards — BS EN 12464-1 and the IES Lighting Handbook — are paid, copyrighted documents; their tables are deliberately not reproduced here, and if you need to design to a code or a client specification you must work from the standard itself, not from this site.

What this calculation cannot tell you

• It is an average, not a map. The lumen method gives the mean illuminance across the plane. It says nothing about glare, shadows under cabinets, or the dark corner behind the door.
• Surface colour matters enormously. The same fixtures in a dark room can deliver 20–30% less usable light.
• Beam angle is not modelled. A narrow spot and a wide flood with identical lumen output light a room very differently.
• Layered lighting beats ceiling lighting. Nearly every good room uses task and accent light as well. This site sizes the ambient layer only.
• Daylight is ignored. Rooms are sized as if it were night.

Planning guidance only, not an electrical design. Fixture counts are a starting point — follow your local wiring rules and use a licensed electrician for the installation.

Corrections

If a layout here looks wrong to you and you design lighting for a living, I would genuinely like to hear about it — get in touch.

SOURCE·IES lumen-method recommendations·published recommendations, quoted as ranges·REVIEWED JUL 2026