Room Lighting Calculator (Lux and Lumen Method)
Room Lighting Calculator (Lux and Lumen Method)
The lumen method, worked properly: room index, the total lumens a room needs at your target lux, how many fittings that is, the whole grid you can actually install, and the illuminance that grid really delivers — new and after maintenance losses.
Lumen method: fittings for a room
A 6 m × 4 m classroom at 300 lx, fittings 3 m up, working plane 0.85 m, 3,400 lm per fitting, UF 0.5, MF 0.8
The lumen method
Φtotal = (E × A) ÷ (UF × MF) N = ⌈ Φtotal ÷ Φfitting ⌉
Eactual = (N × Φfitting × UF × MF) ÷ A
- L, W
- room length and width, in metres
- A
- floor area, L × W, in square metres
- Hm
- height of the fittings ABOVE THE WORKING PLANE, not above the floor — the commonest mistake in this calculation
- k
- room index: how much of a room’s surface is wall rather than floor and ceiling. A long low room has a high k, a tall narrow one a low k, and the luminaire’s utilisation factor table is indexed by it
- E
- maintained illuminance you are aiming for, in lux
- UF
- utilisation factor: the share of a fitting’s light that reaches the working plane, from the luminaire’s own table against k and the room’s reflectances
- MF
- maintenance factor: LLMF × LSF × LMF × RSMF, the light left at the end of the cleaning and replacement cycle
Worked example
A 6 m × 4 m classroom at 300 lx, fittings 3 m up, working plane 0.85 m, 3,400 lm per fitting, UF 0.5, MF 0.8
Mounting height above the working plane: Hₘ = 3 − 0.85 = 2.15 m; area A = 6 × 4 = 24 m²
Room index: k = 24 ÷ (2.15 × 10) = 1.116 — the row to read the luminaire's UF table at
Flux needed: Φ = (300 × 24) ÷ (0.5 × 0.8) = 7,200 ÷ 0.40 = 18,000 lm
Fittings: 18,000 ÷ 3,400 = 5.294, so 6 — laid out 3 along the length × 2 across, which is 6 fittings
What they give: (6 × 3,400 × 0.5 × 0.8) ÷ 24 = 340 lx maintained, 425 lx when new
Spacing 2.00 m × 2.00 m, so the largest spacing is 0.93 × the mounting height — comfortably inside a typical SHR limit of about 1.5
Maintained illuminance to BS EN 12464-1
| Space or task | Eₘ (lux) |
|---|---|
| Corridor; warehouse store | 100 |
| Staircase | 150 |
| Classroom; retail sales area | 300 |
| Office (writing, reading, data processing); meeting room; food-service kitchen | 500 |
| Fine workshop tasks | 500–1,000 |
Room index for a 2.15 m mounting height
| Room | Area | k = LW ÷ (Hₘ(L+W)) | What it means |
|---|---|---|---|
| 3 m × 2 m | 6 m² | 0.558 | tall and narrow — walls absorb most of the light |
| 4 m × 3 m | 12 m² | 0.797 | tall and narrow — walls absorb most of the light |
| 6 m × 4 m | 24 m² | 1.116 | typical room |
| 10 m × 8 m | 80 m² | 2.067 | typical room |
| 20 m × 15 m | 300 m² | 3.987 | large and low — little is lost to the walls |
Sizing a lighting scheme
The lumen method answers one question: how many fittings does this room need to average a given number of lux on the working plane? It works backwards from the target. Multiply the target illuminance by the floor area to get the lumens that must land there; divide by the utilisation factor, because most of a fitting’s output never reaches the working plane; divide again by the maintenance factor, because a scheme must still hit the target at the end of its cleaning cycle, not only on the day it is switched on. A 6 m × 4 m classroom at 300 lx needs 7,200 lm on the plane, and 18,000 lm of installed fitting output to deliver it.
The room index is not decoration. A utilisation factor is a property of a luminaire in a room, not of the luminaire alone, and the manufacturer’s table is indexed by the room index k and the ceiling, wall and floor reflectances. k = LW / (Hₘ(L + W)) compares the floor area with the wall area: a tall narrow room has a low k and loses a lot of light to its walls, a large low room has a high k and loses little. This room is k = 1.116. Take UF from the row nearest that, at the reflectances your room actually has — not the 70/50/20 the table is headed with, unless your ceiling really is 70% reflective.
Mounting height means above the working plane. Hₘ is the height of the fitting above the plane you are lighting, not above the floor. Fittings at 3 m over a 0.85 m desk give Hₘ = 2.15 m. Using 3 m instead would put the room index at 0.800 and send you to the wrong row of the UF table. For a corridor or a warehouse aisle the working plane is the floor, so enter 0.
A whole number, then a rectangle. The exact answer is rarely an integer and almost never a good grid. This page rounds the count up, then lays it out as the nearest rectangle to the room’s own proportions and reports what that grid actually delivers: 3 × 2 here, 340 lx rather than the 300 asked for. Overshooting is normal and is the honest number to design to; the chart shows how each extra fitting moves it, and the gap between the maintained curve and the as-new one is everything maintenance costs you.
What this does not do. It gives an average, so it says nothing about uniformity: check the spacing against the luminaire’s own maximum spacing-to-height ratio, usually around 1.5, or the room will be bright under the fittings and dim between them. It says nothing about glare, which BS EN 12464-1 limits with a UGR figure, nor about colour rendering, nor about daylight. And it assumes an empty room — shelving, partitions and machinery all take their cut. For the lamp side of the arithmetic, use the lumens to watts converter; for what the scheme costs to run, the electricity consumption calculator; for the circuit that feeds it, the MCB size calculator.
Frequently asked questions
How many lights does a room need?
Total lumens = target lux × floor area ÷ (UF × MF), then divide by the output of one fitting. A 6 m × 4 m room at 300 lx with UF 0.5 and MF 0.8 needs 18,000 lm, which is 5.29 fittings of 3,400 lm — 6 once you round up to a 3 × 2 grid.
What is the room index and why do I need it?
k = (L × W) ÷ (Hₘ × (L + W)), where Hₘ is the height of the fittings above the working plane. It is how much of the room is wall rather than floor, and it is the row you read the luminaire’s utilisation factor table at. This page’s default room is k = 1.116.
What utilisation factor should I use?
The one in your luminaire’s own photometric data, read against your room index and your surface reflectances — it is measured, not estimated. If you have no data sheet yet, 0.4 to 0.6 covers most general-purpose fittings in a normal interior, and the lower end is the safer guess.
What is a maintenance factor?
The fraction of the original light still reaching the working plane at the end of the maintenance cycle: MF = LLMF × LSF × LMF × RSMF — lamp lumen depreciation, lamp survival, dirt on the luminaire and dirt on the room surfaces. About 0.8 for a clean interior, lower for a dusty one.
How far apart should ceiling lights be?
No further apart than the luminaire’s maximum spacing-to-height ratio allows, usually about 1.5 times the mounting height above the working plane. The default scheme here spaces fittings 2.00 m × 2.00 m at a mounting height of 2.15 m, a ratio of 0.93.
Related calculators
References
- University College London, Department of Electronic and Electrical Engineering, Lighting Design Guide (teaching-laboratory technical note): the lumen method, E = F × n × N × MF × UF / A; room index K = L × W / ((L + W) × Hₘ); maintenance factor MF = LLMF × LSF × LMF × RSMF; and the spacing-to-height ratio SHRₘₐₓ.
- BS EN 12464-1, Light and lighting — Lighting of work places, Part 1: Indoor work places: maintained illuminance Eₘ. Values quoted here as reproduced by TECHLUMEN, Lighting standards guide (corridors 100 lx, staircases 150 lx, classrooms and retail sales areas 300 lx, offices, meeting rooms and food-service kitchens 500 lx, fine workshop tasks 500–1,000 lx) and by Fagerhult, Standard EN 12464-1 in brief (500 lx for a normal office workplace).
- Commission Internationale de l’Éclairage. CIE S 017, ILV: International Lighting Vocabulary — definitions of illuminance (lux), luminous flux (lumen), maintained illuminance and maintenance factor.
- Bureau of Indian Standards. IS 3646, Code of practice for interior illumination — the Indian schedule of recommended illuminance, which differs from BS EN 12464-1 in places; unverified here, cited as where to look rather than quoted.