Lighting calculator: how many luminaires does a room need

What the lighting calculator shows

The calculator answers one question: how many luminaires a particular room needs to be lit properly. Enter the length, width and height, pick what the room is for — living room, office, warehouse or workshop — and you get the number of luminaires required, the average illuminance in lux (lx), the uniformity U0 and a suggested layout on the room plan.

It calculates with real catalogue luminaires: their luminous flux in lumens (lm), and where we hold a manufacturer photometric file, their actual light distribution too. So the result is not an abstract number — you see how many of a specific luminaire you would need, can download a PDF report, or add the whole set straight to the cart.

How the calculation works

The lumen method: how many luminaires

The number of luminaires is calculated with the lumen method:

n = (E × A) / (Φ × UF × MF)

where E is the target illuminance in lux, A the floor area in square metres, Φ the luminous flux of one luminaire in lumens, UF the utilisation factor (how much of the luminaire's light reaches the working plane) and MF the maintenance factor (how much light remains as the luminaire ages and collects dust).

The utilisation factor is not read off a standard table — it is derived from the luminaire's own light distribution together with the ceiling, wall and floor reflectances you enter. That makes the method work for the whole catalogue, not just for the few luminaires that happen to match a published class.

Illuminance map: point-by-point calculation

When a luminaire has a manufacturer photometric file (IES or EULUMDAT), illuminance is calculated point by point: for every point of the working-plane grid, each luminaire's contribution is added according to distance and angle of incidence. Those values give the average illuminance Em, the minimum Emin, the maximum Emax and the uniformity U0 = Emin / Em.

With no photometric file but a known beam angle, the distribution is modelled from that angle. With luminous flux alone, only the quantity is calculated — no illuminance map is drawn, because the distribution is unknown.

Interreflected light

Light arriving directly from the luminaire is added to light that has bounced. The first bounce off the walls and ceiling is resolved per surface patch, so points near a wall receive a different contribution from those in the middle of the room; later bounces are spread uniformly. This is why dark walls or a dark floor visibly worsen the result — change the reflectances in the advanced settings and watch the difference.

Calculation grid and working plane

Illuminance is measured on the working plane rather than the floor — 0.8 m by default, as is usual for workplaces. A 0.5 m band along the walls is excluded, because illuminance is always lower there and would skew the average. The grid spacing follows the size of the room. For corridors, stairs and warehouse aisles the working plane can be set to 0 — there, what matters is the light on the floor.

Where the target illuminance values come from

Every room type comes with a target illuminance. Values for workplaces are based on the illuminance levels published in EN 12464-1 "Light and lighting. Lighting of work places. Part 1: Indoor work places". Homes — living rooms, bedrooms, kitchens, hallways — fall outside that standard, so those values follow ordinary design practice and are marked as such in the table.

The result is not a confirmation of compliance with the standard. Besides illuminance levels, the standard sets requirements this calculator does not assess — glare limitation, colour rendering index, surrounding-area illuminance. Always work from the current edition of the standard.

How much to trust the result

The results are checked against six reference rooms: a square office, a narrow corridor (room index below 1, illuminance measured on the floor), a tall warehouse with narrow-beam luminaires, a room with dark surfaces, a room lit by an asymmetric floodlight, and a room with a suspended luminaire that sends a quarter of its light to the ceiling.

The same rooms with the same photometric files were recalculated in Radiance (Lawrence Berkeley National Laboratory), an independent ray-tracing program. Average illuminance Em and uniformity U0 agree to within 10 % between the two. The check is repeated automatically on every change to the calculator — had they disagreed, it would not have been published.

Practical accuracy depends on the data available: ±5–10 % with a manufacturer photometric file, ±15–25 % from a beam angle, and with luminous flux alone an indicative quantity with no illuminance map. The catalogue currently holds measured photometry for 710 of 960 luminaires, with a further 779 calculated once a bulb is chosen.

Recommended illuminance by room type

The target illuminance values the calculator offers for each room type. Em is the maintained average illuminance on the working plane, U0 the lowest recommended uniformity, UGR the glare rating limit.

Values marked "practice" do not come from the standard: EN 12464-1 applies to work places, not to homes. Every value in the table is a recommendation; the requirements for a specific project are set by its design and by the current edition of the standard.

Lumens, lux and watts — the difference

• A lumen (lm) is how much light the luminaire emits in total. It is printed on the box and it is the number to choose a luminaire by.
• A lux (lx) is how much of that light lands on a surface: 1 lx = 1 lm per square metre. The same 1000 lm luminaire delivers far more lux in a small kitchen than in a warehouse.
• A watt (W) is power drawn, not light produced. Judging light by wattage only worked in the era of incandescent bulbs; LED efficacy ranges from roughly 80 to 150 lm/W, so two 10 W luminaires can differ in light output by nearly a factor of two. Choose by lumens and look at watts only when working out running costs.

A quick rule of thumb: lumens needed ≈ floor area (m²) × target illuminance (lx) ÷ 0.6. Dividing by 0.6 roughly accounts for the light that never reaches the working plane. A 20 m² living room at 150 lx works out at about 5000 lm — four 1200 lm luminaires, say. The calculator does the same thing more precisely: with the real distribution of the luminaire, your room's reflectances and a maintenance factor.

Related pages: Indoor luminaires · Outdoor lighting · LED bulbs · Lighting advice

Frequently asked questions