Room Lighting Calculator (lux → lumens, number of bulbs)

Understanding the room lighting calculator — concept and principle

This calculator estimates how much light a room needs. You enter the floor area, the illuminance you want on the working surface (in lux) and the light output of one bulb (in lumens), and it returns the total lumens to install, the number of bulbs and a rough LED wattage. It is useful when planning a renovation or checking why a room feels dim.

A 15 m² kitchen at 300 lx needs 7,500 lm installed — ten 800 lm bulbs.

How it is calculated

Total lumens = A × E ÷ (UF × MF), where A is the floor area in m², E is the target illuminance in lux and UF × MF is the combined utilisation and maintenance factor.
Bulbs = total lumens ÷ lumens per bulb, always rounded up to a whole bulb.
LED watts ≈ total lumens ÷ 100, assuming a typical LED efficacy of about 100 lm/W.

Lux is lumens per square metre (1 lx = 1 lm/m²), so area × lux gives the lumens that must actually land on the floor or desk.

The principle

This is the lumen method used in lighting design. Not every lumen a lamp emits reaches the working plane: some is absorbed by the fitting, the walls and the ceiling, or falls outside the useful area. The utilisation factor (UF) expresses the fraction that arrives, and it is higher in rooms with light-coloured surfaces and wide, low proportions. The maintenance factor (MF) allows for dirt on fittings and the gradual loss of output as lamps age. Dividing by UF × MF scales the delivered lumens back up to the lumens you must install. A combined value of 0.5–0.7 is a common starting point for homes with open fittings.

Worked example

Inputs: area 15 m², target 300 lx (kitchen/office), 800 lm per bulb, UF × MF = 0.6.
Lumens needed on the surface: 15 × 300 = 4,500 lm.
Lumens to install: 4,500 ÷ 0.6 = 7,500 lm.
Bulbs: 7,500 ÷ 800 = 9.375, rounded up to 10 bulbs.
LED power: 7,500 ÷ 100 = 75 W in total.

Each point is the calculator re-run with only “Utilisation × maintenance factor” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Utilisation × maintenance factor	Total lumens
0.15	30,000
0.3	15,000
0.45	10,000
0.6	7,500
0.75	6,000
0.9	5,000
1.2	3,750

How to read the result

The result is an average across the room, not the value directly under each lamp. Typical targets are around 100 lx for corridors, 150 lx for living rooms and bedrooms, 300 lx for kitchens and general office work and 500 lx for desks and detailed tasks. Round the bulb count to a number that fits an even layout. Dark walls, high ceilings or deep shades call for a lower factor and therefore more lumens.

Common mistakes

Confusing watts with light output; compare lamps by lumens, because efficacy varies widely between technologies.
Using the lamp's lumens when a fitting with a diffuser or shade absorbs part of it.
Forgetting that task lighting can supplement a lower general level instead of lighting the whole room to 500 lx.

Keep in mind: The lumen method gives an average illuminance and ignores glare, uniformity and colour quality. For workplaces or complex rooms, a professional lighting calculation based on photometric data and local standards is needed.

Frequently asked questions

How many lumens does a 60 W incandescent bulb equal?

Roughly 800 lumens, which is why 800 lm LED lamps are often sold as "60 W equivalent".

Is more lux always better?

No. Too much light wastes energy and can cause glare; aim for the level the task needs and add dimming where the use of the room changes.

Sources: EN 12464-1, Light and lighting — Lighting of work places — Part 1: Indoor work places · CIBSE / Society of Light and Lighting, SLL Code for Lighting

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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