Room Lighting Calculator: Lumens, Lux & Kelvin

Calculate required room lumens, lux standards, bulb wattage, and ideal color temperature (Kelvin) for any living space.

How many lumens to light a room: lux, watts and kelvin

One lux is one lumen per square metre: the tool works out the required light output as floor area × target illuminance, then adjusts it for wall colour and ceiling height. A 20 m² living room aimed at 200 lux therefore needs 4,000 lumens, about 44 W of LED at 90 lm/W.

The calculation

  • Target illuminance by room: hallway 100 lux, bedroom 150, living room 200, kitchen 350, bathroom 400, home office 500.
  • Total light output (lm) = area (m²) × lux × wall factor (light 1, medium 1.15, dark 1.35) × height factor (1 up to 2.5 m, then +20% per metre above).
  • Power (W) = output ÷ luminous efficacy: LED 90 lm/W, compact fluorescent 55, halogen 15. The incandescent equivalent uses 12 lm/W.
  • Suggested colour temperature: 2,400-2,700 K in bedrooms, 2,700-3,000 K in living rooms, 3,500-4,000 K in kitchens and bathrooms, 4,000-5,000 K in offices.

Worked examples

20 m² living room, 2.5 m ceiling, light walls, 4 LED spots: 20 × 200 = 4,000 lm, 1,000 lm per spot, 44 W in total (11 W per spot), equal to 333 W of incandescent bulbs. 12 m² office, 2.8 m ceiling, medium walls: 12 × 500 × 1.15 × 1.06 = 7,314 lm, or 81 W of LED or 488 W of halogen.

Assumptions and limits

  • The tool assumes all the lamp output reaches the working plane. A lighting designer also divides by a utilisation factor (fittings, distance) and a maintenance factor, often 0.5 to 0.7 combined, so allow a margin or use dimmable fittings.
  • The lux values are comfort guidelines. EN 12464-1 (2021 edition) covers workplaces and sets 500 lux for writing, reading and screen work; it says nothing about homes.
  • The 90, 55 and 15 lm/W figures are averages; check the lumen rating on the bulb's packaging.

Sources and dates: in France, the Labour Code, art. R4223-4 (in force since 01/05/2008) sets much lower minimums: 40 lux for circulation areas, 120 lux for workrooms, 200 lux for windowless rooms in permanent use. EN 12464-1:2021 cited without a link (paid standard, not consulted online) (checked 14/10/2026).

Frequently Asked Questions

How many lumens does a 20 m² room need?

Multiply the area by the target illuminance: 20 m² × 200 lux = 4,000 lumens for a living room, 20 × 150 = 3,000 lumens for a bedroom. With dark walls the tool adds 35%.

What is the difference between lumens and lux?

Lumens measure the light a lamp gives out, lux the light falling on a surface: 1 lux = 1 lumen per m². A 1,000-lumen bulb spread over 5 m² gives at most 200 lux.

How many LED watts replace a 60 W bulb?

A 60 W incandescent bulb gives roughly 800 lumens. With an LED at 90 lm/W, as in the tool, about 9 W is enough (800 ÷ 90); go by the lumen rating on the pack rather than the wattage.

Which colour temperature suits a home office?

The tool suggests 4,000 to 5,000 K, a neutral to cool white that helps concentration, and 500 lux on the desk. For a bedroom it recommends a warmer 2,400 to 2,700 K.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.