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Construction & Homeelectrical

Lighting and Lumens Calculator

Lighting in lumens from room area and use. Watts measure power consumed and lumens measure light produced, and since LEDs the two have no fixed relationship, so buying by wattage no longer works.

Also called: lighting calculator, lux to lumens.

m²
lm/W
Lumens needed
3,000

3,000 lumens for a 20 m² living room at 150 lux. That is 4 bulbs of 800 lm, drawing 30 W at 100 lm/W. Lumens measure light and watts measure power. Since LEDs the two have no fixed relationship, so buying by wattage no longer works.

Target illuminance
150
Bulbs needed
4
Total power
30
Old incandescent equivalent
214.3
Room type
living room
On lumens against watts
Lumens measure light and watts measure power. Since LEDs the two have no fixed relationship, so buying by wattage no longer works.
How this is calculated
Method and background

How this is calculated

Each room type has a target illuminance in lux, which is lumens per square metre, so total lumens is that target times the area. A workshop needs three times a bedroom. The habit worth breaking is buying by wattage: an incandescent bulb produced about 14 lumens per watt and an LED produces 80 to 120, so a 10 W LED replaces a 75 W incandescent. Wattage now tells you the running cost and nothing about brightness.

lumens are total light output and lux is lumens per square metre, so the area sets the requirement
E
Illuminance in lux
A
Area

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a 20 m² living room

Room area
20 m²
Room
Living room
Lumens per bulb
800
Bulb efficacy
100 lm/W

Lumens needed3,000

20 x 150 lux; 3000/800 rounded up

Open this example

a workshop needs far more

Room area
20 m²
Room
Workshop
Lumens per bulb
800
Bulb efficacy
100 lm/W

Lumens needed10,000

boundary: over three times a living room

Open this example

Method and limits

What it assumes

  • General ambient lighting with reasonably light surfaces.

What it deliberately does not model

  • Dark walls and floors absorb light and can need thirty percent more.
  • Task lighting is additional to ambient, not a substitute.
  • Fitting design affects how much light reaches the working plane.

Formula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator

Frequently asked questions

How many watts is a 60 W bulb in LED?
About 8 to 10 W. The relevant figure is lumens: a 60 W incandescent produced roughly 800 lumens, which an LED does at a tenth of the power.
What is the difference between lumens and lux?
Lumens are the total light a source emits. Lux is lumens landing per square metre, so the same bulb gives fewer lux in a bigger room.