How this calculator works
Lighting design starts from how much light the task needs and how big the surface is. Illuminance, measured in lux, is lumens per square metre, so multiplying the target by the floor area gives the lumens the room needs. A fixture does not deliver all of its light to the work surface: some is lost in the fitting and some is absorbed by walls and ceilings. The coefficient of utilization is that fraction, typically 0.5 to 0.9 in a home. Dividing the required lumens by the lumens each fixture delivers after that loss gives the number of fixtures, rounded up. The targets come from the Illuminating Engineering Society residential guidelines, where the values are given in foot-candles; one foot-candle is about 10.76 lux.
Formula
required lumens = target illuminance in lux x area in m2. fixtures = ceiling(required lumens / (lumens per fixture x coefficient of utilization)). power = fixtures x lumens per fixture / efficacy.
Worked example
A 12 square metre kitchen at the 5 foot-candle general target of about 54 lux needs 54 x 12 = 646 lumens. With 800 lumen fixtures and a utilization of 0.7, each fixture delivers 560 lumens, so two fixtures are enough. At 100 lumens per watt those two fixtures draw 16 W.
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Frequently asked questions
Why are the kitchen counter targets so much higher than the general ones?
The general target is for moving around the room, while the counter target is measured at the work surface for chopping and reading. Task lighting, such as under-cabinet strips, is the usual way to reach it without making the whole room glaringly bright.
What is a coefficient of utilization in practice?
It combines the fixture efficiency and how much of the light bounces off the room surfaces onto the work surface. A closed fixture in a dark room might deliver less than half of its nominal lumens; an open fixture in a white room can deliver most of them.
Do these figures account for the age of the occupants?
No. The IES values used here are for people aged 25 to 65. Older eyes generally need more light, often double the figure for detailed tasks, and the guideline gives higher recommendations for those age groups.
Should I add a margin?
Yes, for maintenance. Lamps dim as they age and fittings collect dust, so a design that just meets the target when new will fall below it later. Adding one extra fixture or choosing higher-output ones is common practice.