Lighting Calculator — How Many Lumens & Bulbs for a Room

Formula

How this is calculated

The Lumen Method (standardised in EN 12464-1 for indoor workplaces and widely adopted for residential design) calculates how much total luminous flux a room needs to achieve a target illuminance at the working plane. The formula is: total lumens = (floor area × target lux) / utilisation factor. The utilisation factor (UF) captures how much of the lamp's light actually reaches the working plane after losses to ceiling, walls, and room geometry — typically 0.5 for domestic rooms with average-coloured walls and a 2.5 m ceiling, up to 0.8 for open-plan spaces with light walls.

Target illuminance (in lux) is the recommended horizontal illuminance for the activity. EN 12464-1 and similar standards suggest roughly 100 lux for corridors and storage, 150–200 lux for bedrooms and living rooms, 300 lux for bathrooms, and 500 lux for kitchens and office desks. These are design values — you may want more light over a task area and less in an ambient zone.

The calculator divides the total lumens needed by the output of your chosen bulb or fixture to give a ceiling-rounded count. It also estimates LED power draw at a nominal 80 lm/W, which is conservative for modern LED products (which range from 70 to 130+ lm/W). For accurate energy estimates, use your bulb's actual wattage. If you plan multiple fixture types or task lighting, calculate each zone separately and add the bulb counts.