Home Improvement
Lighting Lumen Calculator
Enter room size and a brightness target to see total lumens and a rough bulb count.
How bright should a room be?
Watts no longer tell you how bright a lamp is — lumens do. This calculator helps you estimate total lumens for a room from its area and a target lux (lumens per square metre) level, so you can choose LED lamps or downlights that add up to the job. Living rooms often sit lower than kitchens and bathrooms; workshops and reading corners need more. Fixtures, shades and dark wall colours reduce useful light, so treat the result as a planning total rather than a guarantee of sparkle. Layer ambient, task and accent lighting instead of one harsh central wattage. For running cost after you pick wattage, see appliance running cost. Colour temperature (kelvin) and CRI are separate choices from lumen count. Guidance only — not an electrical design or Building Regulations lighting calculation for commercial spaces. Measure existing rooms with a cheap lux meter app if you want a reality check before you rebuy every bulb.
Worked example
A 4 × 3.5 m living room is 14 m². At a modest 150 lux target you want about 14 × 150 = 2,100 lumens total. Three 800-lumen LED lamps give 2,400 lumens — a sensible ambient start before task lamps. A kitchen at 300 lux on the same footprint needs about 4,200 lumens.
Limits and assumptions
Ignores beam angle, mounting height, reflectance and daylight. Not a photometric layout. Dimmers and smart drivers change perceived brightness without changing rated lumens.
Frequently asked questions
What is the difference between lumens and lux?
Lumens measure total light output; lux is lumens per square metre on a surface.
How many lumens do I need per square metre?
It depends on the room — living spaces often need less than kitchens or workshops. Use a target lux for the activity.
Do dark walls change the maths?
Yes — they absorb light. Aim higher total lumens or add task lighting.
Can I convert old watt ratings?
Prefer the lumen figure on the LED pack. Watt-to-lumen rules of thumb for incandescent bulbs mislead on LEDs.
What about outdoor lighting?
Use higher outputs and weather-rated fittings; glare and neighbours matter as much as raw lumens.