Calculators
Room lighting lumen calculator
Lumens are the only lighting number worth shopping by, and the formula everyone quotes is missing a term.
On this page (3 sections)
Short answer
Multiply floor area by the target illuminance for the room's use: about 15 foot-candles in a living room, 40 in a kitchen, 45 at a desk. Then plan for 1.3 to 2 times that figure in installed lumens, because not every lumen reaches the surface.
The numbers, at a glance
- Living room
- 10 to 20 fc (110 to 215 lux)
- Kitchen, general
- 30 to 40 fc (320 to 430 lux)
- Home office
- 40 to 50 fc (430 to 540 lux)
- Ambient share of total
- About 55%
Room lighting calculator (lumens)#
Converts room size and use into a total lumen target, then splits it across the layers a room actually needs: ambient, task and accent.
Nothing you type leaves your browser: the maths runs on this page, and no values are stored or sent anywhere.
What the calculator is actually doing#
It runs the lumen method in its simplified consumer form:
Target lumens = floor area (sq ft) x target illuminance (foot-candles)
then applies two multipliers that most online calculators leave out: 1.15 for a ceiling above 9 ft and 1.3 above 10 ft, and 1.25 for dark walls and floors or 0.92 for light ones. Finally it splits the result into layers, because a room needs its light distributed as much as it needs it totalled.
The term the simple formula is missing#
The full engineering version is:
Lumens = (illuminance x area) / (coefficient of utilisation x light loss factor)
The coefficient of utilisation is the fraction of a fixture's output that reaches the working plane after the shade, walls and ceiling have taken their share. In a domestic room with mid-tone finishes it is often 0.4 to 0.6. The light loss factor accounts for dirt and lumen depreciation over the lamp's life, typically 0.8 to 0.9.
Multiply those together and you get 0.35 to 0.5, meaning the installed lumens should be roughly two to three times the naive figure in an unfavourable room. This site does not bury that: the calculator gives you the simple target and this page tells you to treat it as a floor, then to put the light on dimmers so the extra capacity is available rather than mandatory.
Turning the number into fixtures#
- Divide by 800 lm for a rough count of standard lamps, or by 400 to 500 lm for typical recessed downlights.
- Never put more than about 60% of the total on one switch.
- Get the ambient layer from at least two positions, ideally at different heights.
- Deliver task lumens close to the task: a 400 lm under-cabinet strip 18 in above a worktop outperforms 1,500 lm on an 8 ft ceiling.
- Specify CRI 90 or better. See colour temperature and CRI.
Frequently asked questions#
How many lumens for a 12 x 12 ft room?
144 sq ft at 15 foot-candles is about 2,200 lm for a living room, about 5,800 lm for a kitchen at 40 fc, and about 6,500 lm for a home office at 45 fc. Add 15 to 30% for a ceiling above 9 ft or dark walls, and split the total across at least three sources.
Is one ceiling light enough for a room?
It can deliver the lumens and it will still light the room badly. A single overhead source produces one flat layer, hard shadows under the eyes and no modelling. The same lumen total split between a ceiling fixture, a table lamp and a floor lamp reads as a different room.
What is the difference between lumens and watts?
Lumens measure light output, watts measure electricity consumed. A modern LED produces 80 to 120 lumens per watt, so an 800 lm lamp draws roughly 7 to 10 W. Watt-equivalence labels on packaging are marketing shorthand for the old incandescent scale: 800 lm is the traditional 60 W bulb.
Why does my room feel dark even though I have enough lumens?
Usually one of three things: the light is all coming from one height, the walls and floor are absorbing more than the calculation assumed, or the colour rendering is poor enough that materials look grey. Dark matt surfaces can absorb 80 to 90% of what falls on them, which is why the calculator adds 25% for a dark room.
Sources and standards referenced#
- The IES Lighting Handbook, 10th edition Illuminating Engineering SocietyIlluminance recommendations by task and space
- IES RP-11 Lighting for Interior and Exterior Residential Environments Illuminating Engineering Society
- Lighting design basics Trade practiceCoefficient of utilisation and light loss factor as applied in room-cavity calculations
We cite published codes, industry planning guidelines and manufacturer specifications. Where a figure is our own recommendation rather than a published standard, the text says so. See how we measure.