Calculator
Room lighting calculator
If a room still feels dim with the lights on, count the lumens. Enter the room type and size to get a range of total lumens, a number to aim for, a color temperature and how many sources to spread it across. It updates as you type and nothing leaves your device.
2,500 lumens to aim for (range 1,700 to 3,350)
Worked example (shown when scripts are off): a 14 by 12 ft living room is 168 square feet. At 10 to 20 lumens per square foot the range is about 1,700 to 3,350 lumens. With mixed surfaces we aim for the middle, about 2,500, spread over at least four sources, which is about four 800 lumen bulbs.
A starting point
Published levels differ a little from one source to the next. Treat the range as a guide and use dimmers or lamps you can switch separately to tune the room.
How the calculator works
A lumen is a measure of the light a bulb gives off, printed on the bulb box. One footcandle is one lumen spread over one square foot, so the calculation is area times lumens per square foot. The ranges used are 10 to 20 for living rooms, bedrooms and dining rooms, 30 to 40 for general kitchen light, 30 to 50 for an office or reading room, and 5 to 20 for halls and entries. Metric sizes are converted to square feet first.
The aim sits inside the range. With mostly light surfaces we use a point a quarter of the way up the range, with mixed surfaces halfway, and with mostly dark surfaces about 80 percent of the way up, since dark finishes absorb light. A room that is dim even by day moves the point up a further tenth of the range. Those adjustments are our own assumptions, and the result is never outside the published range.
Worked example
A 12 by 10 ft kitchen is 120 square feet. At 30 to 40 lumens per square foot, general light should be 3,600 to 4,800 lumens. That is for the room light only. Counter and stove task light is on top of it, because work surfaces need more than the room as a whole. A reader who adds up the bulbs in a ceiling fixture and finds 1,800 lumens knows at once why the kitchen feels dim.
Spread it over layers
The calculator suggests at least three sources even for a small total. One light in the middle of the ceiling lights the room evenly but flatly. Adding a lamp at eye level or below and a task or wall light gives the room depth, and it matches the layered approach in the lighting guide. If the room still feels gloomy after the count, the dark room guide covers shades, bulb color and wall color.
For color temperature, warm white at about 2700K to 3000K suits living areas and bedrooms, 3000K to 3500K is a common choice for kitchens, and 3500K to 4000K suits reading and work. Pick one range for the whole room, since mixed bulbs look patchy.
Limits and safety
- This counts total lumens. It does not model glare, shades, ceiling height or how light falls on a specific task.
- Check each fixture label for the maximum wattage and whether it takes enclosed or dimmable bulbs. Do not exceed it.
- Turn off the power at the switch and let bulbs cool before changing them. A licensed electrician should do any wiring, new fixture or dimmer installation.
- This is general information, not a lighting design or professional advice.
Where this comes from
The ranges are the residential levels from the Illuminating Engineering Society as summarized in lighting guides, with the equivalence of one footcandle to one lumen per square foot. We read these summaries and did not read the original standard, so treat the figures as secondary. The size-based total of 1,500 to 3,000 lumens for many living rooms in the small-room guide on this site fits the same range. The surface and daylight adjustments, the minimum of three sources and the 800 lumen bulb example are our own simplifications.