Lumens, lux & footcandles
Room lighting calculator
How much light does a room need? Enter its size and what you use it for to get the lumens, and how many bulbs of a given brightness add up to that.
1,200–2,400 lumens
about 2 to 3 bulbs of 800 lm (60 W replacement)
- Floor area
- 12 ft (3.66 m) × 10 ft (3.05 m) = 120 ft² (11.15 m²)
- Target
- 10–20 footcandles (108–215 lux), living room, bedroom or relaxing area
- Sum
- 120 ft² × 10–20 fc = 1,200–2,400 lumens
The lower count reaches the bottom of the range; the higher count reaches the top. Lamps on dimmers let you use both ends.
How the lumens are worked out
A footcandle is one lumen spread over a square foot, and a lux is one lumen over a square metre. So the light a room needs is its floor area times the target level: a 12 × 10 ft room is 120 ft², and at 10 to 20 footcandles that is 1,200 to 2,400 lumens. The bulb count rounds up.
This simple sum assumes all of the bulbs' light reaches the room. Lampshades, fittings, dark walls, dust and older bulbs all lose some, so if your lamps are shaded or the walls are dark, add your own allowance or choose the top of the range. Daylight, pale walls and bare bulbs help the other way.
Spread the lumens between several lights rather than one bright ceiling fitting: a ceiling light for general light, plus lamps where you read, cook or work.
Sources
- Target levels: the Illuminating Engineering Society (IES) residential recommendations in footcandles, as published in Penn State's course EGEE 102, Lesson 8.3: How Much Lighting is Needed, which also lists fixture efficiency, lumen depreciation, dirt, surface reflectance and daylight as factors in how many lamps a room needs.
- Bulb brightness: ENERGY STAR, Learn About Brightness: minimum lumens to replace a 40, 60, 75, 100 or 150 W incandescent bulb (450, 800, 1,100, 1,600 and 2,600 lm).
- 1 footcandle = 1 lm/ft² = 10.764 lux, from the foot of exactly 0.3048 m: NIST Special Publication 811, Appendix B.8.
Questions
Why is it a range?
The IES gives a range for each kind of room. Penn State notes that the light needed also depends on the task and on the age of the people using the room, so aim higher for close work and older eyes. Dimmable lights let you cover both ends.
Does a studio flat need one figure?
Work out each zone on its own: the kitchen corner at kitchen levels, the sofa and bed area at living levels, then add them up.
What if I know the lux I want?
Choose "My own target" and enter it in lux or footcandles. The calculator converts between the two.