How to use the room lighting (lumens) calculator
- Pick feet or meters and enter the room length and width.
- Press a room button such as Kitchen or Bedroom to fill in an example for lumens per square foot, or type your own.
- Enter the number of bulbs or fixtures and the efficiency in lumens per watt printed on the bulb box.
- Read the total lumens, lumens per bulb and the watts needed, and copy or print the result.
Formula
Area = length x width (square meters x 10.7639 = square feet)
Total lumens = area in square feet x lumens per square foot
Lumens per bulb = total lumens / bulbs, and watts = total lumens / (lumens per watt)
One lumen per square foot is one foot-candle. The lumens per square foot you choose is the main input and is yours to set.
A 14 by 12 foot room
The area is 14 x 12 = 168 square feet. At 20 lumens per square foot you need 168 x 20 = 3,360 lumens in total. Spread over 4 bulbs, that is 840 lumens each.
At 90 lumens per watt, 3,360 lumens needs 3,360 / 90 = 37.3 watts in total, or 9.3 watts per bulb. A 4 by 3 meter room is 129.2 square feet and needs 2,584 lumens at the same 20 lumens per square foot.
What a lumen is
A lumen measures how much visible light a bulb produces. Watts measure how much electricity it uses, which was a good guide to brightness when all bulbs were the same type. With LEDs, the lumens on the box tell you the brightness, and the watts tell you the energy use. Divide lumens by watts to get efficiency, which is the number you enter. Compare this value on the bulb packages you are considering.
The amount of light a room needs is a choice, not a fixed number. Rooms for reading, cooking or detailed work usually want more light than a hallway or a place for relaxing. The buttons for each room fill in an example number, and you can type a different one. They are starting points and not a rule.
Things the formula leaves out
Ceiling height, dark walls and floors, large windows, shades, the shape of the fixtures and how much light reaches the work surface all change what you need. A dimmer lets you set the level after the bulbs are in, so planning for a little more light and dimming it is often more flexible than planning for less. Layers of light, such as ceiling lights plus lamps, also make a room feel better than one bright source.
To find the room area first, use the square footage calculator. To see what the lights cost to run, try the electricity cost calculator. For other room planning, see the rug size calculator and the TV size and viewing distance calculator.
Choosing bulbs
Check the box for the lumens, the color temperature and whether the bulb works with your dimmer. Warmer colors suit living spaces and cooler colors are often chosen for work areas. The tool does not choose bulbs for you. It gives a total to match against the lumens printed on the packages.
Assumptions and limits
- The lumens per square foot you enter, or fill in with a room button, are example values and not a required level.
- The room is treated as one rectangle, with lighting spread evenly.
- Bulb efficiency is the value you enter. The 90 lumens per watt in the form is an example for LED bulbs.
- Ceiling height, finishes, windows and fixture shape are not modeled.
Frequently asked questions
How many lumens do I need for a room?
Multiply the room area in square feet by the lumens per square foot you want. A 168 square foot room at 20 lumens per square foot needs 3,360 lumens. The number per square foot is your choice and depends on how the room is used.
How many lumens is a 60 watt bulb?
Lumens depend on the type of bulb, not only on watts. Read the lumens on the package. An LED bulb that gives the same light as an old 60 watt bulb uses far fewer watts, which you can see by dividing lumens by watts.
What is a foot-candle?
A foot-candle is one lumen of light spread over one square foot. That is why lumens per square foot and foot-candles are the same number.
Should I use fewer brighter bulbs or more dimmer ones?
More bulbs spread the light more evenly and avoid glare. The calculator shows lumens per bulb so you can compare a few large bulbs with many small ones.