A lumen is a unit of measurement indicating the visible light output of a light source. In practical terms, lumens tell you how bright a light bulb is. Light bulbs and some lighting fixtures carry a specific lumen value. The higher the lumens, the brighter the bulb. Lighting designers often use lumens per square foot to ensure a home has sufficient lighting. The basic calculation is very simple:
- Lumens per square foot = total lumens divided by the total square footage of the area.
You can use this calculation to determine how much light per square foot you currently have in a room or to find out how much light you need (or should have) in a given area.
Measuring Square Footage
Square footage is the same thing as wall-to-wall floor space. To measure the square footage of a room, measure the width and the length of the room, then multiply the numbers. For example, a 10-foot-wide, 12-foot-long room is 120 square feet (10 x 12 = 120).
If you're curious to know how much light a room is getting, simply add up the lumens of all the light bulbs in the room, and then divide by the room's square footage to find the lumens per square foot. For example, for a 120-square-foot room with a single overhead fixture with two light bulbs, each producing 800 lumens:
- 800 + 800 = 1,600 lumens
- 1,600/120 = 13.3 lumens per square foot.
Finding the lumens is easy. Most energy-efficient (LED and fluorescent) light bulbs have their lumens value printed right on the bulb base. Incandescent bulbs list the lumens on the bulb packaging; if you can't find that, look for the wattage rating on the bulb's base and convert that to lumens using a wattage–lumens chart.
Recommended Lumens By Room
Lighting experts recommend the following levels of light for various rooms in a home. These guidelines relate to general room lighting, such as that provided by standard ceiling fixtures or multiple lamps. In areas intended for work or reading, you would likely include additional, focused light (called task lighting) to brighten specific locations where light is crucial, such as kitchen countertops.
- Living room: 10–20 lumens per square foot
- Dining room: 30–40 lumens per square foot
- Bedroom: 10–20 lumens per square foot
- Bathroom: 70–80 lumens per square foot
- Hallways: 5–10 lumens per square foot
- Kitchen (general lighting): 30–40 lumens per square foot
- Kitchen (task areas): 70–80 lumens per square foot
- Laundry room: 70–80 lumens per square foot.
Watts are no longer used as the standard "measure" of light bulb brightness, because watts actually indicate electricity usage, not light. In the old days, when most light bulbs were incandescent, a "100-watt" bulb indicated a certain level of brightness. But with an energy-efficient LED bulb, for example, you can get the same brightness with only 14 to 16 watts of electricity.
Philip Schmidt is author of Install Your Own Solar Panels, The Complete Guide to Treehouses, and 18 other home-related how-to books. A former carpenter, he has been a full-time writer and editor for over two decades, teaching DIYers about houses and everything we do with them.