Lumens to Watts Calculator
Enter a brightness in lumens and pick a bulb type to find the wattage. The calculator works in both directions: switch to reverse mode to go from watts to lumens instead. Efficacy values are mid-range typical figures from the U.S. Department of Energy; use the custom option to enter your bulb's exact lm/W from the packaging.
How lumens and watts are different
Watts measure the electrical power a bulb consumes; lumens measure the brightness it produces. Before modern energy-efficient bulbs, wattage was a reliable shortcut for brightness because all incandescent bulbs had roughly the same efficacy (about 15 lm/W). Today a 10 W LED can match the brightness of a 60 W incandescent, so comparing bulbs by watts alone misleads you. The right unit for shopping decisions is lumens, and the conversion between them always requires one more piece of information: luminous efficacy, the ratio of light output to power input expressed in lumens per watt (lm/W).
The formula
The conversion is a single division: Watts = Lumens / Efficacy (lm/W). Going the other direction: Lumens = Watts x Efficacy. For example, a 10 W LED rated at 90 lm/W produces 10 x 90 = 900 lm. Conversely, if you want 800 lm from a CFL rated at 60 lm/W, you need 800 / 60 = 13.3 W. The efficacy figure to use is printed on the bulb packaging as lumens and watts; divide one by the other to get lm/W, or just read it directly if the spec sheet provides it.
Choosing the right bulb for your room
A good rule of thumb for residential spaces: kitchens and bathrooms need 70-80 lumens per square foot (750-860 lm/m2), living rooms need 10-20 lumens per square foot for ambient light, bedrooms need 10-20 lumens per square foot, and task areas such as desks need 50-75 lumens per square foot. Multiply your room area by those targets to find the total lumens needed, then divide by your bulb output and round up to get the number of bulbs. Pair this calculator with a room-lux or lighting calculator to plan a full installation.
Energy savings when switching bulb types
The biggest real-world use of this calculation is estimating the savings from upgrading bulb types. An 800 lm incandescent needs about 53 W; an 800 lm LED needs about 9 W. Over a year at 5 hours per day: Incandescent uses 53 x 5 x 365 / 1000 = 96.7 kWh; the LED uses 9 x 5 x 365 / 1000 = 16.4 kWh. That is an 80 kWh difference per bulb per year. Multiply by the number of bulbs in your home to see total savings. Use the "Number of bulbs" and "Hours per day" inputs in this calculator to run that comparison for your specific situation.
Bulb type efficacy and wattage equivalents
| Bulb type | Efficacy (lm/W) | Watts for 800 lm | Efficiency tier |
|---|---|---|---|
| Incandescent | 12-18 | 53 W | Very low |
| Halogen | 16-24 | 40 W | Low |
| Mercury vapor | 35-65 | 16 W | Low |
| CFL (compact fluor.) | 45-75 | 13 W | Moderate |
| Fluorescent tube | 40-105 | 11 W | Moderate |
| Metal halide | 75-100 | 9 W | High |
| LED | 80-120 | 9 W | High |
| High-pressure sodium | 85-150 | 7 W | Very high |
| Low-pressure sodium | 100-200 | 5 W | Very high |
Typical efficacy midpoints (U.S. DOE). Equivalent wattages shown for 800 lm (standard "60 W incandescent" replacement output).
Frequently asked questions
Can I convert lumens to watts directly without knowing the bulb type?
No. Lumens measure brightness and watts measure power, so neither can be converted to the other without knowing luminous efficacy (lm/W). The conversion formula is: Watts = Lumens / Efficacy. If you do not have the exact figure, use a typical preset from the bulb-type dropdown as a close estimate.
What is luminous efficacy and where do I find it?
Luminous efficacy is the ratio of light output to electrical input, expressed in lumens per watt (lm/W). Modern LED packaging always lists both lumens and watts; divide the lumen figure by the watt figure to get lm/W. LEDs typically fall in the 80-120 lm/W range, compact fluorescents in the 45-75 lm/W range, and incandescents around 12-18 lm/W.
How many lumens do I need to replace a 60 W incandescent bulb?
A standard 60 W incandescent produces about 800 lumens. When buying an equivalent LED or CFL, look for a bulb labeled 800 lm. A good LED achieves this at roughly 8-10 W, and a CFL at about 13-15 W.
What is the watt equivalent for 1000 lumens from an LED?
Using a typical LED efficacy of 90 lm/W: 1000 lm / 90 lm/W = 11.1 W. Higher-quality LEDs reaching 100-120 lm/W would need as little as 8.3-10 W for the same 1000 lm.
Why do my LED bulbs feel dimmer than the lumen rating suggests?
Several factors can reduce perceived brightness: the direction of light output (LEDs are more directional than incandescents), the color temperature (warmer 2700 K light can appear dimmer than cooler 4000 K light at the same lumen output), shade or diffuser losses, and aging. Lumen measurements are taken under controlled laboratory conditions; real-world output can be 10-20 percent lower after accounting for fixture losses.
How do I calculate energy savings when switching from incandescent to LED?
Find the wattage difference between the two bulbs for the same lumens, then multiply by hours used per year and divide by 1000 to get kWh saved. For example, replacing a 60 W incandescent (800 lm) with a 9 W LED saves 51 W per bulb. At 5 hours per day: 51 x 5 x 365 / 1000 = 93 kWh per year per bulb. Use the energy inputs in this calculator to do this automatically.