Enter a light source's luminous flux and luminous efficacy to estimate its power draw in watts.
How to Use the Lumens to Watts Calculator
Enter the bulb's Luminous Flux rating, then adjust Luminous Efficacy to match its actual technology — try 60 for a CFL or 14 for an incandescent instead of the LED-typical default of 100 to estimate a different bulb's draw.
Every other calculator in this category converts between lumens, candela, and lux using fixed geometry — a beam angle, a distance, an area. This one is different: there is no physical constant linking lumens to watts. The relationship depends entirely on the bulb's technology, which is why Luminous Efficacy is a field set by hand rather than a number baked into the calculation.
watts = lumens ÷ efficacy (lm/W)| Technology | Typical efficacy (lm/W) |
|---|---|
| Incandescent | 14 |
| Halogen | 20 |
| CFL | 60 |
| LED | 100 |
These are illustrative, typical ranges, not a guarantee for any specific product — check the actual bulb's packaging or datasheet for its rated efficacy before relying on the output.
Run 800 lm through each row of that table and the gap gets concrete: about 8 W as an LED, 13 W as a CFL, 40 W as a halogen, and close to 57 W as an incandescent — same light output, four very different electricity bills.
That table is also the real reason LEDs replaced incandescent bulbs. It was never about brightness — a 60 W incandescent bulb and an 8 W LED bulb can both output roughly the same 800 lumens, and that isn't a coincidence or a marketing trick. It's the roughly sevenfold efficacy gap between the two technologies, the same 14 versus 100 lm/W in the table above, doing the same division this calculator just did.