Watts to Lumens Calculator
Turn a wattage into an estimated brightness, and see how many watts each bulb technology needs for the same light.
Light output
Same brightness, other bulb types
| Technology | Efficacy | Watts for 855 lm |
|---|---|---|
| LED | 95 lm/W | 9 W |
| CFL | 60 lm/W | 14.3 W |
| Halogen | 20 lm/W | 42.8 W |
| Incandescent | 14 lm/W | 61.1 W |
Efficacy varies by product; check the lumen figure on the box for an exact value. These presets are typical mid-range numbers, not the record efficacies of premium LEDs.
Watts measure power, lumens measure light
For a century a bulb's wattage doubled as its brightness rating, because nearly every bulb was an incandescent with roughly the same efficiency. That shortcut broke the moment LEDs and CFLs arrived: a modern 9-watt bulb can outshine an old 60-watt one. Watts now describe only what a bulb costs to run.
The number that describes brightness is lumens, and the ratio that links the two is luminous efficacy — lumens per watt. Multiply your wattage by the efficacy of the technology and you get the light output; the equivalent-wattage table then shows the same brightness expressed in each other kind of bulb, which is exactly the comparison you make when swapping an old lamp for an LED.
Frequently Asked Questions
How do you convert watts to lumens?
Multiply the wattage by the bulb's luminous efficacy in lumens per watt: lumens = watts × lm/W. A 9-watt LED at about 95 lm/W emits roughly 855 lumens. Watts alone only tell you power draw; efficacy is what turns that into brightness.
What is luminous efficacy?
Luminous efficacy is how efficiently a source turns electrical power into visible light, measured in lumens per watt. Higher is better. Typical values are around 90–100 lm/W for LEDs, 60 for CFLs, 20 for halogens, and just 14 for old incandescent bulbs — which is why LEDs give the same light for a fraction of the watts.
Why can a 9 W LED replace a 60 W incandescent?
Because they produce similar lumens. A 60 W incandescent at ~14 lm/W makes about 800 lumens; a 9 W LED at ~95 lm/W makes a similar 855 lumens. Same brightness, far less power — the efficacy gap does all the work.
Should I shop by watts or lumens?
Shop by lumens for brightness and use watts to judge running cost. Wattage used to be a proxy for brightness back when almost every bulb was incandescent, but across today's mix of LED, CFL, and halogen the same wattage can mean wildly different light output.
Is the estimate exact?
No — real efficacy depends on the specific product, its colour temperature, and its age. The presets here are representative mid-range figures. For an exact number, read the lumen value printed on the packaging.
Educational estimate. Efficacy varies by product; the lumen figure on the packaging is the definitive value.