Watts to Lumens Calculator: Luminous Efficacy by Lamp Type

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How to use this tool

Pick the technology first, because that sets the efficacy and the efficacy is the whole conversion. Then type a wattage and read the lumens, or hit swap and work backwards from the lumen figure you need. The chart behind the numbers plots every technology at once, so you can see how far apart they sit before you commit to one.

The field people leave alone when they should not is the efficacy box. The preset is a sensible middle of the range value, not a specification. Two LED lamps both sold as LED can sit at 80 and 140 lumens per watt, a difference of nearly two to one, and the only way to know which you have is the data sheet or the lighting facts label on the box. If you have that number, type it in. The preset is for when you do not.

What the calculation is doing

Lumens equals watts multiplied by luminous efficacy. That is the entire arithmetic. Everything interesting about this conversion lives in the efficacy figure, which is not a constant of nature but a property of how a particular lamp turns electrical power into visible light.

There is a hard ceiling on it. A source emitting only at 555 nanometres, the wavelength the human eye responds to most strongly, would produce 683 lumens per watt. Nothing can beat that without changing the definition of the lumen, because the lumen is defined from that response curve. A source that emits usable white light has to spread its output across the visible band, which drags the theoretical maximum down to somewhere around 250 to 370 lumens per watt depending on how you define acceptable colour. Commercial LEDs at 140 lumens per watt are therefore running at roughly half of what white light physically allows, and an incandescent at 14 is running at about four percent.

That four percent figure is not a manufacturing failure. A tungsten filament makes light by being hot, and a hot object radiates mostly in the infrared. The visible output is a thin slice off the top of a distribution that is overwhelmingly heat. An incandescent bulb is an electric heater that happens to glow, and treating it as a lighting product was always a compromise the technology could not escape.

Why there is no single incandescent lumens per watt figure

The equivalence ladder everyone quotes comes from the US Federal Trade Commission lighting facts rules and the Energy Star programme. It says 40 watts is 450 lumens, 60 is 800, 75 is 1100, 100 is 1600 and 150 is 2600. Those are the numbers printed on replacement LED packaging, and they are why a box says sixty watt equivalent.

Divide each pair and the implied efficacy is not constant. It runs 11.25, 13.33, 14.67, 16.00 and 17.33 lumens per watt as you climb the ladder. A 150 watt incandescent is more than fifty percent more efficient than a 40 watt one, from identical technology.

The reason is filament temperature. A bigger bulb runs a thicker filament hotter, and the fraction of blackbody radiation that lands in the visible band climbs steeply with temperature. It is the same physics that makes a hotter star bluer. Small bulbs cannot run as hot without the filament evaporating too fast to give a usable life, so they sit lower on the curve and waste proportionally more of their input as heat.

This tool carries the FTC ladder separately from the efficacy field and interpolates between the rungs, so the incandescent equivalent wattage it quotes follows the real curve rather than a single averaged figure. If you use one flat number for incandescent you will be wrong by up to a fifth at the ends of the range.

A worked check you can repeat on paper

Take the default. A 60 watt LED at 95 lumens per watt gives 60 times 95, which is 5700 lumens. That is a floodlight, not a bedside lamp, and it is worth pausing on because it shows how badly the old wattage habit misleads. Sixty watts of LED is an enormous amount of light.

Work the useful direction instead. You want to replace a 60 watt incandescent, which the ladder says is 800 lumens. At 95 lumens per watt the LED needs 800 divided by 95, which is 8.42 watts. That is why every nine watt bulb on the shelf is sold as a sixty watt replacement. Do the same for the 100 watt rung and you get 1600 divided by 95, which is 16.84 watts, and the shelf offers you a seventeen or eighteen watt lamp.

Now the money. The difference between 60 watts and 8.42 watts is 51.58 watts. Run it eight hours a day for a year and that is 51.58 divided by 1000, times 8, times 365, which is 150.6 kilowatt hours. At fifteen cents a unit the saving is 22.59 a year from one bulb. The lamp itself costs a few dollars, so it pays back in a couple of months and everything after that is profit.

Mistakes that catch people out

Buying on watts. Wattage tells you what the lamp costs to run, not how much light it makes. Two LED lamps at the same wattage can differ by seventy percent in output. The lumen figure is the one that matters and it is printed on every box by law in most markets.

Assuming all the lumens reach the room. The figure on the box is bare lamp output. Put it behind a shade, inside a recessed can or under a diffuser and you lose anywhere from ten to forty percent. Fixture manufacturers quote a separate delivered lumen figure for exactly this reason.

Comparing efficacy at different colour temperatures. A 6500 K lamp will always beat a 2700 K one on lumens per watt from the same chip, because the eye response curve favours the cooler spectrum. That is not a better lamp, it is a different colour, and comparing them on efficacy alone rewards the one most people find unpleasant indoors.

Ignoring lumen depreciation. LED output falls over life. The L70 rating tells you the hours until output drops to seventy percent of new. Design a space to exactly the target lux on day one and it will be under target for most of its service life.

Using the equivalence ladder outside its range. It is defined from 40 to 150 watts. Below and above that the tool extrapolates on the end slope, which is reasonable but not standardised, and a 300 watt incandescent equivalence figure is an estimate rather than a published value.

How this was checked

The engine was written twice and the two versions compared on the same cases. The FTC ladder reproduces its published rungs exactly, 40 watts to 450 lumens and 150 watts to 2600 lumens, and interpolates linearly between them, so 50 watts returns 625 lumens. Round trips through every technology at 0.5, 5, 60, 1000 and 25000 return to the starting value within one part in a billion.

The equivalence results were checked against what the market actually sells. At 95 lumens per watt the calculation says a 60 watt replacement needs 8.42 watts and a 100 watt replacement needs 16.84 watts, which matches the nine watt and seventeen watt lamps on the shelf. The chart is drawn from the same efficacy table the readout uses, so the lines cannot disagree with the numbers.

Popular conversions

Each link loads the tool with that figure already in place. The wattage links assume an LED at 95 lumens per watt unless the technology is named, and you can change it once the page opens.

Watts to lumens for LED. 1 W to lumens · 2 W to lumens · 3 W to lumens · 4 W to lumens · 5 W to lumens · 6 W to lumens · 7 W to lumens · 8 W to lumens · 9 W to lumens · 10 W to lumens · 11 W to lumens · 12 W to lumens · 13 W to lumens · 14 W to lumens · 15 W to lumens · 18 W to lumens · 20 W to lumens · 23 W to lumens · 25 W to lumens · 30 W to lumens · 32 W to lumens · 36 W to lumens · 40 W to lumens · 45 W to lumens · 50 W to lumens · 60 W to lumens · 65 W to lumens · 70 W to lumens · 75 W to lumens · 80 W to lumens · 100 W to lumens · 120 W to lumens · 125 W to lumens · 150 W to lumens · 175 W to lumens · 200 W to lumens · 250 W to lumens · 300 W to lumens · 400 W to lumens · 500 W to lumens · 600 W to lumens · 750 W to lumens · 1000 W to lumens

Lumens to watts for LED. 100 lumens to watts · 200 lumens to watts · 250 lumens to watts · 300 lumens to watts · 350 lumens to watts · 400 lumens to watts · 450 lumens to watts · 500 lumens to watts · 600 lumens to watts · 700 lumens to watts · 800 lumens to watts · 900 lumens to watts · 1000 lumens to watts · 1100 lumens to watts · 1200 lumens to watts · 1300 lumens to watts · 1500 lumens to watts · 1600 lumens to watts · 1800 lumens to watts · 2000 lumens to watts · 2200 lumens to watts · 2500 lumens to watts · 2600 lumens to watts · 3000 lumens to watts · 3500 lumens to watts · 4000 lumens to watts · 4500 lumens to watts · 5000 lumens to watts · 6000 lumens to watts · 7000 lumens to watts · 8000 lumens to watts · 10000 lumens to watts · 12000 lumens to watts · 15000 lumens to watts · 20000 lumens to watts

Watts to lumens for incandescent. 15 W to lumens · 25 W to lumens · 40 W to lumens · 60 W to lumens · 75 W to lumens · 100 W to lumens · 150 W to lumens · 200 W to lumens · 300 W to lumens

Watts to lumens for halogen. 10 W to lumens · 20 W to lumens · 25 W to lumens · 35 W to lumens · 50 W to lumens · 75 W to lumens · 100 W to lumens · 150 W to lumens · 300 W to lumens · 500 W to lumens

Watts to lumens for CFL. 5 W to lumens · 7 W to lumens · 9 W to lumens · 11 W to lumens · 13 W to lumens · 15 W to lumens · 18 W to lumens · 20 W to lumens · 23 W to lumens · 26 W to lumens · 32 W to lumens · 42 W to lumens

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