Electrical & Lighting · Conversion · Free

Lumens to Watts Calculator

Convert luminous flux in lumens (lm) to electric power in watts (W). Enter the luminous flux, pick a light source to fill a typical luminous efficacy — or type your own lm/W value — and press Calculate to get the power in watts. Everything runs locally in your browser.

STEP 1

Enter luminous flux

Type the luminous flux of the lamp in lumens (lm). A 100 W-equivalent LED bulb puts out around 1500 lm, a bright 150 W-equivalent around 2250 lm.

STEP 2

Set the luminous efficacy

Pick a light source — tungsten incandescent, halogen, fluorescent, LED, metal halide, high/low pressure sodium or mercury vapor — to fill a typical efficacy in lumens per watt (lm/W), or type your own value for more accuracy.

STEP 3

Press Calculate, read watts

Power appears in watts (W). Copy the value to your clipboard, try another lamp type to compare efficacies, or press Reset to start over.

How lumens become watts

What is the lumens to watts formula?: The power P in watts (W) is equal to the luminous flux ΦV in lumens (lm), divided by the luminous efficacy η in lumens per watt (lm/W): P(W) = ΦV(lm) / η(lm/W). For example, 1500 lm from an LED lamp at η = 60 lm/W needs 25 W.
What luminous efficacy does each light source use?: This calculator fills a typical / average efficacy per lamp type: tungsten incandescent 15 lm/W, halogen 20, fluorescent and LED 60, metal halide 90, high pressure sodium 100, low pressure sodium 150, mercury vapor 50 lm/W. Real lamps vary by model — type your own lm/W for an exact figure.
How many watts is a 900 lumen incandescent bulb?: At the typical tungsten efficacy of 15 lm/W, 900 lm corresponds to 60 W — about the old 60 W incandescent class. The same 900 lm from an LED at 60 lm/W would draw only 15 W.
Can I use my own lumens-per-watt value?: Yes — the lm/W field is editable. With a typed η of 8 lm/W, 1000 lm works out to 125 W (1000 / 8). A value you type is used until you pick another light source from the list. Example: 2250 lm at the fluorescent preset of 60 lm/W is 37.5 W.
Why do lumens and watts not convert with one fixed number?: A watt measures electricity consumed; a lumen measures visible light produced. The bridge between them is luminous efficacy, and it differs hugely by technology — roughly 15 lm/W for tungsten versus 60+ lm/W for LED. Energy-saving lamps have high luminous efficacy: more lumens per watt, so fewer watts for the same light.

Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Copy buttons use your clipboard only. The per-lamp efficacies above are typical / average values for orientation; fixture-specific datasheets take precedence for engineering use.