Watts to Lumens Calculator
Convert electric power in watts (W) to luminous flux in lumens (lm). Enter the power in watts, pick a light source to auto-fill its typical luminous efficacy — or type your own in lm/W — and press Calculate. The lumens result comes from Φv(lm) = P(W) × η(lm/W), computed entirely in your browser.
Enter the power in watts
Type the real power P the lamp consumes, in watts (W) — the number printed on the bulb or its datasheet. Zero and any positive value are accepted; 0 W simply gives 0 lm.
Pick a source or type the efficacy
Choosing a light source (tungsten, halogen, fluorescent, LED, metal halide, sodium or mercury vapor) fills in its typical luminous efficacy in lm/W. Or ignore the list and type your own η — the last value in the lm/W box is what gets used.
Press Calculate, read lumens
The luminous flux Φv appears in lumens (lm) as P × η. Copy the result to your clipboard, or press Reset to clear the watts, the light source, the efficacy and the result.
Watts to lumens calculation
- Watts to lumens formula
- The luminous flux Φv in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W): Φv(lm) = P(W) × η(lm/W). For example, a 60 W LED lamp at a typical 90 lm/W gives 60 × 90 = 5400 lm.
- What efficacy values does each light source use?
- The presets are typical / average values: tungsten incandescent 15 lm/W, halogen 20, fluorescent 60, LED 90, metal halide 87, high pressure sodium 117, low pressure sodium 150, mercury vapor 50. Real lamps vary by model and power rating, so you can always overwrite the lm/W field with the datasheet value.
- Why do watts not convert to lumens directly?
- A watt measures power consumed; a lumen measures visible light output. The bridge between them is luminous efficacy (lm/W), which depends on the lamp technology — which is exactly what the light-source list estimates for you.
- Which lamps give the most light per watt?
- Energy saving lamps have high luminous efficacy (more lumens per watt): low pressure sodium tops the presets at 150 lm/W and LEDs reach about 90 lm/W, while a tungsten filament bulb manages only about 15 lm/W — the same 60 W that makes 900 lm in tungsten makes 5400 lm in LED.
- Limits of this conversion
- The preset efficacies are typical / average values, not measurements of your specific bulb, and efficacy shifts with wattage, driver quality and color temperature. The calculator also assumes steady-state operation; it does not model beam distribution, which is a separate conversion (lumens depend only on total output, not on direction).
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Copy result uses your clipboard only. Preset efficacies are typical / average values; verify real luminaires from their datasheets or an integrating sphere.