Lumens to Watts Calculator
Enter the luminous flux in lumens and the bulb's efficacy in lm/W to find the estimated wattage — the key figure for comparing an old incandescent to a modern LED.
Works for any bulb technology
The formula W = lm divided by lm/W applies to incandescent, halogen, CFL, and LED bulbs alike. Simply enter the efficacy for your bulb type to get the estimated wattage.
Efficacy varies by model and conditions
Published efficacy figures are nominal lab values. Real-world output depends on colour temperature, ambient temperature, age, and driver losses. See the limitations section for details.
What is luminous efficacy?
The key metric for comparing bulb technologies
Luminous efficacy measures how much visible light a bulb produces for every watt of electricity it consumes, expressed in lumens per watt (lm/W). A higher efficacy means more light for the same power. Dividing a target brightness in lumens by the efficacy of a chosen bulb type gives the wattage that bulb needs to reach that brightness — the core calculation behind the global shift from incandescent lamps to LEDs.
Enter the lumens and the efficacy in lm/W to find the estimated wattage instantly.
Divide the luminous flux in lumens by the luminous efficacy in lm/W to get the estimated wattage.
W = lm ÷ (lm/W)To replace a 60 W incandescent bulb (≈ 800 lm) with an LED rated at 100 lm/W, the calculation is 800 ÷ 100 = 8 W. A brighter 1,600-lumen lamp at the same efficacy needs 1,600 ÷ 100 = 16 W. If you are comparing against a halogen at 20 lm/W, 800 lm would need 800 ÷ 20 = 40 W — still less than an incandescent but far more than an LED.
The wattage figure tells you how much power a bulb with the given efficacy must consume to deliver your target brightness. For everyday reference, a typical living-room reading lamp needs around 400–800 lm, a kitchen ceiling fixture 1,500–3,000 lm, and a bright office workspace 3,000–6,000 lm. If the estimated wattage is surprisingly high, it usually means the efficacy value you entered is low — incandescent lamps sit at just 10–17 lm/W, which is why replacing them with LEDs (80–120 lm/W) cuts lighting energy consumption by 75–85 %. A result near zero can occur when lumens are set very low, which is valid — a 10-lumen night-light at 100 lm/W needs only 0.1 W. Use the result to compare two bulb technologies side by side: enter the same lumen target twice with different efficacy values to see the wattage difference directly.
This calculator gives an estimate based on nominal efficacy values.
Efficacy figures are nominal and vary in practice
Manufacturers publish efficacy at standard test conditions (25 °C, full drive current). Real-world efficacy is typically 5–15 % lower because of heat build-up inside the fitting, driver conversion losses in LED lamps, and lumen depreciation as the bulb ages. Always check the wattage printed on the bulb's packaging rather than relying solely on calculated estimates for purchasing decisions.