Watts to Lumens Calculator
Enter the bulb's wattage and its efficacy in lm/W to find the estimated luminous flux — the key figure for comparing an old incandescent to a modern LED.
Works for any bulb technology
The formula lm = W multiplied by lm/W applies to incandescent, halogen, CFL, and LED bulbs alike. Simply enter the efficacy for your bulb type to get the estimated brightness in lumens.
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. Multiplying a bulb's wattage by its efficacy gives the estimated luminous flux in lumens — the core calculation behind the global shift from incandescent lamps to LEDs. For example, an 8 W LED at 100 lm/W delivers the same 800 lm that a 60 W incandescent once required.
Enter the wattage and the efficacy in lm/W to find the estimated brightness in lumens instantly.
Multiply the wattage in watts by the luminous efficacy in lm/W to get the estimated luminous flux in lumens.
lm = W × (lm/W)To find the brightness of a 9 W LED at 100 lm/W, the calculation is 9 × 100 = 900 lm. A 60 W incandescent at 15 lm/W gives 60 × 15 = 900 lm — the same brightness but using nearly seven times the electricity. A 10 W high-efficacy LED at 120 lm/W reaches 10 × 120 = 1,200 lm, making it suitable for a reading lamp or small ceiling fixture.
The lumens figure tells you how much visible light the bulb produces. For everyday reference, a bedside reading lamp typically needs 250–450 lm, a standard living-room lamp 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 lumens are lower than expected, it usually means the efficacy value is low — incandescent lamps sit at just 10–17 lm/W, compared with 80–120 lm/W for modern LEDs. That difference is why switching from a 60 W incandescent to an 8 W LED cuts lighting energy use by roughly 85 % while maintaining the same 800 lm. A result of zero lumens is valid when the wattage is zero, representing a bulb that is off. Use the calculator to compare two technologies side by side: enter the same wattage twice with different efficacy values to see the brightness 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. For purchasing decisions, check the rated lumens printed on the bulb's packaging rather than relying solely on calculated estimates.