Lumens to Watts Converter
Our home economics calculator computes lumens watts instantly. Get useful results with practical tips and recommendations.
Formula
Watts = Lumens / Efficacy (lm/W) or Lumens = Watts x Efficacy
Luminous efficacy (lumens per watt) defines the conversion between electrical power consumption (watts) and visible light output (lumens). Each bulb type has a characteristic efficacy: LED ~90 lm/W, CFL ~60 lm/W, Halogen ~18 lm/W, Incandescent ~14 lm/W.
Worked Examples
Example 1: Finding the Right LED to Replace a 100W Incandescent
Problem: A 100-watt incandescent bulb produces approximately 1,600 lumens. What wattage LED, CFL, and halogen bulb would produce equivalent brightness?
Solution: Target: 1,600 lumens\nLED: 1,600 / 90 lm/W = 17.8W (use 18W LED)\nCFL: 1,600 / 60 lm/W = 26.7W (use 27W CFL)\nHalogen: 1,600 / 18 lm/W = 88.9W (use 90W halogen)\nIncandescent: 1,600 / 14 lm/W = 114.3W (actual = 100W)\n\nAnnual electricity cost (5 hrs/day, $0.15/kWh):\nLED 18W: $4.93/yr | CFL 27W: $7.39/yr\nHalogen 90W: $24.64/yr | Incandescent 100W: $27.38/yr
Result: 18W LED = 27W CFL = 90W Halogen = 100W Incandescent (all ~1,600 lumens)
Example 2: Calculating Lumens from a 15W LED Bulb
Problem: You have a 15-watt LED bulb. How many lumens does it produce, and what is the incandescent equivalent?
Solution: LED efficacy: 90 lumens per watt\nLumens: 15W x 90 lm/W = 1,350 lumens\nIncandescent equivalent: 1,350 / 14 lm/W = 96.4W\nSo a 15W LED is equivalent to roughly a 100W incandescent\n\nEnergy savings: 100W - 15W = 85W saved per bulb\nAt 5 hrs/day: 85W x 5 x 365 / 1000 = 155 kWh/yr saved\nCost savings: 155 x $0.15 = $23.29/year per bulb
Result: 15W LED = 1,350 lumens = ~100W incandescent equivalent | Saves $23.29/year
Frequently Asked Questions
What is the relationship between lumens and watts?
Lumens and watts measure fundamentally different things. Lumens measure the total amount of visible light emitted by a source, essentially how bright it appears to the human eye. Watts measure the electrical power consumed by the bulb, how much energy it uses. The relationship between the two is defined by luminous efficacy, measured in lumens per watt (lm/W). Different bulb technologies have vastly different efficacies: incandescent bulbs produce about 14 lm/W, halogen about 18 lm/W, CFL about 60 lm/W, and LED about 80-100 lm/W. This means a 10-watt LED produces the same 800 lumens as a 60-watt incandescent. When shopping for bulbs, lumens tell you how bright it will be, while watts tell you how much it will cost to run.
How do I convert lumens to watts for different bulb types?
To convert lumens to watts, divide the desired lumens by the efficacy (lumens per watt) of the bulb type. For LED bulbs: divide lumens by 90. For CFL: divide by 60. For halogen: divide by 18. For incandescent: divide by 14. For example, if you need 800 lumens of light: LED requires 800/90 = 8.9 watts, CFL requires 800/60 = 13.3 watts, halogen requires 800/18 = 44.4 watts, and incandescent requires 800/14 = 57.1 watts. To convert watts to lumens, simply multiply watts by the efficacy value. These are approximate values since efficacy varies by specific product, color temperature, and manufacturer. Check the bulb packaging for exact lumen output, which is required by FTC Lighting Facts labels.
Why did the lighting industry switch from watts to lumens for measuring brightness?
The lighting industry switched from watts to lumens because watts never actually measured brightness; they measured energy consumption. In the era when incandescent bulbs were the only option, consumers could use watts as a reasonable proxy for brightness since all incandescents had similar efficacy around 14 lumens per watt. A 60-watt bulb always meant roughly 800 lumens. However, with the introduction of CFL and LED technologies that produce the same light at dramatically lower wattages, using watts to indicate brightness became misleading and confusing. The FTC updated Lighting Facts labels in 2011 to require lumens as the primary brightness measurement on packaging, similar to how nutrition labels standardized food information. This change helps consumers compare bulbs across different technologies and make informed purchasing decisions based on actual light output.
How do lumens relate to foot-candles and lux?
Lumens, foot-candles, and lux all relate to light measurement but from different perspectives. Lumens measure total light output from a source regardless of direction. Foot-candles measure the illuminance (light intensity) falling on a surface in imperial units, where one foot-candle equals one lumen per square foot. Lux is the metric equivalent, where one lux equals one lumen per square meter. Since one square meter equals 10.764 square feet, one foot-candle equals 10.764 lux. To calculate the foot-candles on a surface, divide the total lumens reaching that surface by the area in square feet. For practical room lighting design, you need 15-20 foot-candles (150-200 lux) for living rooms, 50 foot-candles (500 lux) for kitchens and offices, and 70 foot-candles (750 lux) for workshops. Understanding these relationships helps convert between lumen output of bulbs and the resulting light levels on work surfaces.
Does color temperature affect the lumens-to-watts ratio?
Color temperature has a small but measurable effect on luminous efficacy. Warm white LEDs (2700K) are typically 5-10 percent less efficient than cool white LEDs (5000K) of the same technology because the phosphor conversion process for creating warm tones introduces slightly more energy loss. For example, a warm white LED might produce 85 lumens per watt while the same design in cool white produces 95 lumens per watt. However, this difference is relatively minor compared to the vast efficiency gap between bulb technologies. Color temperature is measured in Kelvin and describes the appearance of light: 2700K appears warm and yellowish like incandescent, 3000-3500K is neutral, 4000-4500K is bright white, and 5000-6500K is daylight blue-white. Choose color temperature based on room function and personal preference rather than efficiency, as the difference is negligible for residential applications.
How do I measure the actual lumens output of a bulb in my home?
Measuring actual lumen output requires specialized equipment, but you can estimate the effective illumination using a smartphone light meter app. Apps like Lux Light Meter (iOS) or Light Meter (Android) use the phone camera sensor to measure approximate lux levels at a surface. To estimate total lumens from a measurement, take lux readings at multiple points in the room and use the relationship: average lux times room area in square meters approximately equals total useful lumens. For a more precise test of a single bulb, measure lux at exactly one meter distance directly below the bulb in a dark room; this reading in lux roughly approximates the lumens divided by 12.57 (surface area of a sphere at 1m radius). Professional lighting consultants use integrating spheres for precise lumen measurements. Note that bulbs lose 15-30 percent of their initial lumen output over their lifespan, a process called lumen depreciation.