Lumen to Watt Calculator
Convert lumens (lm) to watts (W) using luminous efficacy: W = lm ÷ η (lm/W). Reverse watts to lumens. Includes typical LED, CFL, halogen, and incandescent.
Trust summary Engine tested · Specification checked · v1.1.0
- Input interpretation
- Enter values to calculate.
- Result
- —
- Model
- Electrical power, luminous flux, or luminous efficacy from any two of lm, W, and η.
- Scope
- η is constant for the chosen lamp / operating point
- Verification
- Engine tested · Specification checked · v1.1.0
- Named expert review
- Optional · Not performed
- Specification basis
- P (W) = Φ (lm) / η (lm/W)
Formulas
Core equations used by this calculator.
How to use
Choose direction
lm → W, W → lm, or η = lm ÷ W to find efficacy from the label.
Enter lumens, watts, and/or efficacy
Use package lm/W when listed, or a typical value for the lamp type.
Read watts, lumens, or efficacy
Compare technologies at the same lumen target — LEDs use fewer watts.
Example calculations
Common configurations with formula and result.
1000 lm · LED 100 lm/W
Default
900 lm · incandescent 15 lm/W
Classic “60 W” class
900 lm · LED 90 lm/W
Same brightness, less power
1500 lm · LED 100 lm/W
~100 W incandescent equivalent
W → lm
10 W · 100 lm/W
Find η
1000 lm · 15 W
Typical luminous efficacy (guide)
Common values at a glance.
| Light source | Approx. η (lm/W) | Notes |
|---|---|---|
| Incandescent (tungsten) | 12–18 | Legacy bulbs |
| Halogen | 16–30 | — |
| CFL / fluorescent | 40–75 | Varies by tube vs compact |
| LED | 80–120+ | Some exceed 150–200 |
| Metal halide | 75–100 | HID |
| High-pressure sodium | 85–150 | Street / industrial |
| Low-pressure sodium | 100–190 | Very efficient, poor CRI |
Lumen to Watt calculator specification
Version 1.1.0 · Engine tested
- Engine tested 4 published cases
- Named expert review Not performed
- Calculation version 1.1.0
- Definition
- Watts (W) measure electrical power; lumens (lm) measure visible light output. Luminous efficacy η (lm/W) links them: W = lm ÷ η and lm = W × η. Higher η means more light per watt — LEDs typically need far fewer watts than incandescent bulbs for the same lumens.
- What it calculates
- Electrical power, luminous flux, or luminous efficacy from any two of lm, W, and η.
- Inputs
- Any two of: lumens (lm), watts (W), efficacy η (lm/W)
- Outputs
- Watts, lumens, or efficacy (lm/W)
- Formula
P=Φ/η; Φ=P·η; η=Φ/P- Assumptions
- η is constant for the chosen lamp / operating point
- Units
- lm, lm/W → W
- Boundary conditions
- η = 0 → undefined (UI returns 0)
- Example
- 1000 lm, 100 lm/W → 10 W
- Validation cases
4 published on this page
- 1000 lm, 100 lm/W → 10 W
- 900 lm, 15 lm/W → 60 W
- 10 W, 100 lm/W → lm → 1000 lm
- 1000 lm, 15 W → η → ≈ 66.6667 lm/W
- Specification basis
- P (W) = Φ (lm) / η (lm/W)
- Calculation version
- 1.1.0
Background
Interpretation and common distinctions.
Convert lumens (lm) to watts (W) using luminous efficacy eta in lm/W.
P((W)) = (Phi((lm)))/(eta((lm/W))) Phi((lm)) = P((W)) × eta((lm/W))
Default: 1000 lm · 100 lm/W → 10 W (typical LED-class estimate).
Lumens vs watts
| Quantity | Unit | Meaning |
|---|---|---|
| Luminous flux | lm | How much visible light the lamp emits |
| Power | W | How much electricity it draws |
| Luminous efficacy | lm/W | Light per watt — efficiency of making visible light |
Shop by lumens for brightness; use watts for energy use and circuit loading.
Typical replacement guide
Same approximate brightness, different power (order-of-magnitude):
| Target lumens | Incandescent (~15 lm/W) | LED (~90–100 lm/W) |
|---|---|---|
| ~400–450 lm | ~25–30 W | ~4–5 W |
| ~700–900 lm | ~40–60 W | ~7–10 W |
| ~1100–1500 lm | ~75–100 W | ~11–15 W |
| ~2000–2500 lm | ~150 W | ~20–25 W |
Exact wattage depends on the product’s rated η. Prefer the lumen number on the box.
Related tools
Other calculators in this family: Watt to Lumen, Lumen to Lux, Watts to Lux, Watts to Amps .
Frequently asked questions
Key distinctions behind the calculation.
How do I convert lumens to watts?
Divide by luminous efficacy: W = lm ÷ η (lm/W). Example: 1000 lm ÷ 100 lm/W = 10 W.
What is luminous efficacy?
How many lumens you get per watt of electrical power (lm/W). Higher efficacy = more light for the same electricity.
Are lumens and watts the same thing?
No. Watts are power; lumens are visible light output. Modern LEDs deliver similar lumens at much lower watts than old incandescent bulbs.
What wattage LED replaces a 60 W bulb?
Match lumens, not watts. A ~60 W incandescent is often ~700–900 lm; an LED at ~90–100 lm/W needs roughly 8–10 W for that brightness.
What efficacy should I enter for LED?
Use the package value if available. Otherwise start near 80–100 lm/W for common lamps (some products are higher).
Can I find efficacy from watts and lumens?
Yes: η = lm ÷ W. Example: 1000 lm ÷ 15 W ≈ 66.7 lm/W.
Why do replacement tables disagree?
Efficacy ranges and marketing “equivalents” differ by brand. Prefer stated lumens over “60 W equivalent” alone.
Does fixture rating limit LED wattage?
Yes — don’t exceed the fixture’s maximum wattage or thermal design, even though LEDs run cooler at the same light level.