Watts to Lux Calculator
Turn bulb watts into room lux in one step: watts × bulb efficacy (lm/W) ÷ room area. Includes reverse mode — enter your target lux and get the watts you need.
Watts → Lux
100 lux
Target Lux → Watts
60 watts
Worked examples
Example 1 — watts to lux: 10 W LED at 100 lm/W produces 1,000 lumens. Spread over 10 m² → 1,000 ÷ 10 = 100 lux.
Example 2 — target lux to watts: want 300 lux in a 20 m² room → 300 × 20 = 6,000 lumens needed. With LED at 100 lm/W → 6,000 ÷ 100 = 60 W of LED.
Example 3 — square feet: 10 W LED (100 lm/W = 1,000 lm) over 100 ft² = 9.2903 m² → 1,000 ÷ 9.2903 ≈ 107.64 lux.
Bulb efficacy reference (lm/W)
| Bulb type | Typical efficacy |
|---|---|
| LED | ≈ 100 lm/W |
| Linear fluorescent (T8/T5) | ≈ 90 lm/W |
| Metal halide | ≈ 95 lm/W |
| High-pressure sodium (HPS) | ≈ 120 lm/W |
| CFL | ≈ 60 lm/W |
| Halogen | ≈ 20 lm/W |
| Incandescent | ≈ 15 lm/W |
Typical values; real bulbs vary by brand and age. Pick "Custom" to enter an exact figure from the bulb's datasheet.
Recommended lux by room
| Room | Target lux |
|---|---|
| Bedroom | 100 |
| Living room | 150 |
| Hallway / stairs | 100 |
| Bathroom | 200 |
| Garage | 150 |
| Warehouse | 200 |
| Kitchen | 300 |
| Classroom | 300 |
| Office / home office | 500 |
| Workshop | 500 |
| Retail | 500 |
Method
lux = watts × efficacy (lm/W) ÷ area (m²)
Watts measure electrical power; lux measures light on a surface. The bulb's efficacy bridges the two: watts × lm/W gives total lumens, and dividing by the area (converted to m² via ft² × 0.09290304 when needed) gives lux. Reverse mode runs the same chain backwards: target lux × area = lumens needed, ÷ efficacy = watts needed. Results are ideal lamp output; real rooms measure 10–30% lower due to shades, dirt, and aging.
Scope boundary
This tool = the single-step shortcut watts → lux (and reverse target lux → watts), composing bulb efficacy + area in one screen.
- Lighting converter — the hub: 19 units across 6 lighting dimensions, 2-way watts↔lumens tab, and a full room planner.
- Lumens to lux — starts from a known lumen output (no efficacy step); use it when the bulb box already lists lumens.
- Candela to lux — point-source illuminance (E = I/d²) for spotlights and beam angles, not area-based.
Frequently asked questions
What is the formula to convert watts to lux?
lux = watts × efficacy (lm/W) ÷ area (m²). First turn watts into lumens (watts × lumens-per-watt for your bulb type), then divide by the room area in square metres. Example: 10 W of LED at 100 lm/W gives 1,000 lumens; over 10 m² that is 100 lux.
Why can't you convert watts directly to lux?
Watts measure electrical power, while lux measures light falling on a surface — different physical quantities. You need two missing pieces: the bulb's efficacy (how many lumens it produces per watt) and the area the light spreads over. Without those, any watts-to-lux number is a guess.
How many lumens per watt do common bulbs produce?
Typical efficacies: LED ≈ 100 lm/W, linear fluorescent (T8/T5) ≈ 90, metal halide ≈ 95, high-pressure sodium ≈ 120, CFL ≈ 60, halogen ≈ 20, incandescent ≈ 15 lm/W. That is why a 10 W LED (≈1,000 lm) outshines a 60 W incandescent (≈900 lm) while using one-sixth of the power.
How many LED watts do I need for 300 lux in a 20 m² room?
Multiply: 300 lux × 20 m² = 6,000 lumens needed. Divide by LED efficacy (100 lm/W): 6,000 ÷ 100 = 60 W of LED. Use the reverse mode above with your room's area and bulb type to get the exact number.
How do I calculate lux if my room area is in square feet?
Convert square feet to square metres first: m² = ft² × 0.09290304. Example: a 10 W LED bulb (100 lm/W = 1,000 lm) lighting 100 ft² (9.2903 m²) gives about 107.64 lux. This calculator accepts ft² directly and converts for you.
What lux level should each room have?
Common recommendations: bedroom 100 lux, living room 150, hallway 100, bathroom 200, kitchen 300, classroom 300, office 500, workshop 500, retail 500, warehouse 200, garage 150 lux. Use these presets in reverse mode to size your lighting.
Do multiple bulbs just add up?
Yes — enter the total watts of all bulbs in the room (or the combined lumens). Two 10 W LED bulbs at 100 lm/W give 2,000 lumens total, so in a 10 m² room that is 200 lux.
Does fixture efficiency or light loss change the result?
Yes in the real world: shades, diffusers, dirt, and aging lamps absorb light, so measured lux is often 10–30% lower than this ideal calculation. Treat the calculator's result as the lamp output; for critical designs apply a light-loss factor (e.g. multiply by 0.8).
What is the difference between lux and lumens?
Lumens measure total light a bulb emits in all directions; lux measures how much of that light lands on each square metre of surface. The same 1,000-lumen bulb gives 100 lux in a 10 m² room but only 50 lux in a 20 m² room — double the area, half the lux.
How is this different from a lumens-to-lux calculator?
A lumens-to-lux calculator starts from a known lumen output and only divides by area. This tool adds the step before that: it converts watts to lumens using your bulb type's efficacy first, so you can start from the wattage printed on the bulb box. For lumens input directly, see our lumens-to-lux calculator.
How is this different from the lighting converter?
The lighting converter is a unit-conversion hub (watts↔lumens, lux↔foot-candle, and 19 units across 6 dimensions) plus a room planner. This page is the single-step shortcut for one specific job: watts → lux (or target lux → watts) in one screen, with bulb-type efficacy presets built in.
Is this calculation exact or an estimate?
The math is exact (lux = lumens ÷ area), but the input efficacy is a typical value — real bulbs vary by brand, age, and colour temperature. The result is accurate enough for room planning and bulb shopping; for certified lighting designs, use the fixture's photometric data sheet.
Free tool by NRTechStudio