Lux, lumens and watts explained
Updated
A bulb box shows lots of numbers, but only three matter to know whether your room will be well lit:
- Lumen (lm): how much light the bulb emits. This is the number to compare bulbs by.
- Lux (lx): how much light lands on a surface, per square meter (1 lux = 1 lumen per m²; 1 foot-candle ≈ 10.8 lux). Room recommendations use lux.
- Watt (W): how much power the bulb uses. With LEDs, watts say little about light, because efficacy varies.
One way to remember it: lumens are the water coming out of the shower head, lux is how much water lands on each tile, and watts are the bill.
Equivalence chart: incandescent vs LED
| Incandescent | Lumens | Equivalent LED (≈ 95 lm/W) |
|---|---|---|
| 25 W | ~250 lm | 3 W |
| 40 W | ~450 lm | 5 W |
| 60 W | ~800 lm | 9 W |
| 75 W | ~1,100 lm | 12 W |
| 100 W | ~1,600 lm | 15–18 W |
| 150 W | ~2,600 lm | 24–28 W |
How many lumens each LED bulb gives
| LED | Typical lumens | Lights, at 150 lux (light walls) |
|---|---|---|
| 5 W | ~480 lm | ≈ 1.1 m² |
| 7 W | ~670 lm | ≈ 1.5 m² |
| 9 W | ~860 lm | ≈ 1.9 m² |
| 12 W | ~1,150 lm | ≈ 2.6 m² |
| 15 W | ~1,450 lm | ≈ 3.2 m² |
| 18 W | ~1,700 lm | ≈ 3.8 m² |
| 24 W | ~2,300 lm | ≈ 5.1 m² |
| 40 W | ~3,800 lm | ≈ 8.5 m² |
The last column shows how many m² each bulb covers at 150 lux, a living-room level, losses included, in a typical room of about 12 m² (130 sq ft) with light walls and a 2.7 m (9 ft) ceiling. For other rooms, use the calculator.
From lumens to lux
lux ≈ lumens × CU × 0.8 ÷ area
This is the lumen method. CU (coefficient of utilization) is the share of the light that reaches table height; the 0.8 covers LED depreciation and dust. In a 4 × 3 m (13 × 10 ft) room with a 2.7 m (9 ft) ceiling and light walls, CU is about 0.42, so roughly 0.34 of the lumens arrive; with dark walls, about 0.25. That is why two rooms of the same size with the same bulbs can look very different.
Read this before you buy
These numbers are an estimate for planning and comparing, not a lighting design. The math accounts for wall color, light loss over time and an average LED efficacy (95 lm/W), but it does not know which fixture you will use, its beam angle or where the furniture is.
- In the store, compare lumens on the box, not watts. Efficacy ranges from 80 to over 110 lm/W.
- Closed fixtures, frosted diffusers and dark shades deliver less light than the bare bulb.
- When in doubt, aim for the middle of the range and add a dimmer: you can turn light down, not up.
- For construction, major renovations or commercial spaces, talk to a lighting designer, architect or electrician.
Where the values come from
The math follows the lumen method in the IES Lighting Handbook (Illuminating Engineering Society) and CIE 52 on utilization factors. The lux ranges are comfort references for homes, based on ANSI/IES RP-11 (residential lighting), ISO/CIE 8995-1 / EN 12464-1 and the residential values of the former Brazilian standard NBR 5413. Workplace lighting standards do not regulate homes; treat the values as a starting point.
- The Lighting Handbook, 10th ed. (2011)
Illuminating Engineering Society (IES) · Lumen (zonal cavity) method, room index and coefficient of utilization. - CIE 52:1982 — Calculations for interior lighting: applied method
Commission Internationale de l’Éclairage (CIE) · Utilization factor from the room index and surface reflectances. - ANSI/IES/ALA RP-11-26 — Lighting for Interior and Exterior Residential Environments
IES / American Lighting Association · Recommended practice for home lighting: lux levels per room and task. - ABNT NBR ISO/CIE 8995-1:2013 — Iluminação de ambientes de trabalho, Parte 1: Interior
ABNT (Brasil) · ISO/CIE 8995-1 · Current Brazilian/ISO standard for indoor illuminance; basis for home office and circulation areas. - EN 12464-1:2021 — Light and lighting: Lighting of work places, Part 1: Indoor
CEN (Europa) · European counterpart, same concept of maintained average illuminance. - ABNT NBR 5413:1992 — Iluminância de interiores (cancelada em 2013)
ABNT (Brasil) · Replaced by NBR ISO/CIE 8995-1; still the most quoted residential lux reference in Brazil.
Frequently asked questions
How many lumens is a 9 W LED bulb?
About 800 to 900 lumens, equivalent to a 60 W incandescent. The exact value is on the box and varies by brand.
Are more watts always more light?
Only within the same technology and brand. An efficient 12 W LED can give more light than another 15 W one. Always compare by lumens.