Why watts stopped meaning anything
Watts measure electricity consumed, not light produced. Under incandescent lamps the two tracked each other closely enough that “a 60-watt bulb” was a useful description. With LEDs, efficacy ranges from about 70 to 160 lumens per watt across products on the same shelf, so two 10 W fittings can differ by more than double in output. Ask for the lumen figure and the efficacy; a supplier who can quote neither is selling on price alone.
| Fitting | Typical output | Efficacy |
|---|---|---|
| 10 W COB downlight, good brand | 850–1,100 lm | 85–110 lm/W |
| 10 W COB downlight, budget | 600–750 lm | 60–75 lm/W |
| 14.4 W/m LED strip, CRI 80 | 1,100–1,400 lm/m | 75–95 lm/W |
| 19.2 W/m LED strip, CRI 90+ | 1,300–1,700 lm/m | 68–88 lm/W |
| 36 W linear office luminaire | 3,600–4,700 lm | 100–130 lm/W |
| 9 W retrofit bulb | 800–950 lm | 90–105 lm/W |
Converting a room into a lumen number
Multiply your target lux by the floor area in square metres, then divide by a utilisation factor — about 0.5 for a typical room with a mid-tone finish, 0.6 for a pale one, 0.4 where walls are dark. A 12 m² bedroom at 150 lux needs roughly 150 × 12 ÷ 0.5 = 3,600 lumens, which is four 1,000-lumen downlights plus a bedside layer. The lumens calculator runs that for a whole flat.
Two traps
- Fitting lumens versus chip lumens. Some sellers quote the raw LED output before the reflector, diffuser and driver losses. Delivered output can be 15–25% lower. Ask for luminaire lumens.
- High efficacy at the cost of colour. The cheapest way to raise lumens per watt is to drop CRI. A 140 lm/W fitting at CRI 70 will make your veneer look grey. For homes, treat 90–110 lm/W at CRI 90 as the sensible trade.
Lumens belong on the lighting schedule alongside beam angle, colour temperature and CRI — those four numbers describe a fitting completely. Our layered lighting guide shows how the layers add up, and the residential service issues the schedule as part of the working drawings.
Frequently asked questions
How many lumens do I need per square foot?+
Around 15–20 lumens per sq ft for living areas and 30–40 over a worktop, before allowing for room losses. Those are rules of thumb; converting a lux target with a utilisation factor is more reliable, especially in dark-finish rooms.
Is a higher lumen rating always better?+
No. Over-lighting is as common a fault as under-lighting, and lumens gained by sacrificing colour rendering make a room look worse, not better. Match output to the task and control it with dimming.
What is a good lumens-per-watt figure in 2026?+
For residential fittings at CRI 90, 90–110 lm/W is good. Office linear luminaires reach 120–140 lm/W at CRI 80. Anything claiming 160 lm/W at CRI 90 in a cheap fitting is a specification worth verifying.
Do lumens decrease over time?+
Yes. LEDs depreciate rather than fail — an L80 rating means 80% of initial output remains at the stated hour count, usually 25,000–50,000 hours. Design with 10–15% headroom so the room is still right in year five.