Lighting Converter (Lumens to Watts)
Brightness is measured in lumens and paid for in watts, and the exchange rate depends on the lamp. Convert a lumen rating into the watts each type needs, go back from an existing bulb's wattage to the light it makes, and check whether the room gets enough — with the lamp count to fix it.
Efficacies are industry-typical midpoints: LED lamps run about 90–110 lm/W (quality ones past 130), CFLs 60–70 and incandescent 14–17 including the halogen tail. Fixtures, diffusers, dimming and age all pull the real output below the rating, and the room targets are ordinary residential design advice, not code — task lighting wants two to five times these levels. Lux here assumes every lumen reaches the surface. Manufacturer photometric data and your local rules govern anything that has to pass a check.
Formula and calculation
Worked example
An {{lumens}} lm lamp in anything as efficient as a {{efficacy}} lm/W LED draws only {{watts}} W — the light a 60 W incandescent bulb used to make. Spread over a 12 m² living room it lands at {{lux}} lx, and {{fixtures}} of them reach the 150 lx design target for the room in the ideal, no-loss case.
Lumens, not watts
The wattage on an old bulb told you roughly how much light came out, because every incandescent converted electricity at about the same miserable 15 lm/W. With LEDs that shortcut is dead: a 9 W LED and a 60 W incandescent both make around 800 lm, at one-seventh the power. So compare lamps by lumens per watt — the efficacy — and by colour temperature if the look matters. The box figure is for the bare lamp; inside a fixture with a thick diffuser, expect a good chunk of it to disappear before it reaches the room.
Why a room needs more light than the arithmetic says
The lamp count this page prints assumes every lumen reaches the floor. Real rooms do not work that way: dark walls, a shaded pendant, a lampshade and the geometry of the fitting all take a cut, which is why designers divide by a utilization factor of about 0.5–0.7 and then add margin. Extra lamps also let you stop relying on one bright ceiling point — a mix of ambient, task and accent light feels better and dims better than a single over-driven fixture. If you are lighting for a task rather than a mood, the target is much higher: a kitchen counter or a desk wants 300–500 lx on the work surface itself.
What the results mean
- Light output per lamp
- Light output per lamp: the figure you entered, or what the wattage and type produce. Total lumens, not per square metre.
- Power per lamp
- Power per lamp: the figure you entered, or what the light output costs in the chosen type's watts.
- Efficacy used
- Lumens per watt for the lamp type used: LED about 100, CFL about 65, incandescent about 15. The higher it is, the less power each lumen costs.
- Illuminance
- Illuminance if the lamp's lumens spread evenly over the room area — lumens ÷ m². Assumes no losses; real rooms keep roughly 60% of the light.
- Lamps for the room target
- How many lamps of this output the room target needs in the ideal case. Real rooms need about 1 ÷ 0.6 of that once absorption and distribution are allowed for.
- The same light from each type
- The same light output priced in each lamp type's watts, so the incandescent-to-LED gap is visible at your numbers rather than in a table.
- Room plan
- A sentence of room advice: the target lumens for the room and area, the ideal lamp count and a practical budget with losses included.
- Substituting your numbers
- The one-line arithmetic with your numbers substituted, so the conversion can be checked by hand.
Frequently asked questions
How many lumens do I need for a room?
Multiply a target illuminance by the floor area: 100 lx is a reasonable hallway or bedroom level, 150 lx suits a living room, 300 lx a working kitchen. A 12 m² living room therefore wants about 1,800 lumens of light in the ideal case, before allowing for what the walls and shades absorb.
How many watts is 800 lumens?
It depends on the lamp type: about 8 W for a 100 lm/W LED, 12.3 W for a 65 lm/W CFL and 53.3 W for a 15 lm/W incandescent. That 800 lumen figure is the classic 60 W incandescent replacement level, which is why LED packaging advertises '60 W equivalent' next to the actual wattage.
What is luminous efficacy?
Lumens of light produced per watt of power drawn — the lamp's exchange rate. Typical midpoints: LED about 100 lm/W, CFL about 65, incandescent about 15, so the same brightness costs roughly six to seven times more electricity in an incandescent than in a good LED. Higher efficacy means more light for the same bill.
What is the difference between lumens and lux?
Lumens are the total light a lamp makes; lux is how much of it lands on each square metre, so lux = lumens ÷ area. An 800 lm lamp spread over 12 m² gives about 67 lx, while the same lamp concentrated on a 2 m² desk gives 400 lx. That is why task lighting is about putting the lumens where the work is.
Is the room lighting advice a code requirement?
No. The 100–400 lx targets are ordinary residential design advice, in the ballpark of published lighting handbooks but not an enforceable standard anywhere. Commercial buildings, workplaces and stairwells have their own requirements, and emergency lighting is a separate code matter entirely — check the applicable standard and a lighting designer's numbers.