Lumens vs watts: how to choose a bulb that is bright enough

Watts tell you how much electricity a bulb uses; lumens tell you how much light it gives. When every bulb was incandescent the two rose together, so "60 watts" became shorthand for brightness. LEDs broke that link: an LED gives the light of an old 60 W bulb from roughly 8 to 10 W. So compare lumens when you shop, then choose the lowest wattage that delivers them. I built the lumens calculator to do both conversions and to estimate how much light a room needs.

What lumens and watts measure

A lumen measures luminous flux: the total visible light a source sends out, weighted for how sensitive the human eye is to each colour. A watt measures power, the rate at which the bulb draws energy. The ratio between them, lumens per watt, is called luminous efficacy, and it is the number that changed. A classic incandescent bulb manages about 13 lm/W because most of its energy leaves as heat; halogen manages about 16, compact fluorescent around 60, and a typical household LED about 100, with the best well above that. Those are the efficacies the calculator uses, and they are typical figures: the lumens printed on the box always win.

The equivalence table

Packaging still quotes "equivalent" wattages, and they come from the light output of standard incandescent bulbs:

• 25 W ≈ 250 lumens
• 40 W ≈ 450 lumens
• 60 W ≈ 800 lumens
• 75 W ≈ 1,100 lumens
• 100 W ≈ 1,600 lumens

Larger incandescent bulbs were a little more efficient, which is why the table is not a straight line: going from 40 W to 60 W adds 50% more power but nearly 80% more light. In LED terms, 800 lumens at 100 lm/W is about 8 W; the same light takes about 13 W from a CFL and 50 W from a halogen. In the EU, a bulb that claims to replace a 60 W incandescent must meet a minimum output of roughly 700 to 810 lumens, so the claim on the box is not entirely marketing. If a bulb gives only a wattage, divide the lumens you want by its efficacy rather than trusting an "equivalent" sticker.

How many lumens a room needs

Light falling on a surface is measured in lux, which is one lumen per square metre. To light a room, multiply its floor area by a target lux level for what you do there. Living rooms and bedrooms are comfortable at roughly 100 to 300 lux, kitchens at 300 to 500, and a desk for reading or detailed work at about 500. A 16 m² living room at 150 lux needs about 2,400 lumens, or three 800-lumen bulbs. A 10 m² kitchen at 400 lux needs about 4,000 lumens: five 800-lumen bulbs, or about 40 W of LED in total. In imperial units a foot-candle is one lumen per square foot, about 10.76 lux. Treat the result as a starting point: dark walls, high ceilings and lampshades absorb light, so add 20 to 30% in those rooms, and several smaller sources light a space more evenly than one bright ceiling bulb. The area calculator helps with L-shaped rooms.

Colour temperature and other label details

Brightness is only half of the label. Colour temperature, in kelvin (K), describes how warm or cool white light looks:

• 2,700 K — warm white, close to an old incandescent; cosy for living rooms and bedrooms.
• 3,000 K — slightly crisper warm white, a common all-rounder.
• 4,000 K — neutral or cool white, popular for kitchens, bathrooms and workspaces.
• 5,000 to 6,500 K — daylight white, bluish; useful for task lighting and garages.

A higher kelvin number does not mean more lumens, although cooler light can feel brighter. Also check the colour rendering index (CRI): around 80 is typical, and 90 or more shows skin tones and food more naturally. Not every LED is dimmable, and dimmable ones need a compatible dimmer, or they may flicker or buzz.

What switching saves

Because an LED needs about a seventh of the power for the same light, the saving adds up quickly. Swapping one 60 W bulb for an 8.5 W LED used four hours a day saves about 75 kWh a year; ten bulbs save around 750 kWh, roughly £190 at 25p per kWh. The LED savings calculator adds payback time and bulbs avoided over the LED's life, and the electricity cost calculator prices any single appliance. If kilowatt-hours are unfamiliar, What is a kilowatt-hour, and how do you work out what an appliance costs? explains them, and Which appliances actually use the most electricity shows where lighting sits next to heating and cooking.