Watt to lumen equivalent

Watts measure what a bulb consumes. Lumens measure what it produces. For a century the two tracked each other closely enough that people used watts as a brightness scale — and LEDs broke that relationship completely.

The equivalence table

These are the figures the FTC lighting label is built on, and they are the reference every "60W equivalent" claim points back to:

Note the curve: doubling from 40 W to 75 W does not double the light, and 100 W gives twice the light of 60 W rather than 1.7 times. Wattage was never a linear brightness scale even when it worked.

Efficacy is the number that replaced it

Lumens per watt. An incandescent managed 12–17 lm/W. A modern LED lamp runs 80–120 lm/W, and the efficient end of the market pushes past that.

lumens ÷ watts = lm/W

That single division tells you more about a lamp than the equivalence claim on the front of the box, because "60W equivalent" is a marketing statement while lm/W is a measurement.

Where the equivalence quietly fails

An equivalence claim matches total lumen output. It says nothing about where the light goes. A bare incandescent radiated in every direction; many LED lamps put most of their output forward.

So a "60W equivalent" LED in a table lamp with a fabric shade often reads dimmer than the incandescent it replaced — same lumens, different distribution. In a downlight it reads brighter, for the same reason.

Beam angle decides whether a swap that matches on lumens also looks equivalent in the fitting — an omnidirectional bulb and a reflector lamp of the same output light a room very differently.

The base has to match before anything else does

Lumens are the interesting part of the decision and the base is the part that stops it working. The common domestic fittings:

Two traps live in that table. E26 and E27 look identical and are very nearly interchangeable, but not quite — an E27 lamp can sit slightly proud in an E26 holder. And GU10 and MR16 look similar from below and are not interchangeable at all: GU10 is mains, MR16 is 12 V and needs a transformer. Fitting the wrong one either does nothing or destroys the lamp instantly.

Shape codes, and why they matter more than they look

The letter-and-number code describes the envelope, and it decides whether a lamp physically fits and how the light leaves it.

A19 / A60 — the classic pear-shaped general bulb. Omnidirectional. GLS — the same thing under the older British name. BR30 / R63 — reflector shapes, output directed forward. PAR16 / PAR20 / PAR38 — pressed-glass reflectors with a defined beam angle. Candle, golf ball, globe — decorative envelopes, usually lower output. ST64, G80, G125 — filament-style decorative shapes.

The number is the diameter in eighths of an inch (A19 = 19/8 in ≈ 2.4 in) or in millimetres (A60 = 60 mm). It is the number that tells you whether the lamp will foul a shade or stand proud of a fitting.

Reading a lighting label

Most packaging carries a standardised panel. The five fields worth reading, in the order they matter:

Brightness, in lumens. The only measure of how much light you get.

Estimated yearly cost. Derived from the wattage and an assumed tariff, so it is really the wattage in different units.

Life. Almost always stated at L70B50 even where that is not printed — half the units still above 70% output at that point.

Light appearance, in kelvin. A scale from warm to cool.

Energy used, in watts. Divide the lumens by this and you have efficacy in lm/W, which is the honest comparison between two lamps.

What the panel does not carry is the three specifications that separate a good lamp from a cheap one: CRI and R9, flicker percentage, and SDCM. Their absence from the standard label is precisely why cheap lamps compete on the fields that are on it.

Dimmable, enclosed-rated, and the two ratings people miss

Two lines of small print decide whether a lamp survives where you are putting it.

Dimmable. Not all LED lamps are, and a non-dimmable lamp on a dimmer will flicker, buzz, or fail early. Some dimmable lamps also state a minimum dimming level — 1% and 20% are both sold as dimmable, and the difference is the whole evening.

Suitable for enclosed fixtures. A sealed shade traps heat around the driver, and heat is what kills LED lamps. Most lamps are not rated for it, and using an unrated lamp in an enclosed fitting both shortens its life and voids the warranty.

Also worth checking where relevant: damp or wet location rating for bathrooms and outdoors, and operating temperature range for an unheated garage.

Use the calculator above to convert between the wattage you remember and the lumens you now have to buy, then check the base, the shape and those two ratings before ordering.