The formula
lumens = area × the lux recommended for that room
Where it comes from
The old confusion is buying bulbs by the watt. A watt measures what it consumes, not what it lights, and the move to LED broke that habit: a 9 W bulb can put out as much light as an old 60 W one. What to look at is the lumen, the light coming out, and the lux, the light landing on the table: a lux is one lumen spread over a square metre. The sum follows from that, and it is a multiplication: the square metres times the lux the use of the room calls for.
How to work it out by hand
- Measure the square metres of the room
- Look up the lux its use calls for: 150 a living room, 500 a desk
- Multiply: those are the total lumens
- Divide by the lumens of each bulb
What is worth knowing
The levels in this table are the ones in ordinary use and they cover general lighting. Task lighting is a different thing and adds on top: a kitchen worktop or a desk wants its own light source, closer and more directed, and no ceiling fitting solves it. Wall colour counts too — a dark room swallows a good share of the light — along with ceiling height and colour temperature: 2,700 K to relax, 4,000 K to work. And spreading the light over several fittings always beats concentrating the same lumens in one, even though the sum comes out the same.