Total lumens needed to light a room to a target illuminance.
Typical targets: living 150–300, kitchen 300–500, office 500 lux. Add fixtures to share this total.
A room needs enough lumens to reach the light level that suits its use. The simplest way to estimate it is to multiply the floor area by the target level, in lumens per square foot or lux. Common rules of thumb for homes suggest about 10 to 20 lumens per square foot for living rooms and bedrooms, 30 to 40 for kitchens, and more for bathrooms and workspaces.
These figures are a starting point. The real need depends on the colors of the room, since dark walls absorb light, and on the tasks you do. Spaces for detailed work or for older people need more light, and rooms used for relaxing can have less. Dimmers and several light sources let you adjust the level for each activity.
Divide the total lumens by the lumens of each fixture to count how many you need, and spread them evenly around the room, mixing ambient light with task light in places such as counters and desks. Remember that decorative fixtures often give less usable light than their size suggests, so use the lumen rating on the product.