So I asked Google the same question the client asked:
Client: “I want some lights in my room…how many should I have for the right lighting effect?”
I got 1,375 Million results, many refer to some crude formula. As a curious person I followed the results for a simple kitchen I was planning…
This 4 step formula is quoted across the World on web sites: (step 5 is mine!)
Step 1: Multiply the room’s width by its depth to ascertain the square footage.(or convert this to Sq meters if you prefer)
Step 2: Multiply the square footage by 1.5 (or 2.5 for task areas like Kitchens) to give the total wattage needed in the room. For example in the kitchen below 4.2mtrs X 4.2mtrs (thus 190SqFt x 2.5 = 475 watts)
Step 3: Divide the total wattage previously calculated by your chosen bulb wattage to work out how many fittings you need. If using LEDuse their equivalent wattage in these sums.
Step 4: Spacing,Recessed downlights are generally positioned 1.5 to 2 ft. away from walls with a space of 3 to 4 feet between each light.
“Using a best selling 7 watt downlight this tells me to use 19 No. (I actually cant see how to fit them all in so I used only 16 ! )”
Step 5: Throw it in the Bin
and design something you will be proud of instead.