The necessary light depends on the activity that is performed in a particular place, and is easier to express it in lumens per square meter (or 'Lux' value).
Where people read or do detailed work, such as in an office or in a kitchen, one should have between 400-500 lux. In a hall, bathroom, etc 250-300 lux could do.
Based on this data, we have to calculate the necessary light: necessary light value in lumens = area space x required light value in LUX.
On each lamp is stated how much lumen it gives. If we know how many lumens we need, we can divide this value by the lumen value that the chosen lamp gives, and so we get the number of necessary lamps.
Ex: I would like to place LED tubes of 18W and 1.2m long in my garage, where I want to do repairs, so I have to be able to see clearly. The garage is 8m long at 5m wide.
Calculation: Because I have to be able to see well, I choose 450 lux value. The surface of the garage is 8m x 5m = 40m2. The total required light in lumens will be 450 lux x 40m2 = 18,000 lumens. When I look at the lumen value of the chosen LED tube, I see in the specifications that such a tube gives 2018 lumen. Now I can calculate how many tubes I need: 18,000 / 2,018 = 9, so I have to hang 9 LED tubes of 1.2m.
This calculation is not 100% correct because we do not take into account the color of the walls, ceiling, windows etc, but is good enough to give a reasonable idea.
