It is often desirable to calculate the amount of light that will result from a design. While seldom required in residential design, lighting calculations are critical to the success of lighting designs in most nonresidential buildings— schools, offices, stores, and most other commercial and institutional building types.

In modern design, it is common to talk about foot-candles of light (or lux, if working metric) rather carelessly. It is important to remember that the stated required number of foot-candles for a space generally refers to the average light level requirement measured in the horizontal plane at desk height. However, sometimes the criterion is for light only at the task or for light measured
in the vertical plane (as for artwork).