The lumen method, sometimes referred to as 'lumen input method' or 'total flux method', is based on the principle that the light reaching a particular point in a room is a function of the amount available outside of the window. In a simplified form, the lumen method can be expressed as:

DF_{average} = F x U x (A_{glazing} / A_{floor}) x 100%

where

- F
- Window factor, giving the ratio of the average vertical illuminance on the window plane from the sky to the external horizontal illuminance
- U
- Coefficient of utilisation. The ratio of the flux reaching the reference plane to the total flux entering the room.
- A
_{glazing}/A_{floor} - Area of glazing relative to the area of the floor or reference plane

In order to use the formula, the window factor and the coefficient of utilisation need to be known. For a completely unobstructed site, F is equal to 0.5 for side-lit rooms. U varies between 0.2 for low internal reflectances and 0.5 for high ones. Exact values are published in table form.

The lumen method for daylight is derived from calculations for artificial lighting, where it is used in slightly modified form.