In architecture, a Daylight Factor (DF) is the ratio of the light level inside a structure to the light level outside the structure. The only problem – you must know both the illumination level on the inside, on the working plane and simultaneous outdoor illuminance on a horizontal plane from a unobstructed hemisphere of overcast sky.

A Daylight factor can be expressed as an average, using experimental formulas. Here are some of them:

IES formula: DFm, IES = ( Awindow ε U * 100 ) / ( Afloor )

BRE formula: DFm, BRE = ( Awindow α M t ) / ( Atotal ( 1 – ρm² ) )

Sumpner formula: DFm, Sumpner = ( Awindow α M t ) / ( 2 Atotal ( 1 – ρm ) )

Italian formula: DFm, Italy = ( Awindow ε ψ t ) / ( Atotal ( 1 – ρm ) )

SBEM formula: DFm, SBEM = ((45 × Awindow × t ) + (90 × Aroof × t )) / ( Atotal × 0,76 )

Where:
Awindow – surface area of the window, excluding frame, bars and other obstructions, m². For SBEM it is the total window area, including frame.
Aroof – surface area of the rooflight, excluding frame, bars and other obstructions, m². For SBEM it is the total rooflight area, including frame.
Afloor – floor area of the room, m²
Atotal – total internal surface area of the room (ceiling, floor, walls and
windows), m²
ε – factor to account for external obstructions
U – utilisation factor
α – angle of visible sky from the mid-point of the window, °
M – maintenance factor of the window
t – transmission factor of the glazing. For the SBEM formula, t / 0,76 can be ommited for low-e double glazing.
ρm – average reflection factor of all internal surfaces
ψ – factor to account for thickness of the window wall