Daylight Factor Calculator

A daylight factor (DF) is the ratio of internal illuminance at a point on a horizontal surface to the simultaneous external illuminance under overcast sky conditions, expressed as a percentage. A DF of 2% means the internal light level is 2% of the outside level. BS 8206-2 recommends a minimum average DF of 2% for kitchens, 1.5% for living rooms, and 1% for bedrooms.

Approved Document L and Approved Document O reference daylight provision. BS 8206-2 recommends minimum average daylight factors of 2% for kitchens, 1.5% for living rooms, and 1% for bedrooms. The BRE recommends that at least 80% of a room's area should receive direct skylight. Planning guidance (BRE Report 209) often uses a minimum DF of 1% for habitable rooms.

The simplified daylight factor formula is: DF = (Ag x T x M) / (A x (1 - R²)) where Ag is the net glazed area, T is the glass transmittance (typically 0.6-0.7 for double glazing), M is a maintenance factor (typically 0.8), A is the total room surface area, and R is the average reflectance of room surfaces. For a simplified calculation: DF = (Window area x Transmittance) / (Floor area x 2) x 100.

Key factors include: window size and position, glass transmittance (clear vs tinted), room depth (deeper rooms get less light), surface reflectances (lighter walls bounce more light), external obstructions (nearby buildings, trees), window head height, and frame factor (how much of the window opening is glass vs frame). Roof lights provide approximately 3x more daylight per m² than vertical windows.

Daylight factor is a ratio (percentage) that remains constant regardless of sky brightness. It measures the building's inherent ability to admit daylight. Lux is an absolute measure of illuminance (lumens per m²). On a typical overcast day in the UK (around 10,000 lux outside), a room with a 2% daylight factor would receive approximately 200 lux internally. DF is for design-stage; lux is measured on-site.

Yes. Roof lights (skylights) typically provide 2-3 times more daylight per unit area than vertical windows because they see more of the sky vault. A roof light of 1m² delivers roughly the same daylight as 2-3m² of vertical glazing. However, roof lights can also cause overheating in summer, so Approved Document O (Overheating) must be considered in the design.