Daylight Factor & Window Area Calculator

How it works

DF (%) = A_glass × T × θ × M ÷ [A_total × (1 − R²)] (Lynes/BRE), with θ the visible sky angle in degrees and A_total all room surfaces; interior lux ≈ DF × 10,000 lux overcast sky

The daylight factor is the ratio of interior illuminance to the simultaneous exterior illuminance under an overcast sky, so it describes the room rather than the weather: 2% is the usual threshold for a room that feels daylit, 5% for one where electric light is rarely needed. The BRE average-daylight-factor formula captures the physics compactly — more glass and clearer glass admit more light, a wider view of sky (less obstruction) matters proportionally, and light rooms bounce daylight deeper, which is what the (1 − R²) term rewards. The approach is used for planning-stage design and for daylight credits in BREEAM and LEED's older paths; detailed design uses climate-based simulation, but the factor remains the quickest way to see whether a proposed window is enough.

Worked example

A 6 × 5 × 2.7 m room (total surface 119 m²) with 4 m² of 70% VLT glazing, a 65° sky angle and 50% reflectance has an average daylight factor of 1.8% — poorly daylit, about 180 lux under a standard overcast sky. Reaching 2% needs 4.4 m² of glass (15% of the floor), and with a 2.2 m window head the daylight only reaches 4.4 m into the 6 m deep room.

Frequently asked questions