Calculate the ratio of indoor to outdoor illuminance.
Illuminance Parameters
lux
lux
Calculation Results
Enter parameters to calculate
About this calculator
Quick Reference
DFDaylight Factor
E_iIndoor Illuminance
E_oOutdoor Illuminance
10,000 luxStandard overcast sky outdoor level
Overview
The Daylight Factor (DF) is a common numerical indicator used to determine the amount of daylight available in a room. It is the ratio of the indoor light level at a point to the outdoor light level under an overcast sky.
💡
Pro Tips
DF < 2%: The room will feel gloomy, and electric lighting will likely be needed during the day.
DF 2% - 5%: Good balance; the room feels well-lit, but some tasks may need extra light.
DF > 5%: Very bright; potentially increases heat gain in summer or glare issues.
!
Fun Facts
"Daylight factors are always measured under an 'overcast sky' (CIE standard) to ensure consistency, as clear skies are too variable."
"Strategic window placement and light-colored interior surfaces can significantly increase the daylight factor of a room."
