The daylight factor (DF) is a percentage that expresses the ratio between the indoor illuminance at a specific point in a room and the simultaneous unobstructed outdoor illuminance under a standardised overcast sky.
How the daylight factor is calculated
The daylight factor is measured or calculated under a CIE standard overcast sky, which provides a uniform reference condition. The formula is straightforward: DF = (indoor illuminance / outdoor illuminance) x 100%. For example, if the outdoor light level is 10,000 lux and the indoor level at a desk is 200 lux, the daylight factor at that point is 2%.
Three components contribute to the total daylight factor: the sky component (direct light from the sky through windows), the externally reflected component (light reflected off neighbouring buildings or surfaces), and the internally reflected component (light bouncing off interior walls, floors, and ceilings).
What counts as a good daylight factor
A daylight factor of 2% or higher is generally considered adequate for living spaces and offices. Kitchens and workspaces benefit from values of 2.5% or more. Values below 1% typically mean a room will feel dark and will depend heavily on artificial lighting during the day.
Building regulations in many countries set minimum daylight factor requirements for habitable rooms. These requirements ensure that occupants receive enough natural light for comfort, health, and energy efficiency.
Application
The daylight factor plays an important role in the design stage of any building. Architects use it to determine window sizes, positions, and glazing types. It also influences the choice of interior finishes, since lighter surfaces improve internal reflections and raise the daylight factor.
For extensions, loft conversions, and dormers, calculating the daylight factor helps ensure the new space meets building regulations and provides a comfortable living environment.
Related terms
- Illuminance (lux)
- Sky component
- Solar gain
- Light transmittance
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