Daylight Factor Calculator
The average daylight factor of a room — the share of outdoor light that reaches the inside — from glazing area, glass transmittance, the room's surfaces and how much sky the window sees.
The standard estimate is DF = T × Aw × θ / (A × (1 − R²)): glass transmittance, window area, the angle of visible sky, divided by total room surface area and a term for how reflective the room is.
How the daylight factor calculator works
The standard estimate is DF = T × Aw × θ / (A × (1 − R²)): glass transmittance, window area, the angle of visible sky, divided by total room surface area and a term for how reflective the room is. It is the formula in every daylight guide and it is accurate to within a factor that does not matter at sketch stage.
Two per cent is the usual minimum for a room to feel daylit; five per cent and the lights stay off on most days. Above that, glare and heat gain start to matter more than daylight.
Formula: DF = T × A_window × θ / (A_total × (1 − R²))
Worked examples
| Inputs | Average daylight factor | Note |
|---|---|---|
| A 27 m² room with 4.2 m² of glass | 2.31% | about 2.4% — daylit |
| An obstructed window | 1.07% | dim |
| A bright room | 4.96% | nearly 5% |
FAQFrequently asked questions
What is a daylight factor?
The ratio of light inside to light outside under an overcast sky, as a percentage. It describes the room, not the weather.
What daylight factor is enough?
Two per cent feels daylit; five keeps the lights off on most days. Living rooms and offices target 2 to 5, kitchens and studios higher.
What is the visible sky angle?
The vertical angle of unobstructed sky seen from the window center. An open site gives 80 to 90; a window facing a building across a narrow street may see 30.
Why does surface reflectance matter?
Because most daylight in a room has bounced at least once. White walls and a pale floor can raize the factor by a third for the same glass.
Is this a compliance calculation?
It is the sketch-stage estimate the compliance methods refine. Climate-based daylight modelling is what a rating scheme will ask for.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- International Code Council — publisher of the model codes most US jurisdictions adopt
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.