Daylight Penetration Calculator
How far into a room daylight from a side window is useful — the rule-of-thumb multiple of the window head height — and what share of the room’s depth that covers, with and without a light shelf.
Daylight from a side window falls off quickly with distance: the usual planning rule is that useful daylight reaches about twice the height of the window head above the floor, or two and a half times with a light shelf or a high, clear window, and up to 1.
How the daylight penetration calculator works
Daylight from a side window falls off quickly with distance: the usual planning rule is that useful daylight reaches about twice the height of the window head above the floor, or two and a half times with a light shelf or a high, clear window, and up to 1.5 times for a tinted or heavily framed one. A 2.4 m head lights about 5 m of depth; deeper rooms need windows on two sides, skylights or electric light through the middle.
Formula: daylit depth ≈ multiplier × window head height
Worked examples
| Inputs | Useful daylight depth | Note |
|---|---|---|
| 2.4 m head, clear glass, 8 m deep | 4.8 m | 4.8 m — 60% of the depth |
| With a light shelf | 6 m | 6 m — 75% |
| A 3.3 m head in a 6 m room | 6.6 m | 6.6 m — the whole room |
FAQFrequently asked questions
Why does head height matter more than window size?
Light enters at the top of the window and travels down into the room at a shallow angle; the higher the head, the further back the sky is visible from. A wide, low window lights a strip by the wall; a tall one lights the room.
What does a light shelf do?
A horizontal reflector at or above eye level bounces sunlight onto the ceiling, which then lights the back of the room, while shading the glass below. It extends useful daylight to about 2.5 times the head height and evens out the glare near the window.
Is the 2× rule accurate?
It is a planning rule for the zone where daylight alone is enough for most of the working day in temperate climates with unobstructed sky. Obstructions, dark finishes and small windows shrink it; daylight-factor calculation or simulation gives the real answer.
What about rooms with windows on both sides?
Each side lights its own zone, so a room up to four times the head height deep can be fully daylit from two walls — the logic of narrow-plan buildings and courtyards.
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.