Window Size for
Daylighting
Enter your room dimensions and target daylight factor — get the minimum window area required, recommended WWR, and suggested window dimensions using the BRE simplified method.
Who it's for: Architects and building designers who need a quick minimum-glazing check at concept stage — supply room dimensions and a target daylight factor to get the required window area and WWR.
The BRE simplified daylight factor method estimates required glazing area from room geometry and surface reflectances. A 2% average daylight factor is the habitable room minimum (CIBSE/BRE); 5% is considered excellent for tasks. This calculator inverts the standard formula: given a target DF, it returns the minimum window area needed. Check results against LEED v4 prescriptive WWR limits (15%–40%).
How to use this calculator
The formula
W = DF × A × (1 – R²) / (Tvis × θ)
W — required window area (m²)
DF — target daylight factor as a decimal (e.g. 0.02 for 2%)
A — total room surface area: floor + ceiling + four walls (m²)
R — average surface reflectance of ceiling, walls, and floor combined (0.0–1.0)
Tvis — glazing visible light transmittance (0.65 for standard double glazing)
θ — sky angle factor; fixed at 0.40 for an unobstructed side-lit room (BRE average)
Worked example
When this estimate will be off
- Uses a fixed sky angle factor (θ = 0.40) assuming an unobstructed sky — a facing building or deep window reveals can cut actual daylight by 20–40%; increase the result by that margin for obstructed sites.
- Assumes uniform surface reflectance across ceiling, walls, and floor. Very dark finishes or a heavily glazed ceiling shift the result significantly; re-run with adjusted values.
- Returns the average daylight factor across the working plane, not a point-by-point distribution — deep rooms have a much lower DF near the back even if the average meets 2%.
- For planning submissions, LEED credit EQ c7, or specialist buildings such as schools and hospitals, a full BRE/CIBSE daylight calculation or simulation tool is required.
Frequently asked questions
A common mistake: Uses a fixed sky angle factor (θ = 0.40) assuming an unobstructed sky — a facing building or deep window reveals can cut actual daylight by 20–40%; increase the result by that margin for obstructed sites.
A 2% average daylight factor is the CIBSE/BRE minimum for habitable rooms. For a home office or other task space, 3–5% is more comfortable; 5% is the BRE excellent threshold. Below 2%, occupants tend to switch on electric lighting for most daytime hours even on a bright day.
Window-to-wall ratio (WWR) is the glazed area divided by the total wall area it sits on, expressed as a percentage. LEED v4 prescriptive daylighting compliance requires WWR between 15% and 40%. Values above 40% risk overheating and glare in most climates; values below 15% rarely achieve a meaningful average daylight factor.
Higher reflectances allow internal surfaces to bounce and distribute light more efficiently, reducing the glazing needed for a given daylight factor. Light neutral finishes — ceiling roughly 0.70, walls 0.50, floor 0.30 — average around 0.50. Dark interiors may need 30–50% more window area to reach the same target DF.
No — it uses θ = 0.40, calibrated for an unobstructed sky angle. A facing building, courtyard, or deep overhang reduces the visible sky and lowers the actual daylight factor. For obstructed sites, increase the calculated window area by 20–40% as a rough allowance, then verify with a simulation tool such as VELUX Daylight Visualizer or DIALux.
Sources
- BRE Digest 309 — Estimating Daylight in Buildings — defines the simplified average daylight factor formula and sky angle correction factor used here
- CIBSE/BRE guidance — 2% average daylight factor as the minimum for habitable rooms; 5% as excellent daylighting
- LEED v4 BD+C Daylight credit — prescriptive window-to-wall ratio limits of 15–40% as one compliance path for qualifying daylight
This free Window Size for Daylighting Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.