Window-to-Wall Ratio Calculator
The window-to-wall ratio of a facade or a whole building — glazed area as a share of the exterior wall — and how it sits against the limits energy codes use.
Divide the glazed area (including frames, as the codes do) by the gross exterior wall area above grade.
How the window-to-wall ratio calculator works
Divide the glazed area (including frames, as the codes do) by the gross exterior wall area above grade. Glass loses and gains far more heat than wall, so codes cap the ratio — 40% is the common prescriptive limit, and 30% or less makes an efficient envelope easy; above 60% is a curtain wall that needs high-performance glass and shading. Daylight, on the other hand, is well served by 25–35% placed high.
Formula: WWR = glazed area ÷ gross wall area × 100
Worked examples
| Inputs | Window-to-wall ratio | Note |
|---|---|---|
| 40 × 12 m facade with 48 windows of 3.6 m² | 36 % | 36% — within the limit |
| A curtain wall: 320 m² of glass on 480 m² | 66.7 % | 66.7% — curtain wall |
| A house wall: 10 × 3 m with 6 m² of windows | 20 % | 20% |
FAQFrequently asked questions
Does it include frames and spandrels?
Frames yes — the glazed area is the rough opening. Opaque spandrel panels in a curtain wall count as wall if they are insulated to the wall standard; otherwize many codes count them as glazing.
What ratio is best?
For energy, as low as daylight allows: 25–40% with the windows placed high. Views and marketing push it up; the answer is usually 30–40% with good glass and shading, and less on west facades in hot climates.
Is the ratio per facade or the whole building?
Codes apply it to the whole building envelope, but design decisions are per orientation: north (or south in the northern hemisphere) can carry more glass, west the least.
What happens above the limit?
A performance path — energy modelling that shows the whole building meets the target — with better U-values, lower solar heat gain coefficients and external shading making up for the extra glass.
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.