How to use the window-to-wall ratio calculator
How much glass a facade carries drives daylight, views, heating and cooling loads, and cost, and energy codes cap it on the prescriptive path. This calculator gives you the window-to-wall ratio (WWR) for each orientation and for the whole building, compares it with the limit you pick, and shows how much glazing you can add or must remove, so you can check a scheme before it goes to an energy consultant.
- Pick the units, square metres or square feet. Use the same unit for every field.
- Choose a code limit: the IECC commercial prescriptive 30%, 40% with daylight responsive controls, ASHRAE 90.1 prescriptive 40%, or your own figure.
- Enter the gross wall area of each facade, measured above grade and including the windows, and the glazing area on it, measured to the outside of the frames. Leave a facade at 0 if it has no exterior wall.
The results show the ratio for each orientation, the overall ratio, the maximum glazing allowed at the limit, and the glazing still available or the amount over the limit.
Window-to-wall ratio formula
WWR = Total glazing area ÷ Total gross wall area × 100
Facade WWR = Facade glazing ÷ Facade gross wall area × 100
Max glazing = Total gross wall area × Limit
Headroom = Max glazing − Total glazing
The gross wall area is the full exterior wall including openings, not the opaque wall left over. Code limits apply to the building total, but the per-facade figures show where glass is concentrated, which matters most on west and south facades for overheating.
| Standard or path | Vertical glazing limit | Notes |
|---|---|---|
| IECC commercial, prescriptive | 30% of gross above-grade wall | Recent editions; check the adopted one |
| IECC with daylight responsive controls | 40% | Most climate zones, controls over at least half the floor area |
| ASHRAE 90.1, prescriptive | 40% of gross wall area | Above this, use the performance path |
| Performance or energy modelling path | No fixed cap | Extra glass traded against other measures |
Worked examples
A mid-rise office
An office has 300 m² of wall to the north and south and 200 m² to the east and west, 1,000 m² in total. Glazing is 90, 120, 50 and 40 m², 300 m² in total, so the overall ratio is exactly 30%. That meets the IECC 30% limit with no headroom; the south facade is at 40% and the west at 20%.
A detached house
A two-storey house has 2,000 ft² of exterior wall and 360 ft² of windows and glass doors. The ratio is 18%, well under any commercial limit and typical for housing, leaving room to enlarge the living room glazing if overheating is controlled.
A glass-heavy scheme over the limit
A building with 1,200 m² of wall and 600 m² of curtain wall has a 50% ratio. Under ASHRAE 90.1 prescriptive at 40%, the maximum glazing is 480 m², so the design is 120 m² over. The team can cut glass, add spandrel panels, or take the performance path with better glazing and shading.
Who this calculator helps
- Architects checking facade concepts against energy code limits early in design.
- Facade and envelope consultants doing quick comparisons between options.
- Energy modellers and sustainability consultants confirming inputs before a model run.
- Developers weighing the cost of extra glass against the views it buys.
- Students learning how glazing, orientation and energy performance connect.
Tips for glazing ratios
- Measure glazing to the outside of the frame, not the visible glass; frames can add 10 to 20 percent.
- Put more glass where it helps, such as north light in the northern hemisphere, and shade south and west glazing.
- Get facade areas quickly with the square footage calculator, and price the envelope with the construction cost calculator.
- Check the building's footprint and envelope first with the setback calculator.
- Confirm the code edition and any local amendments; some jurisdictions set tighter limits than the model codes.
Want to see how the facade reads at night with the glazing lit? Turn a daytime render into an evening view with Day to Dusk.
