What WWR is recommended?

20–40% usually balances daylight and heat loss.

What does ASHRAE 90.1 allow?

The prescriptive path caps vertical fenestration at 40% of gross wall area.

Does orientation matter?

A lot — more glazing south with shading, less west.

Are more windows always better?

No — more glass means more light but also more heat loss and gain.

What WWR do passive houses use?

Often 15–25%, concentrated on the south side.

Window-to-wall ratio calculator

Gross facade and window area: you get the window-to-wall ratio, opaque area and the window area for your target ratio.

Window-to-wall ratio

30 %

Window-to-wall ratio30 %
Opaque area753.474 ft²
Window area for target376.737 ft²

Use my prices

Your prices per language/currency. Used when you enable “Use my prices” in calculators. My prices →

Calculate labour, margin & VAT

How it's calculated (visible formula)

WWR = window area ÷ gross wall area × 100

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure gross area of each facade.
Add up its windows and glazed doors.
Work out WWR per facade.
Compare with the design or code target.
Adjust by orientation (more south, less west and north).

Worked example: 1,076 sq ft facade with 323 sq ft of windows

323 ÷ 1,076 = 30%
Opaque: 753 sq ft
For 35%: 377 sq ft

→ WWR of 30%.

Common mistakes

Using net wall area as the denominator.
Same ratio on every orientation.
Forgetting shading on south and west faces.

Pro tips

Calculate per orientation — the overall ratio hides overheating issues.
With high ratios, upgrade glazing (low-e, solar control).
Add exterior shading on south and west.
Think about daylight depth in rooms, not just the ratio.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & VAT

FAQs

What WWR is recommended?: 20–40% usually balances daylight and heat loss.
What does ASHRAE 90.1 allow?: The prescriptive path caps vertical fenestration at 40% of gross wall area.
Does orientation matter?: A lot — more glazing south with shading, less west.
Are more windows always better?: No — more glass means more light but also more heat loss and gain.
What WWR do passive houses use?: Often 15–25%, concentrated on the south side.
Does the frame count?: Yes, the whole window unit.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.