Windows & Doors
Window-to-Wall Ratio Calculator
Work out the glazed fraction of a wall
Updated 12 July 2026 · Live
What this tool does
Works out the window-to-wall ratio — the glazed fraction of a façade — and flags whether it sits in the typical range for daylight, heat loss and overheating.
Enter the total glazing area and the total wall area; the calculator returns the window-to-wall ratio as a percentage and an assessment.
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Formula Used
Works out the window-to-wall ratio — the glazed fraction of a façade — and flags whether it sits in the typical range for daylight, heat loss and overheating. It runs entirely in your browser and shows the working, so you can check every step against your own figures.
How the window-to-wall ratio calculator works
The ratio is simply the glazing area divided by the total wall area, expressed as a percentage. A low ratio means a solid, well-insulated but potentially dark wall; a high ratio means plenty of daylight but more heat lost in winter and gained in summer. The tool bands the result so you can see where a façade sits.
What you enter
Enter the total glazing area and the total wall area; the calculator returns the window-to-wall ratio as a percentage and an assessment. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
Most dwellings land between 15% and 40%. Below that a room can feel gloomy; well above it, overheating and winter heat loss become the design problem to solve with better glass and shading.
Safe by design
This tool sizes glazing, cost and thermal performance — not the structural opening. Sizing a lintel or a load-bearing opening is a structural engineer’s calculation, and this page never attempts it.
Assumptions and limits
This is a planning ratio, not a thermal model. The comfort outcome depends on orientation, shading, glazing quality and the room’s use — a south-facing glazed wall behaves very differently from a north one at the same ratio.
Using it with the rest of BuildMetricLab
This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Works out the window-to-wall ratio — the glazed fraction of a façade — and flags whether it sits in the typical range for daylight, heat loss and overheating. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or rates are embedded.
Frequently asked questions
What is a good window-to-wall ratio?
For homes, roughly 15–40% balances daylight against heat loss. The best figure depends on orientation, shading and how the room is used.
Does a higher ratio mean a warmer home?
Not necessarily. More glazing brings more daylight and solar gain but also more heat loss in winter — the net effect depends on the glass quality and orientation.
Should I include doors?
Include glazed doors in the glazing area if you want the full glazed fraction. Solid doors count as wall.
Is there a regulatory limit?
Building energy rules limit overall glazed area indirectly through fabric and overheating checks. Enter your own figures — this tool is a planning ratio, not a compliance check.
Spotted something off?
Calculations or display — let us know.