Windows & Doors
U-Value Contribution Calculator
Area-weighted U-value of glazing plus wall
Updated 12 July 2026 · Live
What this tool does
Combines glazing and wall areas with their U-values into a single area-weighted U-value, and shows how much of the heat loss goes through the glass.
Enter the glazing and wall areas with their U-values; the calculator returns the combined area-weighted U-value and the glazing’s share of the loss.
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Formula Used
Combines glazing and wall areas with their U-values into a single area-weighted U-value, and shows how much of the heat loss goes through the glass. It runs entirely in your browser and shows the working, so you can check every step against your own figures.
How the u-value contribution calculator works
Each element loses heat in proportion to its area times its U-value. The area-weighted average multiplies each area by its U-value, adds them, and divides by the total area — giving the single U-value the wall-plus-glazing behaves as. The tool also shows what fraction of the total loss escapes through the glass.
What you enter
Enter the glazing and wall areas with their U-values; the calculator returns the combined area-weighted U-value and the glazing’s share of the loss. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
A small patch of poor glazing can dominate the heat loss because glass U-values are several times higher than a modern wall’s. The glazing-share figure makes that visible and shows where upgrading pays back fastest.
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 an area-weighted average, not a full fabric assessment. It ignores thermal bridging at junctions and frames, which a detailed U-value or SAP calculation would include. Enter your own U-values from the product data sheets.
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
Combines glazing and wall areas with their U-values into a single area-weighted U-value, and shows how much of the heat loss goes through the glass. 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
Why weight by area?
Heat loss scales with both area and U-value. Weighting each element by its area gives the true combined U-value rather than a naïve average of the two numbers.
Where do I get U-values?
From the product data sheets — windows quote a whole-window U-value, and wall build-ups are calculated or taken from construction guidance.
Why does the glass dominate the loss?
Glazing U-values (often 1.2–1.6) are several times a modern wall’s (around 0.2–0.3), so even a modest glazed area accounts for a large share of the loss.
Does this include the frame?
Use the whole-window U-value, which already includes the frame, for the glazing figure. Junction thermal bridging is separate.
Spotted something off?
Calculations or display — let us know.