Window Glass Area Calculator

Work out window glass area instantly with clear inputs, formula shown and shareable results.

About Window Glass Area Calculator

Glass area is the window opening less the frame on all sides and between panes, so more panes means more frame and less glass. The glazing ratio matters for both daylight and heat loss, since frame and glass have very different U-values.

Formula

Glass area

glass width = opening width - frame width x (panes + 1); glass height = opening height - 2 x frame

Worked example

With the default values below, Window Glass Area Calculator returns Total glass area of 1.31. Change any input above and every figure here is recalculated in your browser.

Inputs used

Window width: 1.2 m
Window height: 1.4 m
Frame width: 60 mm
Panes in the window: 2 panes

Results

Total glass area: 1.31 m²
Frame area: 0.37 m²
Glazing ratio: 77.71% %
Area per pane: 0.65 m²

Step by step

1Values used
2Glass area
3Total glass area= 1.31 m²
4Frame area= 0.37 m²
5Glazing ratio= 77.71 %
6Area per pane= 0.65 m²

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does the glazing ratio matter?

Glass and frame lose heat at different rates, and the whole-window U-value is an area-weighted average. Slim frames improve both daylight and thermal performance.

Does a central mullion cost much light?

More than people expect. A 60 mm mullion in a 1.2 m window removes 5% of the glass area and creates a visible obstruction.