Stained Glass Panel Area Calculator

Ordering too little glass is the classic first-project mistake. Cutting is not 100% efficient: you lose material at every discard, a break goes wrong now and then, and the offcuts left over from curved pieces rarely fit anything else. Reordering halfway through is worse than it sounds, because the replacement sheet comes from a different batch and the colour will not quite match.

Start with the panel:

Panel area (sq in) = Width × Height

Then the shape penalty. Every piece has to come out of a rectangle of glass, so the more your pieces curve, the less of each rectangle you keep. This is the step most calculators skip:

Efficiency is what survives; the multiplier is 1 divided by it, and that is what you actually buy. A 65% pattern needs a bit over one and a half times its own area in glass, not 1.35 times.

Then the safety buffer, which covers breakage and the piece you decide to recut because the grain ran the wrong way:

Glass to order = Panel area × Shape multiplier × (1 + Buffer ÷ 100)

Worked example. A 12" × 16" panel with moderate curves, cut by an intermediate hand, in four colours, on 12" × 12" sheets.

• Panel area = 12 × 16 = 192 sq in
• Curves and S-shapes, 65% efficient: 192 ÷ 0.65 = 295 sq in
• Intermediate buffer of 20%: 295 × 1.20 = 354 sq in to order
• That is 2.5 sheets of glass in total, but split across four colours it is one sheet each, because you cannot buy a quarter of a sheet. Four sheets, 576 sq in, and a full scrap bin afterwards.

That last line is the honest part, and it is why the panel below reports the glass you need and the glass you have to buy as two separate figures. Any project with several colours buys far more than it uses. The fix is not a better calculation, it is designing the next panel around the offcuts.

Glass sheet sizes. Common retail cut sizes are 8 × 10 (80 sq in), 8 × 12 (96), 12 × 12 (144), 12 × 16 (192), 12 × 24 (288) and 16 × 24 (384) square inches. Mill sheets from the factory are much larger but you will rarely be shipped one.

Foil against lead came. Copper foil forgives an untidy cut and is where most people start. Lead came suits straight lines and gives the traditional window look. The glass is not cut quite the same way for each: came has a heart, the central web between the two channels, usually 1.5 to 3 mm thick, and every piece has to be cut back by half of that from the pattern line on each edge. The material saving is tiny and vanishes into the buffer. Getting it right matters anyway, because a panel cut without allowing for the heart comes out oversized and will not drop into the opening it was made for.