Glass Cutting: Measure, Score and Reduce Waste
Glass punishes guesswork. Learn how to measure openings, which glass can be cut at all, and how much extra to order.
Measure the opening, not the old pane
Measure the width and height of the opening at three points each and use the smallest value, because frames are rarely square. Then deduct 1.5–3 mm of clearance so the pane can expand with temperature. A glass cut cannot be patched afterwards — every millimetre of accuracy has to be there before the first score.
Score once, break immediately
Cutting annealed glass is a two-step process: score with a carbide wheel in one continuous, unhesitating pass along a straight edge, then break within a minute or two. Re-scoring the same line crushes the edge and the break will run off. Practise on scrap until the score sounds like a sharp zip, not a gritty scratch.
Tempered and laminated glass cannot be cut
Tempered (safety) glass is under internal stress — score it and it shatters into pebbles on the spot. Laminated glass can be scored on both sides but the PVB interlayer must be split separately, which is a professional job. If you need a tempered pane in a new size, order it cut to size before tempering.
How much waste to plan for
Allow 10–15% extra for straight cuts on annealed glass in a small shop, and 20% or more when the job has notches, holes or curved cuts, because each special operation breaks more panes. Keep offcuts whole — glass nests like sheet goods, and a leftover half-sheet often covers the next small job for free.
Plan cuts like a nesting problem
Treat a glass sheet exactly like a cutting list: list every pane with its finished size, add the clearance and edge trim, then fit the pieces to minimise offcut. The Glass cutting calculator does this layout step for you and shows the waste percentage before you touch a cutter.
Can you cut tempered glass?
No. Toughened (tempered) glass cannot be cut, drilled or edge-worked after tempering — the surface is in compression, and breaking that skin makes the pane disintegrate into small granules. Every cutting operation happens before tempering; a tempered pane in the wrong size is replaced, not resized. Recognise it by the permanent corner stamp (Tempered, Toughened, Safety Glass or EN 12150) or by bright spots through polarised sunglasses. What you can cut at home: ordinary float glass, and laminated glass by scoring and separating both plies before cutting the interlayer — Glass cutting calculator plans the layout so every score line runs edge to edge.
What about laminated glass?
Laminated glass — two panes bonded around a plastic interlayer, the usual safety glass in doors and windscreens — can be cut, but only before lamination. Once the sandwich is bonded, a clean break is a professional job: score both sides, snap each leaf, then cut through the interlayer warm with a blade or heating wire, and the edges will still never match factory-cut float glass. For a DIY project, order laminated panes cut to size and plan ordinary float glass yourself with Glass cutting calculator; the tempered case is covered in the section above.
Can you cut insulating glass units (double glazing)?
No - a sealed double-glazed unit (two panes with a spacer and gas between) cannot be cut at all: cutting through the unit destroys the seal and the gas fill, and the panes themselves are usually toughened. If a window unit is the wrong size, the correct move is to order a new sealed unit made to size. Single panes in the frame - float or laminated - are a different story: measure the opening, subtract a few millimetres of play, and cut or order the single glass to size.