How to Measure for Replacement Window Glass

Cut-to-size glass is made to the numbers you give us and cannot be resold if it is wrong, so this is worth ten careful minutes. Almost every remake we see comes from one of three mistakes.

Measure the glass, not the opening

This is the big one. The frame opening is always larger than the pane, because the glass sits inside a rebate — a recessed lip that holds it. If you measure the opening and order that size, the new pane will not fit.

Where the old glass is still in place, measure the visible glass and add roughly 3mm to each dimension to account for what is hidden in the rebate. If the old pane is already out, measure the rebate depth on all four sides and work from that.

If you are not confident, remove a strip of the beading and measure how deep the rebate actually is. Guessing here is how people end up with a pane 6mm too small that rattles.

Measure at the widest point, in millimetres

Old timber frames are rarely square. Measure width at the top, middle and bottom, and height at both sides and the centre. Take the largest of each — then decide your clearance from there.

Work in millimetres, not centimetres. A decimal point in the wrong place is the second most common cause of a remake.

Allow clearance

Glass expands and contracts, and a pane cut to the exact rebate size will bind or crack. As a general rule, take 2–3mm off each dimension so the pane sits with a small gap all round.

For anything unusual — an arched head, a very large pane, an aluminium frame with a deep channel — send us the measurements and a photo and we will tell you what allowance to use rather than have you guess.

Note the thickness before you order

The new pane should match what came out. Measure the edge of a broken piece with a ruler, or check the rebate depth. Most domestic window glass is 4mm; larger panes and doors are often 5mm or 6mm.

If you are unsure, our thickness guidance covers the common cases, or send us the pane dimensions and we will tell you what we would use.

Check whether it has to be safety glass

This catches people replacing original panels in older homes. Glass that sat in a door for fifty years would very likely not meet today’s requirement. Any pane in a door, beside a door, in a bathroom, or below 500mm from floor level must be safety glass under Australian Standards.

Read when safety glass is required before ordering, because toughening has to be specified up front — toughened glass cannot be cut or drilled afterwards.

Photograph patterned glass

If the broken pane is textured, photograph it before you throw it out, ideally with a coin or tape measure beside it for scale. Patterns look almost identical in a catalogue and are simple to tell apart side by side. In a run of matching panels, a mismatch is obvious from across the room.

A short checklist

• Width and height of the glass, in millimetres, at the widest point
• Rebate depth, if the old pane is out
• Thickness
• Whether the position requires safety glass
• A photo, if the glass is patterned or you are unsure of anything

Enter your measurements on any clear glass product for an immediate price, or email us at info@affordableglassonline.com.au and we will talk it through first. Measuring advice is free, and getting it right at this stage costs a great deal less than a remake.