How much clearance should I allow when measuring for a replacement window?

As a general rule, deduct 10mm from the width and 10mm from the height (roughly 5mm each side, top and bottom) from the smallest aperture measurement. This gives you a fitting gap for packers and a proper sealing joint. On larger frames, out-of-square openings, or where cills and add-ons are involved, allow more — many installers work to a 10–15mm total deduction depending on the situation. Remember: you can always pack a gap, but you can’t pack a frame that’s been made too big.