To measure a window for replacement, measure the width and height of the opening in three places each, use the smallest readings, check the opening is square, and deduct around 10 mm for fitting. This guide explains each step.
Getting the measurements wrong means a window that doesn’t fit, which leads to extra cost and delays.
Quick Answer
- Measure the width at the top, middle and bottom.
- Measure the height on the left, in the centre and on the right.
- Use the smallest width and smallest height.
- Measure both diagonals to check the opening is square.
- Deduct about 10mm from the width and height for fitting.
Tools You’ll Need
- A steel tape measure (fabric tapes stretch)
- A pencil and notepad
- A spirit level
- A step ladder for high windows
- A helper for large windows
Measure the Opening, Not the Old Frame
Old frames can warp or move over time, so always measure the structural opening in the wall, brick to brick.
Measure from outside where possible, because you can see the full opening there. If you can’t reach the outside, measure from inside, but make sure you’re measuring the wall opening, not the window.
Step-by-Step: How to Measure a Window
Step 1: Measure the Width
Measure across the opening at the top, middle and bottom. Openings are rarely perfectly straight, so write down the smallest of the three.
Step 2: Measure the Height
Measure from top to bottom on the left, in the centre and on the right. Again, write down the smallest reading.
Step 3: Check the Opening Is Square
Measure diagonally from bottom left to top right, then from bottom right to top left. If the two readings match, the opening is square. If they differ, the opening is out of square, which is common in older properties. Tell your installer before you order.
Step 4: Deduct the Fitting Allowance
Subtract around 10mm from both your smallest width and your smallest height. This gap lets the installer level the window and seal it properly. If the gap is too small, the window won’t fit. If it’s too big, you’ll get draughts and heat loss.
Measuring by Window Type
The basic method is the same for every window. Some styles need extra details:
- Casement windows: Note which side each opening sash is hinged on, left or right.
- Tilt & turn windows: Note the hinge side and check the opening is level.
- Sash windows (vertical sliders): Check the opening is level so the sashes slide smoothly.
- Bay and bow windows: Measure and record each section separately, as the sections meet at angles. We recommend a professional measure for these.
Common Measuring Mistakes
- Measuring the old frame instead of the wall opening
- Taking only one reading instead of three
- Forgetting the fitting allowance
- Letting the tape sag or measuring at an angle
- Mixing up width and height (write width first, then height)
Measure twice, label every window clearly, and double-check before ordering.
Should a Professional Measure Your Windows?
You can measure yourself to get a price. For the final order, especially for bay windows or openings that aren’t square, have a professional measure. When your installer takes the measurements, they’re responsible if the window doesn’t fit.
See how our process works, from survey to installation.
FAQs
How do I measure a window for replacement?
Measure the width in three places and the height in three places, use the smallest of each, check both diagonals, then deduct around 10mm for fitting.
Should I measure from inside or outside?
From outside where possible, because it shows the full opening. Inside also works if you measure the wall opening and not the old frame.
What if my window opening isn’t square?
Use your smallest width and height and tell your installer the diagonal measurements before you order.
How much should I deduct for fitting?
Around 10mm from both the width and the height.
Get Your Windows Measured and Fitted
Trade uPVC Windows is a family-run company that has supplied and installed A-rated uPVC windows across Glasgow and the Central Belt since 2013, backed by our 10-year warranty.
