How to measure windows that will replace old ones? First decide whether the old frame comes out of the wall or stays behind a renovation frame, because that decides what you measure. Then measure the width and height in three places, take the smallest figure and note the opening side as seen from inside. We do not come out to measure, so here is how to do it yourself or check your fitter's measurements.
Before you pick up the tape: what stays in the wall?
There are two ways to replace a window.
- Full removal. The old window comes out with its frame and the new one goes into the masonry opening. You measure the opening in the wall.
- Renovation frame. The new window is fitted over the old frame without breaking out the plaster (in France, pose en rénovation). You measure the clear opening of the old frame. This only makes sense if the old frame is sound and stable. The renovation frame is in our configurator; availability depends on the system.
A new build is simpler: you measure the masonry opening once the shell is up, not from the plans.
How to measure windows step by step
1. Get a tape measure, a spirit level and a sheet with the windows numbered. Label each window with its room so the sizes do not get mixed up. 2. Expose the edges. For full removal you need the edge of the masonry: remove the trims or a little plaster at the reveal from inside, because the old frame often hides the real opening. For a renovation frame, measure the inner edges of the old frame. 3. Measure the width at the top, middle and bottom. Note the smallest figure. 4. Measure the height on the left, in the middle and on the right. The bottom edge is the masonry or the old frame, not the internal sill. Note the smallest figure. 5. Measure both diagonals. A big difference means the opening is out of square, and your fitter needs to know. 6. Note the opening side standing inside the room: hinges on the left or on the right. With two sashes, mark which one gets the handle. 7. Decide on shutters. A top-mounted roller shutter adds a box above the window, so the window and the box together must fit in the opening. 8. Convert the opening size into the frame size. Deduct the fitting gap you agree with your fitter from the smallest figure; it depends on the wall, the system and the fitting method. For a renovation frame, agree the new frame size with your fitter or with us as well. 9. Check the limits before ordering. An opening sash must be at least 450 mm wide and a door sash at most 2300 mm high (above that we add a fanlight). The configurator checks this for you.
If you are unsure, send us photos of the openings and the raw measurements through our contact page. We will go through them with you.
The most common measuring mistakes
We have been working with windows since 1997 and have completed more than 5,000 orders. These mistakes keep coming up:
- The opening size entered as the frame size. The window turns out too big and will not fit with a fitting gap.
- The opening side described from outside. The window arrives mirrored.
- A shutter added after measuring. The box adds height and the sizes have to be worked out again.
- One measurement instead of three. Walls are rarely straight, and the difference only shows during fitting.
- Measuring to the plaster around the old window. The real edge of the masonry sits deeper, behind the old frame.
That is why we check the sizes, opening sides and descriptions of every window before quoting. Before production you receive a drawing and specification to approve, and once you have paid the sizes are locked. Made-to-measure windows cannot be returned within 14 days (Directive 2011/83/EU, Article 16(c)), so the drawing is your last chance to correct anything.
From measurements to a price
Enter the frame sizes in our window configurator: you see the price straight away, after every change, and a sash that is too narrow is flagged immediately. Compare profile systems on the uPVC windows page, and read how pricing works in Online window configurator. Fitting stays with you: do it yourself or with a fitter you choose.