A guide to measuring your old windows for replacement

a man measuring his windows with a tape measureMeasuring up for new windows is not quite as simple as it sounds. But you do need to get your measurements exactly right otherwise you risk being lumbered with a lovely new window that doesn’t fit.

Here we’ve provided some guidelines on how to measure up your home for new windows. But if you’re not sure, it makes sense to get a professional to help you.

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  1. Use metric not imperial
  2. The first rule of thumb is to make sure you take accurate measurements in millimetres not inches. This gives you a finer level of accuracy and means there’s no room for error from the window manufacturer when they have to convert your measurements to metric.

  3. Measure the opening outside
  4. So firstly step outside. You need to take height and width measurements for the brickwork opening. Bear in mind that the height measurements must be from underneath any existing removable windowsill. If your property is rendered, simply take off a little of the render and measure to the brickwork edge.

  5. Measure brick to brick in at least three different places
  6. Remember the opening may not be exactly square so take a minimum of three measurements – top, middle and bottom, for the height and the same again for the width. This means your window manufacturer can get an accurate view of the exact size of the space the finished product will be fitted into. The final width and height measurement the manufacturer will use is the smallest one.

  7. Deduct 10mm (yes, really!)
  8. This sounds like odd advice but you need to take off 10mm from the final height and width measurements. This is to allow for a small fitting allowance around the new frame and means the fitter can make sure the new window frame sits squarely when it’s in place. Any gaps are then fitted with spacers and filled to keep the frame in just the right position.

  9. Never measure the old frame
  10. This is a common error and certainly one to avoid. Ignore the old window frame – this is going because it is no longer suitable. Consider the fact that it may well have warped so the last thing you want to do is take measurements from it!

  11. Be sure of your measurements
  12. Make sure you check (and double-check) your measurements so you are 100% sure you have them correctly. This is important because no window manufacturer will accept responsibility for incorrect measurements and you will still have to pay for a window that doesn’t fit. If you are unsure, consult a professional installer or builder and give yourself total peace of mind. It’ll be worth it.

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