How Do I Measure A Window Frame Accurately?

Measuring the exact dimensions of a window frame isn't as straight-forward as you quite rightly suspect.

For windows, especially those in older houses or with wooden frames, cannot be assumed to be completely square or regular in size. Variations in production methods, for example, or contraction of the wood could easily result in the top of the window being a different size from the bottom. While to the eye a window frame may look perfectly square and right angled, a tape measure can often prove the beholder wrong.

As a general rule of thumb always take the measurements twice. For, as the carpenter explains, measure twice, cut once. It is also worthwhile taking the dimensions of the window frame in both centimetres and inches. Clearly mark them as metric and Imperial to avoid confusion later a la' the Hubble space telescope where the lenses ended up the wrong size when US and European engineers used separate systems of measurement. Unfortunately, the error only came to light when the telescope was in space and couldn't focus properly.

Measure all the diagonals, if they don't agree in both centimetres and inches then the aperture is out.