How to Measure Up for Replacement Windows

Coloured timber windows

If you are looking to improve your home’s energy efficiency, or just give it a fresh new look, installing new windows can be quicker and easier than you might think. Ready-to-install replacement windows are easy to work with. The most important thing is that you pick the right size.

How Do You Measure Your Windows?

Before you buy new window kits, you need to check the size of your windows. The size of the replacement unit is based on the dimensions of the existing window frame. To measure the frame, start with the width:

  • Measure the inside of the old frame – from jamb to jamb.
  • Collect this measurement in three places – across the very bottom, across the top, and at the middle of the frame.
  • Note down the smallest of those three measurements.

Next, measure the height:

  • Take the height from the top of the sill to the underside of the head jamb.
  • Again, you will need three measurements – one at the left jamb, one at the right jamb, and one in the middle of the window.
  • Note down the smallest measurement.

Finally, check the squareness of the frame:

  • Measure the diagonals from corner to corner – both ways.

The two diagonals should be the same. If they differ by one quarter of an inch or less, then the replacement window can be shimmed to fit the opening. If they differ by more than that, then the frame may require adjustments and this is a more specialist job. You could need a full-frame replacement for your new window.

Measure the Sill

The final job is to use an angle-measuring tool to calculate the slope of the sill. Some frame replacements offer a choice of sill angles.

It is vital that the replacement windows are the right size for the opening, so check your measurements carefully before you buy a new frame!

Looking for new windows for your home? Browse our sliding sash or casement windows or find out about our handmade bespoke windows.