Measuring window opening

You’ve made the decision to replace your home’s windows, and now you’re asking yourself how do I measure them for replacement? Proper measurement of windows is essential in successfully installing new or replacement windows in a home. Because of the importance of having your windows built to the exact size necessary to ensure optimal performance, we always recommend you hire a professional to tackle the job because more goes into the project than many people realize. And, if the replacement window does not fit perfectly, you will be responsible for this, and may then need a new window or the existing opening will need to be modified to make it fit.

Even if you’re planning to hire a contractor to do the job, you may want to know how to measure for replacement windows to ensure you get the right size. Following these Ranebrook Windows guidelines will help to ensure precision and therefore, the best performance of your new windows. These recommendations are based on the installation of a typical replacement wood double-hung window into an existing wood double-hung window frame. Please, do not forget, that you’ll need to add box size to the opening size if you are looking to install a Traditional Box Sash Window.



To get the best result, take measurements from the outside opening (brickwork opening) of both the width and the height.

Note: Never measure the old frame or door, only measure the brickwork opening!

  1. Measure the width of the window or door opening at the top, middle, and bottom.
  2. Take the narrowest measurement and deduct 10mm. This will be the width of the window you order.
  3. Measure the height of the opening at the left side, middle, and right side.
  4. Take the shortest measurement and deduct 10mm. This will be the height of the window you order.
  5. Repeat for each opening



The information above and is generic because there are many things that determine exactly where you measure and how much you deduct or add to the width and height. If you’re considering replacing your home’s windows to boost energy efficiency or eliminate air and moisture leaks for comfort, we recommend you work with a reputable replacement window dealer, like Ranebrook Windows. Want some help from our professional window replacement team? Just fill in the short contact form or reach us immediately by calling 01322 332237.


Source – Blog Post from

2 thoughts on “How To Measure For Replacement Windows

  1. Nrel
    February 24, 2021 at 2:09 am

    Usually I don’t read article on blogs, but I wish to say that this write-up very forced me to try and do it! Your writing style has been surprised me. Thanks, quite nice article.

  2. froleprotrem
    November 29, 2020 at 5:03 am

    Keep working ,fantastic job!

Comments are closed.