Timber casement windows with mullions and transoms

Timber casement windows with mullions and transoms

With hinges on one side, casement windows open wide like a door and are a classic style that has endured throughout the ages – which is why it’s important to keep them looking their best.

Like any window, you’ll probably want to replace them from time to time to upgrade the performance or functionality, and to make sure they’re as thermally efficient as possible.

Our below guide on how to measure casement windows for replacement will take you through all the necessary steps to ensure you’re getting the right size windows for your space.

Double casement window with central mullion

Double casement window with central mullion

Choosing your preferred style of casement window

There is a wealth of options when it comes to finding a casement window that suits your property and your needs. The first step will be to decide whether you are looking for vent or fixed casement windows. The difference is as it sounds – vent casement windows can open from the hinges, whilst fixed casement windows do not open at all.

At Timbawood, we can exactly match your existing casement windows, or can create a new configuration or design depending on the ages and requirements of your property. You can opt for a standard single or double casement window, or you may want to add a transom window – an additional smaller window separated from the main window by a transom (horizontal bar). The main casement windows may also have a mullion (vertical bar) between them.

By measuring the height and width of your windows, as well as taking photos, we will be able to give you a quote for the various options available.

Get ready to measure

Because houses come in all different shapes and sizes, so do casement windows. Whilst some ‘standard’ sizes are available for newer builds, generally speaking each casement window needs to be made to measure to ensure it fits the space exactly.

At Timbawood, we measure your windows twice – the first time during the sales survey, where one of our team visits your property to assess your windows, take measurements, and advise on the type of window that you need.

The second time is during the production survey, which is carried out after you have confirmed your window design and specification, where exact measurements are taken ready for production.

Accurate measurements are key to the final fit, which is why it is best that a professional surveyor takes these measurements.

However, we are able to provide you with an initial quote from your own approximate measurements and photos.

To do this, take the height and width measurements from inside the property. It’s helpful to have a pen and notepad handy to write down the location of the window (you might want to draw out a rough floor plan and label each window A, B, C etc).

Next measure the width and height of the window, measuring past the interior trim to the wall. A good rule of thumb is to think about the size of the hole you would be left with if you removed the window entirely.

Write down the measurements and send them to us at sales@timbawood.co.uk (we would recommend using millimetres/centimetres rather than inches for a more accurate record).

Speak to a professional

Our expert team is available Monday to Friday to talk to you about your project and to help ensure you choose the right casement windows for your property. As experts in windows for period, conservation and listed buildings, we can advise on a range of design options as well as specialist glazing types, timber, paint and hardware finishes. Contact us today to arrange an appointment.


There are a range of terms to be familiar with when it comes to window sizing and installation. We’ve listed a couple of the key ones below:

  • Mullion: A vertical divider separating two windows and providing structural support.
  • Transom: A horizontal divider separating two windows and providing structural support.
  • Jamb: The two vertical sections on each side of the window frame.
  • Head: The horizontal section across the top of the frame.
  • Sill/cill: The horizontal section across the bottom of the frame.
  • Window frame: Consists of the head (top of the window), the sill along the bottom and two vertical jambs.
  • Casement: The casement is the part of the window that opens, in which the glass sits.
  • Stile: The vertical sections of a casement.
  • Rail: The horizontal sections of a casement.