How to measure for replacement windows: a step-by-step guide
When investing in replacement windows, accuracy is everything. A precise measurement ensures a perfect installation, preserving the aesthetic and performance of your home’s windows.
This expert guide will walk you through measuring for timber sash windows with a sash reveal and casement windows and doors.
Note: This guide applies specifically to cords and weights timber sash windows. It is unsuitable for measuring spring/spiral sash or non-timber windows.
Measuring sash windows accurately
Sash windows with a sash reveal
A sash reveal (also known as a check reveal, rebated reveal, or L-reveal) means the external brickwork opening is smaller than the internal wall opening. This design allows the sash box to sit neatly within a recess, ensuring only a small portion of the window frame is visible from the outside.
Using a rebated sash reveal allows the sash box to sit in a recess, hiding both the front-facing sides and the top so that only a tiny part of the window frame is visible or ‘facing’ on the outside.
Sash windows without a sash reveal
Some sash windows are fitted brick to brick without a recessed reveal. In these cases, the external side of the sash box is typically finished with decorative trim for a clean, seamless appearance.
Understanding sash window box depths
The depth of the sash box is just as crucial as its width and height. A proper fit ensures the window sits flush with the interior wall without protruding into the room.
In period properties, sash reveal depths typically range between 145mm and 150mm. Using a deeper frame than the recess allows can result in bulky, ill-fitting architrave.
The picture above shows a 165mm thick sash box which is too deep for the 140mm reveal. Using a deeper frame means architrave will have to be packed out which can look very bulky and not in-keeping with period properties
Sashed’s standard sash profiles
Sashed offers a selection of profile depths to suit both period properties and new builds:
- Profile 137mm – Ideal for listed building and vacuum glazed joinery.
- Profile 145mm – Ideal for period properties with standard reveal depths.
- Profile 150mm – Suitable for both period homes and modern builds. Also available in our PAS 24 offering for new builds.
- Profile 165mm – Best for new builds or period properties with deep reveals.
- Profile 190mm – Designed for triple glazed sash windows, suitable only for new builds.
How to measure for replacement sash windows
Follow this step-by-step guide to ensure accurate measurements:
Step 1: Measure the depth of the existing sash box
- Measure from the outside to the inside of the existing window to determine the depth of your existing sash box.
Step 2: Measure the width of the external opening
- Measure the width in three places – at the opening’s top, middle, and bottom.
- If the measurements vary, identify any visible obstructions (such as uneven bricks).
- Use the narrowest measurement as the final width.
Step 3: Measure the height of the external opening
- Measure the height in three places – at the opening’s left, centre, and right.
- If an arch or curve is at the top, ensure it is less than 80mm. If greater than 80mm (which is unlikely), additional timber may be needed to fill the space.
- Use the narrowest measurement as the final height.
Step 4: Calculate the total height of your new sash box
- Add 80mm to your external height measurement to account for the sash box frame.
- If the recess is tight at the top due to a bowed lintel, reduce this addition to 60mm to ensure a proper fit.
- Example: If your external height is 1920mm, adding 80mm gives you 2000mm – the height of the sash box to order.
Step 5: Calculate the total width of your new sash box
- Add 160mm to your external width measurement to account for the frame.
- Example: If the external width is 1040mm, adding 160mm results in 1200mm – the correct width for your sash box.
How to measure for replacement casement windows and doors
Casement windows and doors are typically easier to measure as they lack sash reveals. These windows are installed from the outside, and the internal opening is often slightly smaller due to plaster build-up.
Step 1: Measure the width of the external opening
- Measure the brick-to-brick width at three points and use the smallest measurement.
Step 2: Measure the height of the external opening
- Measure the brick-to-brick height in three places and take the smallest measurement.
Step 3: Adjust for the final order size
- Deduct 10mm from both the width and height measurements to allow for fitting tolerance.
- If the window or door is larger than 1800mm, deduct 15mm instead of 10mm.
- Example: If the smallest width measurement is 1200mm, subtract 10mm, making the final order width 1190mm.
Why precision matters
Getting your measurements right ensures a perfect fit, preserving the timeless beauty and refined craftsmanship of your new sash or casement windows. If you need assistance, our experts at Sashed are always happy to guide you through the process.
Need help? Contact Sashed today!
For further advice or to discuss your bespoke replacement windows, contact our team for expert guidance and tailored solutions.