How to Measure for Different Types of Replacement Windows

Reading time: 4 minutes

Why accurate window measurements matter

The guide looks at replacing the full window unit, including the frame, and that all measurements should be taken from the brickwork opening rather than the old frame.

Replacement windows are made to order, and even a few millimetres out can cause problems when it comes to fitting. In general, poorly sized windows may result in:

• Draughts and reduced energy efficiency
   

• Increased condensation and damp
   

• Security concerns if locks and frames don’t align
   

• Additional installation costs due to adjustments or reordering

By learning how to measure windows for replacement windows properly, you can be confident the new frames will sit snugly within the existing openings.

Tools you’ll need

Before you begin the process of measuring up, make sure you have the following tools to hand:

• A sturdy steel tape measure (not a fabric one)
   

• A notepad and pencil to record measurements
   

• A spirit level to check squareness
   

• Step ladders if measuring upper sections for safety and ease

General rules for measuring replacement windows

When you find yourself working out how to measure a replacement window, follow these golden rules:

1. Measure in millimetres (mm), as window suppliers tend to work in metric.
 

2. Take three measurements for both width and height. You will need to do this at the top, middle, and bottom (for width) and left, centre, and right (for height).
 

3. Use the smallest measurement, as this will ensure the window will fit within the tightest part of the opening.
 

4. Next, check diagonals. Measuring from corner to corner helps identify if the opening is out of square.
 

5. Finally, deduct around 10mm, as that 10mm gives you enough wiggle room for packers and sealant so the frame can sit perfectly square.

Measuring different types of replacement windows

Casement Windows

Casement windows are the most common style in UK homes, and can be measured by doing the following:

• Measure the width (from brick to brick) in three places: The top, middle, and bottom of the opening
   

• Measure the height (from brick to brick) in three places: The left, centre, and right side
   

• Note the smallest measurement of each and subtract 10mm from both width and height

Doing this ensures the new casement fits neatly, with space for sealing around the edges.

Sash Windows

If you’re looking at window sash replacement measurements, accuracy is especially important, due to traditional sash frames often shifting over time.

• Measure the box frame opening rather than the sliding sashes themselves
   

• Take width measurements at the top, middle, and bottom
   

• Measure the height in three places
   

• Deduct 10mm from the tightest width and height

Also check for squareness, as older properties are more likely to have shifted frames over time.

Bay Windows

Knowing how to measure a bay window for replacement can be one of the trickier tasks. Bays are built from several angled window units, all of which are joined together.

• Measure each individual window section using the same “three-point” method
   

• For angled joins, measure carefully from the internal corner post to the outer edge of each section
   

• Record each panel separately and double-check angles with a spirit level

Due to the complexity, many homeowners choose to confirm bay measurements with a professional installer.

Other window types

• Tilt & Turn Windows: Measured like casements but ensure you check internal clearances for opening space.
   

• Fixed Windows: Straightforward, just use the three-point method.
   

• Roof Windows (e.g. VELUX): Usually ordered by model size but measure the existing frame externally if unsure.

Fitting roof windows? Our guide to roof flashing explains how to keep them watertight.

Common mistakes to avoid

• Only measuring once instead of at three points
   

• Forgetting to deduct fitting allowance
   

• Measuring the old frame instead of the brick opening (always measure the full structural opening when replacing the entire unit)
   

• Not recording whether it’s height x width (suppliers usually ask for width first)

Final checks before ordering

Before placing your order, ask yourself each of the following:

• Have I measured both width and height three times?
   

• Did I use the smallest measurement?
   

• Have I deducted 10mm for fitting space?
   

• Do I need professional help for complex styles like bays?

Taking time now is imperative for preventing expensive errors later.

Replacement windows from Travis Perkins

At Travis Perkins, we stock a wide range of window solutions, from uPVC casement styles to roof windows and contemporary aluminium window options. By following this guide on how to measure for replacement windows, you’ll be ready to order with confidence, ensuring a secure fit and long-lasting performance.

Explore our range of windows and building supplies to find everything you need for your next project.