How to Measure for a Replacement Window (Homeowners Guide)

Thinking of replacing your windows? Whether you’re getting quotes or planning a DIY job, knowing how to measure correctly is the first step — and it can save you time, money, and stress later on.

At Windows and Doors Newcastle, we’ve helped hundreds of homeowners across the region fit new windows that are made to last. And it all starts with an accurate measurement.

Here’s a simple, step-by-step guide on how to measure for a replacement window in your UK home — the right way.

Why Accurate Window Measurements Matter

Getting the measurements wrong, even by a few millimetres, can lead to major issues:

• Windows that don’t fit the opening
• Delays in manufacturing and installation
• Draughts or poor insulation
• Additional labour and extra costs

Most professional installers — including Windows and Doors Newcastle — will double-check your measurements before placing an order. But if you’re planning ahead or comparing quotes, having the right figures upfront puts you in control.

What You’ll Need

Before you start measuring, make sure you have:

• A steel tape measure (not a fabric one)
• A notepad or measuring sheet
• A pencil or pen
• A spirit level (optional but useful)
• A step ladder (if needed for upper frames)

You’ll be measuring the brick opening, not the frame itself.

Always measure in millimetres (mm) — this is the standard for window manufacturers in the UK.

Step-by-Step: How to Measure for a Replacement Window

Step 1: Measure the Width

Start by measuring the brick-to-brick width of your window opening.

Take three measurements:

• Top (just under the lintel)
• Middle
• Bottom (just above the sill)

Use the smallest of the three measurements. Walls and old frames aren’t always perfectly square, so this ensures the new window will fit without issues.

Write this number down as the width.

Step 2: Measure the Height

Now measure the brick-to-brick height of the opening.

Again, take three measurements:

• Left side
• Centre
• Right side

Use the smallest measurement as your final height. This helps avoid problems with uneven sills or older openings that have settled over time.

Write this down as the height.

Step 3: Deduct for Fitting Clearance

To allow room for fitting and expansion, you’ll need to subtract a small amount from your final measurements.

We recommend:

• Subtract 10mm from the width
• Subtract 10mm from the height

This gives the installer enough space to level and secure the new window properly without forcing it into place.

So if your smallest measured width is 1200mm and your height is 1050mm, your order size would be:

• Width: 1190mm
• Height: 1040mm

If you’re unsure, your installer or supplier can confirm the exact clearance based on the frame type.

Step 4: Check Window Depth (for Replacement Frames)

If you’re replacing old windows but keeping the original trim or plaster reveals, you’ll also want to measure the depth of the existing frame.

Use your tape measure to check from the inside edge of the frame to the outside edge, including any cill or trim.

This helps match your new window frame thickness and avoid gaps around the reveal.

Measuring for Bay Windows or Bow Windows

Bay windows are more complex and usually require professional measuring. Each section (left, centre, right) will need to be measured individually, and angles must be considered.

If you have a bay or curved bow window, we recommend booking a survey with a local expert like Windows and Doors Newcastle to ensure everything is accurately captured.

Measuring for Sash or Casement Replacements

Whether you’re switching from timber sash to uPVC, or casement to tilt-and-turn, the measuring method is the same.

What matters is:

• Measuring the opening, not the existing frame
• Using the smallest width and height
• Noting the type of opening (top hung, side hung, fixed, etc.)

This will help your installer recommend the right style and layout to match your property.

Additional Tips

• Always double-check your measurements before ordering
• Label each window clearly, especially if you’re measuring more than one
• Take photos of each window to share with your supplier if needed
• Don’t guess — if you’re unsure, book a professional survey

At Windows and Doors Newcastle, we offer free measuring and site surveys to make sure every installation goes smoothly from day one.

Final Thoughts

Measuring for a replacement window isn’t complicated — but it needs to be done carefully.

By following the steps above and giving yourself a bit of clearance, you’ll be ready to get accurate quotes, place the right order, or prepare for a professional installation.

Still not sure if your measurements are right? Contact Windows and Doors Newcastle today for friendly advice or to book a free site visit. We’ll help you get the perfect fit, every time.