How to Measure for Replacement Doors and Windows (The Easy Way)?

When you’re replacing your doors or windows, the most important step happens before you even place your order — measuring.
Accurate measurements make the difference between a smooth installation and unnecessary delays. The good news? You don’t need specialist tools or experience — just a tape measure, a notepad, and a few minutes of careful checking.

Below, we’ll take you through exactly how to measure for replacement windows and doors step-by-step, so you can order with complete confidence.

Why Measuring Correctly Matters

Every property is slightly different, even if two openings look the same. Walls may not be perfectly square, plaster thickness can vary, and old frames might not have been installed completely level. That’s why the right way to measure is to check multiple points and record the smallest size.

Getting it right now means your new windows or doors will fit perfectly the first time — reducing gaps, improving thermal performance, and making installation straightforward.

What You’ll Need

Before you begin, make sure you have:

• A reliable tape measure (ideally in millimetres)

• A notepad or phone to record each measurement

• A spirit level (optional but helpful for checking squareness)

• A pencil or marker

Always measure in millimetres (mm) to keep things accurate and consistent with manufacturing standards.

Measuring for Replacement Windows

Whether you’re measuring uPVC, aluminium, or timber-look windows, the process is the same.

Step 1: Measure the Width

Take three measurements across the window opening — top, middle, and bottom — from brick to brick or plaster edge to plaster edge (depending on whether the frame is being fully replaced).
Record all three measurements and use the smallest one.

Step 2: Measure the Height

Do the same vertically — left, centre, and right.
Again, take the smallest measurement.

Step 3: Check the Opening Is Square

Measure the two diagonals from corner to corner.
If they differ by more than about 10mm, your opening isn’t square. That’s not a problem, but it’s useful information for the installer or manufacturer.

Step 4: Allow a Fitting Tolerance

When ordering new windows, you’ll generally want to deduct around 10mm from both the width and height. This small gap allows for adjustment during fitting and ensures the frame doesn’t bind against the wall.

Measuring for Replacement Doors

The principle is the same, but there are a few extra things to note — especially for bifold, French, or entrance doors.

Step 1: Decide How You’re Measuring

If the entire frame is being removed, measure brick-to-brick.
If you’re keeping the existing frame, measure inside the current frame (frame-to-frame).

Step 2: Measure Width and Height

Take three width measurements — top, middle, and bottom — and three height measurements — left, centre, and right.
As before, use the smallest figures for accuracy.

Step 3: Consider the Threshold

If your new door will sit on a threshold or cill, make sure you include this in your height measurement.
Note down the total height including the threshold and one without — it’s always best to clarify this with your supplier.

Step 4: Allow for Fitting

Deduct roughly 10mm from both width and height to create a small tolerance for fitting and foam sealant.

Common Mistakes to Avoid

• Measuring from the wrong surface (always measure from the same face, inside or outside).

• Forgetting to check squareness or level.

• Only taking one measurement in each direction.

• Ignoring plaster or render thickness.

• Guessing instead of writing measurements down immediately.

Double-checking now is much easier than replacing a frame later.

When to Call in a Professional

If you’re measuring unusually shaped openings, bay windows, or areas that look out of square, it’s worth arranging a professional site survey. A trained surveyor can take detailed readings and confirm exactly what’s needed before manufacturing begins.

Ready to Order?

With your measurements in hand, you can now request an accurate quotation or place your order online.
If you’re unsure about anything, our team can review your sizes before production to make sure everything fits exactly as it should.

Home Build Doors makes ordering replacement windows and doors simple — from measuring through to delivery.
Explore our full range today or contact our team for expert advice before you buy.