Accurate measurements are essential when ordering made-to-measure doors and windows. A few millimetres can make the difference between a perfect fit and an installation issue. This guide explains the simple steps you need to follow, what tools to use and the common mistakes to avoid.

Why accurate measuring matters

Manufacturers build your product to the exact sizes you provide. Once production begins, the sizes cannot be changed. Measuring correctly ensures:

• A smooth installation

• No unexpected gaps or trimming

• Compliance with building tolerances

• Correct thresholds, cills and frame positions

If you are unsure at any stage, we always recommend using a professional installer or surveyor.

What you need before you start

• Tape measure (preferably a rigid, high-quality one)

• Pen and paper or your phone notes

• A spirit level (helpful but not essential)

• Step ladder for tall openings

Measure in millimetres (mm) — this is the industry standard.

How to measure for a replacement window

1. Measure the width

Measure in three places:

• Top

• Middle

• Bottom

Use the smallest measurement. This accounts for any bowing or uneven brickwork.

2. Measure the height

Measure:

• Left

• Centre

• Right

Again, use the smallest measurement.

3. Check for a cill

If you have an external cill:

• Measure from the very top of the opening down to the cill.
  If you plan to replace the cill, measure the full opening including the new cill height.

4. Deduct your fitting tolerance

Installers typically deduct 10mm total (5mm each side) to allow the frame to fit comfortably.
If you are not using an installer and are unsure, provide us with your full brick-to-brick sizes and request “no deductions” or “exact sizes” — we can guide you.

How to measure for a replacement door

1. Measure the width

Take three measurements across the opening:

• Top

• Middle

• Bottom

Record the smallest number.

2. Measure the height

Measure from the top of the opening down to:

• The floor

• Or the top of your existing cill

If you are replacing the cill, measure the entire opening including space for the new cill.

3. Check your threshold choice

Your measurement may change depending on:

• Standard threshold

• Low threshold

• Part M compliant threshold

If you’re unsure which threshold you need, contact us and we can advise.

4. Deduct fitting tolerance

As with windows, installers usually deduct 10mm overall.
If you want us to manufacture to “opening size with standard tolerance deducted”, let us know when ordering.

What if the walls are uneven?

Uneven brickwork is common. That’s why measuring in multiple places is essential. Using the smallest measurement ensures the new frame will fit without forcing or trimming on site.

If the opening is severely out of square, a survey is recommended.

Special notes for bifold, sliding and French doors

Bifold doors

These require extremely accurate measurements. The threshold height, cill depth, and squareness of the opening all matter. Always measure:

• Internal-to-internal

• External-to-external

• Sub-cill position if applicable

Sliding doors

Check for head height restrictions. Some systems need additional clearance for the track and rollers.

French doors

Check the height of the meeting stile and ensure your opening is level to prevent one sash from catching.

Common mistakes to avoid

• Guessing or rounding numbers

• Measuring only once

• Using inches instead of millimetres

• Forgetting to account for plaster or render removal

• Assuming old frame sizes = brickwork sizes

• Not checking threshold or cill changes

When should you get a professional survey?

A survey is recommended if:

• Your opening is significantly out of square

• You’re installing a large bifold or sliding door

• The property is older or has mixed brickwork

• You’re unsure about cills, thresholds or vents

• You need Part F or Part L compliance guidance

Professional surveys ensure the manufacturer receives precise, installation-ready dimensions.

Need help?

If you're measuring for new doors or windows and you're unsure about anything, our team can talk you through the process or arrange for a professional survey. Accurate measurements ensure a smooth installation and the perfect finish for your home.