How to Measure for Replacement Doors and Windows

If you’re planning to upgrade your home’s doors or windows, getting accurate measurements is the crucial first step. Whether you’re gathering quotes or ordering new products, knowing how to measure for replacement doors and windows ensures a smoother installation, and prevents costly mistakes.

Why Measuring Correctly Matters

Accurate measurements help ensure:

• A perfect fit for your new products
• Reduced installation time and disruption
• No costly adjustments on installation day
• More accurate quotes from suppliers or installers

While it’s always best to have final measurements confirmed by a professional installer, providing approximate figures when requesting quotes can help speed things up and set realistic expectations.

Tools You’ll Need

Before you get started, gather the following tools:

• Steel tape measure (not a fabric one)
• Pencil and paper or a notes app
• Spirit level (optional for checking frame alignment)
• Step ladder (for taller windows or door frames)
• A second person (optional, but helpful for accuracy)

Always measure in millimetres (mm), it’s the standard unit used in the glazing industry.

How to Measure Windows for Replacement

Follow this guide for measuring existing window openings, not new structural openings.

1. Measure the Width

Take three measurements of the window’s width:

• Top
• Middle
• Bottom

Measure from brick to brick (brickwork opening), not the existing frame. Record the smallest measurement, this ensures the new window will fit the narrowest point.

2. Measure the Height

Again, take three measurements:

• Left side
• Centre
• Right side

Measure from top of the sill (or brickwork if the sill will be replaced) to the underside of the top brickwork. Record the shortest height.

3. Note Window Style and Opener Positions

Make a quick sketch of each window, noting:

• Number of panes
• Whether it’s side or top hung
• Which way it opens
• Handle positions

This is especially useful when requesting like-for-like quotes or comparing designs.

How to Measure Doors for Replacement

The process is similar but includes a few extra details.

1. Measure the Width

Take three measurements across the opening:

• Top
• Middle
• Bottom

Measure brick-to-brick, not including the frame. Record the smallest width.

2. Measure the Height

Take three vertical measurements:

• Left side
• Centre
• Right side

Measure from the top of the threshold or external step to the underside of the lintel or top brickwork. Again, record the shortest measurement.

3. Measure the Threshold (Step Height)

If you’re replacing a door with a low threshold or flush entrance, make sure to measure the height of the internal floor to the external ground level.

4. Note Door Type and Hinge Position

• Is the door hinged on the left or right (as viewed from outside)?
• Inward or outward opening?
• Does it include sidelights or top lights?
• Is there existing glass or panels?

This information ensures the replacement suits your opening and layout.

Pro Tips for Accurate Measurements

• Always measure in millimetres for accuracy
• Measure brick-to-brick, not frame-to-frame
• Record the smallest of your three measurements (width and height)
• Don’t worry about accounting for fitting tolerances — the installer will handle that
• If you have unusual shapes (arched tops, bays, or angled frames), take multiple measurements and provide photos

Should You Measure Yourself or Let a Professional Do It?

For initial pricing, DIY measurements are usually fine, just make sure they’re accurate to the nearest 5mm. But for the actual manufacturing and fitting process, a professional surveyor should always confirm final sizes.

Measure With Confidence

Knowing how to measure for replacement doors and windows is a useful first step when planning a home upgrade. Whether you’re refreshing tired windows, improving thermal performance, or boosting your kerb appeal, accurate measurements help you get the right products at the right price.

Once you’ve measured up, why not explore our range of energy-efficient, secure and stylish windows and doors?