How do I measure correctly for new doors or windows?

It’s one of the most common questions we get — how do I measure for a new door or window without getting it wrong?

Getting accurate measurements is the key to a perfect fit. Even a few millimetres off can cause draughts, leaks, or installation headaches later on. The good news? Measuring correctly is simple once you know what to look for.

1. Measure the opening, not the old frame

If you’re replacing an existing door or window, don’t just measure the old frame — that can give you the wrong size.
Instead, measure the brick opening (the structural hole in the wall) once the old frame is removed or exposed.

If you can’t remove it yet, take measurements as accurately as possible from the inside, where the frame meets the plaster, and from the outside, where it meets the brick.

2. Measure in three places

Openings aren’t always perfectly square — especially in older homes.
To get the right size, take three measurements for both width and height:

• Width: Top, middle, and bottom

• Height: Left, centre, and right

Use the smallest of the three as your final size. This ensures the new door or window fits even if the opening isn’t perfectly straight.

3. Deduct the fitting clearance

Once you have the brick opening size, you’ll need to allow for a small fitting tolerance — typically 10mm (5mm per side) to make space for sealant and foam.

So, if your brick opening measures 2100mm x 900mm, your manufacturing size would be 2090mm x 890mm.

This small gap is vital for adjustment and airtight sealing during installation.

4. Don’t forget sill height and threshold type

For doors, pay close attention to:

• Threshold height – Decide whether you want a low threshold (flush with the floor) or a standard one (better for weather protection).

• Floor levels – Measure from the finished floor level inside and outside to ensure the door clears tiles, flooring, or carpets.

For windows, make sure to measure from the sill upwards — not just from the plaster line — to get an accurate height.

5. Double-check diagonals for squareness

Measure from corner to corner diagonally in both directions.
If the two measurements differ by more than 5mm, your opening isn’t perfectly square.
Don’t worry — that’s normal. Professional installers can pack and level the frame to ensure a clean, even fit.

6. When in doubt, ask for help

If you’re unsure about any measurement, it’s always worth speaking to your supplier before ordering.
At Home Build Doors, our team can talk you through the process or check your dimensions before manufacturing — giving you peace of mind that everything will fit perfectly the first time.

Final thoughts

Accurate measurements are the foundation of a perfect installation.
Take your time, measure carefully, and always double-check before ordering.
It’s the simplest way to guarantee your new doors and windows look flawless, seal perfectly, and perform for years to come.