How to measure an internal door

Getting the size right is the most important part of internal door installation. A door that doesn’t fit can cause gaps, sticking, or costly adjustments later. Whether you’re replacing an old door or fitting a new one, this guide explains how to measure correctly and gives you tips for planning your installation.

Why measurements matter

Always measure the door frame, not the existing door. Using the wrong measurements is one of the most common mistakes people make. By checking the frame directly, you’ll avoid poor fitting and get a clear idea of the internal door installation cost.

What you need

Tape measure
Pencil
Paper to record sizes

Step 1: Measure the height

Measure both vertical sides of the frame.
Frames are not always perfectly even, so check both.
Use the longer side as your final door height.

Most UK doors are 1981mm (6’6”), but some are 2040mm.

Step 2: Measure the width

Take three measurements: across the top, middle, and bottom.
Write them all down.
Use the widest size when ordering your new door.

Step 3: Measure the thickness

Measure the rebate (the slot where the door closes).
Standard internal doors are 35mm thick.
Fire doors, often used in kitchens or garages, are 40mm or 44mm.

Extra Tips for internal door installation

Check how the door opens. Uneven floors or changes in floor level can affect swing.
Plan for hinges and handles. These add to the fit and function.
Adjust frames if needed. New frames can be set to suit standard door sizes.

If you’re also asking how to install an internal door frame, accurate measuring is the same starting point.

How much does it cost to install internal doors?

The internal door installation cost depends on the type of door and how complex the job is. Fire doors, or jobs such as installing an internal door to a garage, usually cost more.

If you’re unsure about how to install internal doors, hiring a professional fitter can save time and give peace of mind.

Browse Our Range of Internal Doors

At Howarth, we stock internal doors from leading brands including LPD Doors, Deanta, and JB Kind. You can explore our full internal doors category online or visit your local branch for advice.

For more practical help, check our Inspiration & Advice section, where you’ll find guides on:

And if you have any questions on how to measure an internal door, or would just like to know some further information about the products we sell at Howarth, please visit your local friendly Howarth branch, or contact our customer services on the number 01472 907051.

21st Aug 2025 Kacey Gill