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Upgrading your home with a new external door is a great way to refresh the front of your home or back garden. Before you do that, there’s the first necessary step: the measurement.
Taking the measurements isn’t as scary as it sounds. Even with measurements that are a few mm off, your installer will likely pack or trim the doors slightly to ensure they fit properly.
This guide will walk you through exactly how and where to measure, the tools you’ll need, and how to choose the right size with total confidence.
Measuring for Internal Doors? Take a look at our guide here.
Jump to the section you need:
Tape Measure
Square Measure
Notepad or Note App
Brick to Brick Measurements
For larger external doors, we’ll focus on measuring the structural opening brick to brick, ignoring trims and frames.
If frames are already installed on the openings, then these should be removed, as all of these doors come with their own frame and track.
You’ll be measuring brick to brick for these doors. All of our sets come with framing that will be installed in the brickwork.
You’ll be measuring brick to brick for these doors. All of our sets come with framing that will be installed in the brickwork.
Option 1: Use your square measure
Option 2: Use a tape measure (minimum of two people)
If both measurements are the same, your opening is squared.
If there is more than a 2mm difference in your measurement, then make your builder/ carpenter/ installer aware of this and they will pack out the walls so that they measure square.
You’ll take three measurements in millimetres.
Height of:
The smallest measurement this is your height.
You’ll take three measurements in millimetres.
The width of:
Take the smallest measurement – this is your width.
Head to the product page for the door you’re buying and take a look at the sizing guide.
Compare your measurement to the size measurement for the required opening.
Choose the closest measurement for the required opening.
Note: Each product has two measurements: the required opening and the product size. The required opening is the measurement that you should focus on.
Can’t find your size?
Frame to Frame Measurements
External single doors are measured from frame to frame. Most of our external doors can be fitted to a standard frame. Unlike larger doors, single doors only need one height measurement.
You’ll be measuring brick to brick for these doors. All of our sets come with framing that will be installed in the brickwork.
Option 1: Use your square measure
Option 2: Use a tape measure (minimum of two people)
If both measurements are the same, your opening is squared.
If there is more than a 2mm difference in your measurement, then make your builder/ carpenter/ installer aware of this and they will pack out the walls so that they measure square.
Use your tape measure and make note of the height in millimetres.
You’ll take three measurements in millimetres.
The width of:
The smallest measurement is your width.
Head to the product page for the door you’re buying choose the closest measurement.
If your opening is not square, then let your builder/installer/architect know. They will be able to make the necessary adjustments (Packing or trimming) to the door opening to make it square.
Older properties, and even newer builds, can suffer from shifting, leading to frames that are warped or out of plumb. By using the tightest point, you ensure the new door will actually fit into the opening without getting stuck or requiring massive structural alterations. If you order a door based on the widest measurement, it will need to be trimmed, which is more difficult than packing the frame to fit.
Written by Katja
22nd July 2026
Written by Katja
22nd July 2026
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Our family values mean honest advice, real conversations, and help when you need it.
All of our products are delivered safely and reliably to any Mainland UK postcode.
Our expert support team are always on hand to answer your questions. Drop us a line today.
You’ll need to trim mm off the left & right of each door
You’ll need to pack mm either side of the frame
You’ll need to trim mm off the top and bottom of each door
You’ll need to pack mm above
You’ll need to trim mm off the left & right of each door
You’ll need to pack mm either side of the frame
You’ll need to trim mm off the top and bottom of each door
You’ll need to pack mm above
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