Measuring a door yourself costs nothing. A single mistake in that measurement can cost you over £150 in fees for a door you cannot return. Most UK suppliers charge a 25–50% restocking fee on custom-cut doors, or refuse returns entirely (Wickes returns policy, 2026; B&Q returns policy, 2026; Howdens Joinery returns policy, 2026). The single most common reason for a door not fitting is measuring the opening instead of the door slab itself.
Measuring a door costs nothing, but a wrong measurement can cost over £150 in return fees. Measure the door slab at three points in each direction and always use the smallest width and shortest height. Standard UK internal doors are 35mm or 40mm thick.
- Measure door slab at three points in each direction.
- Always use the smallest width and shortest height.
- Standard UK internal door thickness is 35mm or 40mm.
- Measure the door slab, not the opening, to avoid mistakes.
- Wrong measurements incur 25–50% restocking fees or no returns.
- Measuring a door costs nothing but a wrong measurement can cost over £150 in return fees
- The three measurements you need and the exact order to take them
- Quick numbers – measurement tolerances and typical UK door sizes
- The difference between measuring for a pre-hung door and a door slab
- How to measure a door that is already fitted without removing it
- The direct answer to “how to measure a door” – the featured-snippet method
- What to do if your measurements don’t match a standard UK door size
- How to verify your installer has measured correctly – MCS, TrustMark, and FENSA
The direct answer is to measure the door slab at three points in each direction and always use the smallest width and the shortest height. A typical UK internal door slab is 35mm or 40mm thick, and a replacement blank costs between £30 and £150 (Energy Saving Trust, 2026).
Measuring a door costs nothing but a wrong measurement can cost over £150 in return fees
A replacement internal door blank costs £30–£150, but a door cut to the wrong size is usually non-returnable. Most UK suppliers charge a 25–50% restocking fee on custom-cut doors, or refuse returns entirely (Wickes returns policy, 2026; B&Q returns policy, 2026; Howdens Joinery returns policy, 2026). The single most common reason for a door not fitting is measuring the opening instead of the door slab itself.
The three measurements you need and the exact order to take them
You need three measurements: width, height, and thickness. For width, measure the door slab at the top, middle, and bottom – the smallest width is the correct one. For height, measure the left side, centre, and right side – the shortest height is the correct one. For thickness, measure the door’s edge at three points; standard UK interior doors are 35mm or 40mm (British Woodworking Federation door installation guide, 2026).
Quick numbers – measurement tolerances and typical UK door sizes
| Door type | Typical width range | Typical height range | Standard thickness | Recommended clearance gaps |
|---|---|---|---|---|
| Internal door | 762mm, 838mm, 914mm | 1981mm, 2032mm | 35mm or 40mm | 2–3mm each side and top, 6–10mm bottom |
| External door | 762mm, 838mm, 914mm | 1981mm, 2032mm | 44mm | 2–3mm each side and top, 6–10mm bottom |
| Fire door | 762mm, 838mm, 914mm | 1981mm, 2032mm | 44mm | 2–3mm each side and top, 6–10mm bottom |
The difference between measuring for a pre-hung door and a door slab
A pre-hung door (door plus frame) requires measuring the rough opening width and height, not the door slab. A door slab requires measuring the existing door (not the frame) if replacing, or the frame opening if fitting new. For a pre-hung unit, add 10–15mm to the door width for the frame thickness and packing allowance (Timber Trade Federation technical datasheet, 2026).
How to measure a door that is already fitted without removing it
Measure the visible face of the door slab, not the frame or architrave. For width, measure from the left edge to the right edge at the middle of the door – do not include hinges or handles. For height, measure from the top edge to the bottom edge at the centre – ignore the gap under the door. If the door is painted or warped, use a straightedge to check for bows before measuring (FENSA technical guidance for replacement doors, 2026).
The featured-snippet method
Step 1: Remove the door from its hinges (or use a small spirit level to confirm it is square if left in place). Step 2: Measure width at the top, middle, and bottom of the door slab – record the smallest figure. Step 3: Measure height on the left, centre, and right of the slab – record the shortest figure. Step 4: Measure thickness at three points on the door edge – use the average for ordering. Step 5: Order a door that matches the smallest width and shortest height; do not order a door that is larger than any of your measurements.
What to do if your measurements don’t match a standard UK door size
Standard UK door widths increase in 6-inch steps (762mm, 838mm, 914mm); standard heights are 1981mm and 2032mm. If your measurement is between sizes, order the next size up and have it planed down by a joiner or DIY. Planing more than 6mm from one edge can expose the hollow core in a cheap door – check the manufacturer’s specification (BWF guide to door sizing and planing allowances, 2026).
How to verify your installer has measured correctly – MCS, TrustMark, and FENSA
For replacement external doors, the installer must be FENSA registered (or equivalent building control approval) to self-certify compliance (GOV.UK, 2026). For internal doors, no mandatory certification scheme exists, but TrustMark-registered tradespeople follow the BWF installation standard (TrustMark scheme rules, 2026). Always ask the installer for a written measurement sheet before they order the door – it should show the three width and three height readings.
how to choose the right internal door for your home If you are replacing an internal door, you may also want to check door hinge types and sizing guide to ensure your new door fits the existing frame.
Frequently Asked Questions
Measure the door slab at three points each for width, height, and thickness. Use the smallest width and shortest height. Standard UK internal door thickness is 35mm or 40mm, according to the British Woodworking Federation door installation guide (2026).
Standard UK internal door widths are 762mm, 838mm, and 914mm. Standard heights are 1981mm and 2032mm. Thickness is typically 35mm or 40mm, per the British Woodworking Federation (2026).
No, most UK suppliers charge a 25–50% restocking fee on custom-cut doors or refuse returns entirely. Wickes, B&Q, and Howdens Joinery all state this in their 2026 returns policies.
You need a 2–3mm clearance gap on each side of the door slab. This allows the door to open and close without sticking, as recommended by the British Woodworking Federation door installation guide (2026).
Measuring the opening instead of the door slab is the most common reason for a door not fitting. The opening may be uneven or larger than the slab, leading to gaps or a door that cannot be installed correctly.
