How to Measure for an Internal Door: UK Step-by-Step Guide
Getting the measurements right is one of the most important steps when buying a new internal door. A small difference in width, height or thickness can affect whether the door will suit the existing opening, frame or lining. This guide explains how to measure for an internal door before you order, what details to record and which common mistakes to avoid.
If you are still comparing styles and materials, start with our Internal Doors Buying Guide, then browse our full range of internal doors.
What You Need Before Measuring an Internal Door
Use a reliable tape measure, a pencil or notes app and, where possible, measure in millimetres. It is useful to record more than one measurement for both width and height because older openings are not always perfectly square.
How to Measure an Existing Internal Door
1. Measure the Door Width
Measure across the face of the existing door from one vertical edge to the other. Take the measurement at the top, middle and bottom and record the figures. If they differ slightly, do not assume the largest size is automatically the correct replacement. Compare your measurements with the product dimensions and the available space in the frame.
2. Measure the Door Height
Measure from the top edge of the door to the bottom edge. Check both sides of the door, particularly in older properties where floors or frames may not be perfectly level.
3. Check the Door Thickness
Measure the thickness along the edge of the door. This matters because hinges, latches, existing linings and fire-door requirements can depend on the door thickness. Never assume two doors with the same width and height will have the same thickness.
4. Check the Frame or Door Lining
If you are fitting a door into an existing frame, measure the clear opening and inspect the condition of the lining, stops and architrave. If the frame is being replaced as part of the project, the measurements you need may differ from a simple door-slab replacement.
How to Measure an Opening When There Is No Existing Door
Measure the opening width at the top, middle and bottom, then measure the height on the left and right. Also check the depth of the wall or lining area. The finished door size is not necessarily the same as the raw opening size, so match your measurements to the manufacturer's sizing information or ask your installer to confirm the required door and lining sizes.
Check the Door Style Before Ordering
Once you know the size, consider whether you need a solid, glazed, prefinished, primed or unfinished door. JJ Building Materials stocks options such as the LPD Doors Pre Finished Oak Belize Internal Door and a wide choice of Mendes internal doors in different finishes and configurations. Always use the individual product page as the source of truth for dimensions and specification.
What About Fire Doors?
If the opening requires a fire-rated door, measurement is only one part of the job. The door leaf, frame or lining, hinges, seals, closer and other components may need to form a compatible fire-rated assembly. Follow the manufacturer's instructions and applicable building requirements rather than treating a fire door as a standard replacement door.
Common Internal Door Measuring Mistakes
- Taking only one width or height measurement.
- Confusing the door leaf size with the structural opening size.
- Forgetting to measure door thickness.
- Assuming an old frame is square.
- Ordering before checking whether the product is a door slab or part of a doorset.
- Ignoring fire-rating or hardware requirements.
Before You Buy
Write down the width, height and thickness you need, confirm the opening and frame condition, and compare those details with the exact product specification. If you are uncertain about the finished size, ask your joiner or installer to confirm it before ordering.
Ready to compare options? Shop our internal doors and choose from oak, white, glazed and contemporary designs for projects across the UK.
Frequently Asked Questions
Should I measure the old internal door or the opening?
If you are replacing a door like-for-like, measure both. The existing door gives you a useful reference while the opening and frame help confirm whether that size still suits the installation.
Should internal doors be measured in millimetres?
Millimetres make it easier to record accurate dimensions and compare them with product specifications, although some door ranges may also display traditional imperial sizes.
Can an internal door be trimmed to fit?
Some doors allow limited trimming, while others have strict manufacturer limits. Check the individual product instructions before altering a door.
Do I need to measure the door thickness?
Yes. Thickness can affect compatibility with the frame, hinges, latch and any fire-rating requirements.