How to Measure an Internal Door (UK Step-by-Step)

Quick answer: Measure the opening inside the door lining, not just the old door. Take the width between the jambs at the top, middle and bottom, and the height from the finished floor to the head on both sides, then use the smallest figures. Subtract about 3 mm per side and top, and 10 mm underneath, for the door size.

Measuring the old door and ordering the same again works only if the old one fitted well, and in older houses it often did not. Measuring the lining gives you the true opening, tells you whether the frame is square and shows how much trimming a new door will need. This guide is part of our home measuring guides; for the list of sizes you will be matching against, see our UK standard internal door sizes chart.

Table of Contents

What do you need to measure an internal door?

A steel tape measure, a spirit level, a pencil and somewhere to write six or seven figures down. A steel tape is better than a fabric one because it stays straight across the opening.

Steel tape measure, 3 m or longer, reading in millimetres (see how to read a tape measure)
Spirit level, ideally 600 mm or longer, to check the head and jambs
Pencil and notepad, or a phone photo of the opening to write on

How do you measure an internal door opening?

Measure the width three times and the height three times inside the lining, plus both diagonals, and work from the smallest width and height. Openings are rarely perfectly square, so a single measurement can mislead you by several millimetres.

Measure between the jambs where the door will sit, and from the finished floor surface, not the floorboards.

Take the old door off or open it fully. You need clear access to both jambs and the head of the lining.
Measure three widths. Run the tape between the two jambs near the top (W1), at handle height (W2) and near the floor (W3). Measure on the line where the door sits, just behind the door stops, not across the stops themselves.
Measure three heights. Go from the finished floor to the underside of the head at the hinge side (H1), the centre (H2) and the latch side (H3). If carpet or new flooring is going down, measure from where its surface will be.
Check the diagonals. Measure from the top corner on one side to the bottom corner on the other, then the opposite pair. If they are within a few millimetres of each other, the opening is square enough for a standard door.
Measure the old door. Record its height, width and thickness. It helps confirm whether you have an imperial (1981 mm high) or metric (2040 mm high) opening.
Note the hinge side and hinge positions. Measure from the top of the lining to the top of each hinge recess, so the new door can be cut to match.

How do you work out what size door to buy?

Take the smallest width and height, subtract the gaps the door needs, and choose the standard size that matches or is slightly larger. A slightly larger door can be trimmed; one that is too small cannot be made bigger.

Door maker LPD recommends a gap of about 3 mm between the door and the frame. At the bottom, the ventilation guidance in the Building Regulations (Approved Document F) expects an undercut of at least 7,600 mm² above the finished floor, which is about 10 mm for a standard width door. Those allowances give a simple formula:

Door width = smallest opening width − 6 mm (3 mm each side)
Door height = smallest opening height − 13 mm (3 mm at the top, about 10 mm underneath)

An opening of 768 × 1,994 mm is a perfect match for the most common UK internal door, 1981 × 762 mm.

When the opening is not a standard size, the same arithmetic tells you how much to trim. Say the smallest opening is 740 × 1,970 mm. You need a door of 734 × 1,957 mm, so a 762 × 1,981 mm door would lose 28 mm from its width and 24 mm from its height. Check the maker’s trimming limits before buying: most doors list how much can be planed or cut from each edge, and hollow-core doors have only a narrow solid timber frame around the outside.

Where should you take a little off a new door?

Split the trimming between both sides, and between top and bottom, rather than taking it all from one edge. That keeps the door’s panels and frame looking balanced. LPD notes that if you need to remove less than 5 mm, the hinge side is the better place to take it from.

Mark the cut with the door held against the opening, score the line with a knife to stop the veneer splintering, and offer the door up after each cut. Fire doors are different: their gaps, trimming limits and hardware are set by the manufacturer’s certification, and replacing one is controlled work under the Building Regulations, so follow the instructions or use a qualified installer.

How do you measure for a new door and lining?

If the frame is coming out too, measure the structural opening in the wall and the wall’s full thickness, including plaster on both sides. Lining sets are sold to suit a particular door size, and the maker states the opening they need. The lining’s depth should match the finished wall thickness so the architrave sits flat on both faces.

Remove the architrave on one side so you can see the gap between the lining and the wall.
Measure the structural opening width at top, middle and bottom, and height on both sides, from the bare floor.
Measure the wall thickness from plaster face to plaster face at several points.
Compare with the lining set’s stated opening size, leaving room for packing to set the new lining plumb.

Is my door imperial or metric?

Check the height: about 1981 mm means an imperial door, about 2040 mm means metric. Imperial doors are the norm in England and Wales and are usually 35 mm thick; metric doors, common in Scotland, are usually 40 mm thick with widths ending in 26 mm, such as 726 or 826. The two are not interchangeable without altering the lining, because a metric door is 59 mm taller.

Measured old door	Likely system	Nearest standard size
About 1981 × 686 mm	Imperial	6ft 6in × 2ft 3in
About 1981 × 762 mm	Imperial	6ft 6in × 2ft 6in
About 1981 × 838 mm	Imperial	6ft 6in × 2ft 9in
About 2040 × 726 mm	Metric	2040 × 726 mm
About 2040 × 826 mm	Metric	2040 × 826 mm

If you are working from old plans in feet and inches, the feet to cm converter will turn them into metric figures you can compare with a tape.

Common mistakes to avoid

Measuring only the old door, which copies any mistakes made when it was fitted.
Measuring across the door stops instead of between the jambs.
Measuring the height from bare boards when carpet or a thicker floor is going down.
Taking one width and one height, then finding the opening narrows at the top.
Trimming all the excess from one edge, leaving the door lopsided.

Measuring a doorway for furniture is a different job, because what matters there is the clear width with the door open; our guide on measuring a sofa to fit through a door covers it.

Frequently asked questions

Do you measure the door or the frame when replacing an internal door?

Measure the frame, or lining, as well as the old door. The opening between the jambs and from floor to head tells you the true space; the old door only tells you what was fitted before.

What gap should there be around an internal door?

About 3 mm at each side and along the top, and roughly 10 mm underneath above the finished floor so air can pass between rooms. Fire doors follow the manufacturer’s specified gaps.

What size door fits a 768 mm wide opening?

A 762 mm (2ft 6in) door. Taking 3 mm from each side of a 768 mm opening leaves exactly 762 mm.

Can I trim an internal door to fit?

Yes, within the limits the manufacturer gives for each edge. Split the trimming between both sides and between top and bottom, and take extra care with hollow-core doors, which have only a narrow solid edge.

How do I measure a door for new carpet?

Measure the height from where the top of the new carpet will be, not from the floorboards, and keep roughly a 10 mm gap under the door above the carpet surface.

Sources

LPD Doors: How to fit an internal door
North Tyneside Council building control: Doors, external and internal (undercut and fire door replacement)
GOV.UK: Approved Document F, Ventilation
Howarth Timber: UK door sizes