How-to

How to measure an internal door: a UK guide

Getting the measurements right is the first step to a door that fits properly. This guide explains how to measure an internal door and its opening, how UK standard sizes work and how to avoid the mistakes we see most often.

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  • Updated October 2026

Should you measure the door or the opening?

Measure both, but trust the opening. The opening is the space inside the door lining (the timber frame the door hangs in). It tells you what will actually fit.

Your existing door is a useful guide, but it can mislead you. Doors are often trimmed at the bottom to clear a new carpet, or planed down at the sides because they stuck. If you order a new door based only on the old one, it can come out smaller than the opening needs. Measure the old door first, then check it against the opening.

If there is no door in the opening at all, measure from the inside face of one side of the lining to the other, and from the floor to the underside of the lining head.

How to measure height, width and thickness

You need a steel tape measure, a pencil and somewhere to write the numbers down. Measure in millimetres, as that is how most doors are sold.

Height

Measure from the floor to the underside of the lining head in three places: on the left, in the middle and on the right. Floors and heads are rarely perfectly level, so the three numbers may differ by a few millimetres. Write down the smallest.

Width

Measure across the opening between the inside faces of the lining at the top, the middle and the bottom. Again, use the smallest figure. If the difference between your largest and smallest figures is more than a few millimetres, the frame may be out of square.

Thickness

Measure the edge of the existing door. Most imperial internal doors are 35 mm thick. Fire doors and heavier doors are usually 44 mm, and some metric doors are 40 mm. The thickness has to suit the door stop on your lining, so check this before ordering a thicker door.

Tip: take a quick photo of each opening with the numbers written on a sticky note. It saves a lot of confusion when you are ordering several doors.

Standard UK internal door sizes

Most UK homes, including the older terraces and semis around Newcastle, have imperial-sized openings. Some newer homes use metric sizes. The two are not interchangeable, so it is important to know which you have.

Size (H × W)Imperial equivalentSystemTypical use
1981 × 610 mm6’6” × 2’0”ImperialCupboards, small en suites
1981 × 686 mm6’6” × 2’3”ImperialBathrooms, box rooms
1981 × 762 mm6’6” × 2’6”ImperialBedrooms, living rooms, kitchens
1981 × 838 mm6’6” × 2’9”ImperialWider openings, easier access
2040 × 626 mm–MetricCupboards, bathrooms
2040 × 726 mm–MetricBathrooms, bedrooms
2040 × 826 mm–MetricMain rooms in newer homes

If your height measures around 1981 mm, you almost certainly have imperial openings. If it is closer to 2040 mm, they are probably metric. Openings that don’t match either may need a made-to-measure door or an adjusted lining. You can see the range of sizes and styles we fit on our internal doors page.

Working out the handing and hinge side

Handing describes which side the hinges are on and which way the door opens. It matters for hinge positions, latch and lock positions, and pre-machined or glazed doors that can only be hung one way.

The easiest way to record it is to stand on the side the door opens towards you. If the hinges are on your left, note it as hinges on the left. If they are on your right, note hinges on the right. Different suppliers use different terms for handing, so describe it in plain words rather than relying on “left-hand” or “right-hand” alone.

Also note which way the door swings into the room. A door that hits a radiator, a light switch or the end of the stairs might be better hung the other way.

Allowing for floor clearance and gaps

A properly hung door has small, even gaps all round. As a guide, allow about 2 to 3 mm at the sides and top of a standard internal door. The gap under the door depends on the floor covering, and is often somewhere around 8 to 10 mm over the finished floor.

Always measure to the finished floor level. If you are about to lay thick carpet, tiles or a wood floor, the door needs to clear the new surface, not the old one. Fire doors have tighter limits, usually around 3 mm at the sides and top, with the bottom gap set by the manufacturer’s instructions. Our guide to fire door regulations explains why those gaps matter.

Common measuring mistakes to avoid

  • Measuring only the old door, which may have been trimmed
  • Taking one measurement instead of three for each dimension
  • Forgetting that new flooring will raise the floor level
  • Mixing up imperial and metric sizes when ordering
  • Ordering a 44 mm door for a lining set up for 35 mm doors
  • Not checking that the frame is square and plumb

To check the frame, hold a long spirit level against each side and the head. If the lining is badly out, a new door will need careful trimming or the lining may need replacing. Our frames and linings service can sort this out. If you would rather not measure at all, our supply and fit service includes measuring every opening.

Door measuring questions we’re often asked

Can I just order a door the same size as my old one?

Only if you are sure the old door hasn’t been trimmed. Many older doors have been cut down over the years to clear carpets, so always check the opening as well before you order.

What if my opening is between two standard sizes?

Order the next size up and trim it to fit. Most doors can be trimmed a little on each edge, but check the manufacturer’s limits first, as fire doors and some glazed doors allow very little trimming.

Should I take the old door off to measure the opening?

There is no need. Open the door fully and you can reach the whole height and width of the lining. Taking the door off only helps if it is badly fitted and hides part of the frame.

More guides

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