How to Measure a Door (UK): The No-Mistakes Guide

Measuring a door needs to be done with optimum accuracy if you want to avoid draughts, any sticking and achieve the perfect fit. Our guide will clarify how to go about it, providing expert advice on precise measuring in millimetres for both existing frames and new door sets. We’ll also give direction on gaps, floor coverings and when to go bespoke.

• Do I measure the door or the frame?
  For a typical front door replacement, measure the actual frame, not the existing door, as it may have swelled or shrunk over time. The frame will offer a true opening, helping you get sizing right. But, because we always replace the frame too, we recommend brick-to-brick measuring for this particular step. If no frame exists, use your metal tape measure to get a brick-to-brick recording, deducting the frame size from that measurement so a new door fits seamlessly.
• What gaps should I leave around a door?
  So a door doesn’t catch when being opened and closed, leave some small, even gaps around the edges while recording door measurements. Around 3mm at the top and sides and approximately 9mm at the bottom will suffice to stop it scuffing on your carpet or rug. Most types of doors (specifically new doors) have a degree of lipping (a solid timber strip along the edges) so that a door fitter can slightly plane or trim a door to get the required clearance.
• What are internal doors standard sizes in the UK?
  When looking to learn how to measure a door, be aware that manufacturers generally work to a group of common door sizes. The most typical size is 1981 x 762 x 35 mm (roughly 6’6” × 2’6”), but there can be slight anomalies. Here are a few additional average UK internal door sizes:
  • 1981 × 610 × 35 mm (6’6” × 2’0”)
  • 1981 × 686 × 35 mm (6’6” × 2’3”)
  • 1981 × 838 × 35 mm (6’6” × 2’9”)
  • 2040 × 826 × 40 mm (metric size)

  Consider door thickness too, as fire-rated doors (FD30) exceed the thickness of standard internal doors by around 9 mm (44 mm against 35 mm).

  Work to the closest standard size and confirm trimming options ahead of placing a bespoke order for the strongest front door.

• How do I measure when there’s no frame?
  Without an existing frame, measuring doors must involve direct measurement of the brickwork opening. Get the width brick to brick in a straight line across the top, middle and bottom of the opening and jot down the smallest measurements. For the height, go brick to brick in a straight line on the left, centre and right sides of the opening, recording the smallest measurement again. Using these dimensions, you can calculate the required door set size, so long as you deduct the specified allowance. It’s better to have an oversized opening than undersized, as an installer can pack and trim around an undersized frame if necessary.

Measuring for a doorset, when looking to replace an internal or external door, customarily sees the measuring of the existing opening with the door frame in situ.

How to measure a door frame in this instance calls for measurements to be taken from the inside to ensure the new door fits nicely in the existing frame structure.

Let’s talk you through it in numbered steps:

1. Collect the width with a measurement of the distance between the inside edges of the frame at the top, middle and bottom. The smallest measurement is the crucial recording.
2. Door height is captured by measuring from the top frame to the floor (or threshold, if there is one) at several points. Once more, use the smallest measurement.
3. Don’t forget to also measure the depth of the door frame and the door rebate (this is where the door sits within the frame) for the right alignment.
4. Be mindful of whether the door opens inwardly or outwardly and whether it is left- or right-handed so that it correctly hangs.
5. Finalise the measuring process by checking the frame is perfectly square, which you will uncover by measuring diagonally from corner to corner.