How to Measure a Door and Pick the Right Size
How to Measure a Door: Clearances, Sizes and Floors
Measuring a door accurately is essential if you want to avoid draughts, sticking, and ensure a perfect fit. Our guide explains how to measure a door frame and take precise recording door measurements in metric millimetres, whether you have an existing frame or are measuring for a new door set. We’ll also cover clearances, floor coverings, and when to go bespoke.
- Do I measure the door or the frame?
For most front door replacements, always measure the frame rather than the door itself, as doors can swell or shrink over time. The frame provides the true opening, helping your measurements be accurate. If no frame exists, use a metal tape measure to record brick-to-brick dimensions, then deduct the frame size so your new door fits seamlessly. - What gaps do I need to leave around a door?
When measuring doors, allow small, even gaps so the door opens and closes smoothly. A clearance of around 3mm at the top and sides and 9mm at the bottom usually works. This prevents the door from rubbing on carpets or tiles.Many modern doors feature lipping (a solid timber edge strip), so a fitter can trim the door slightly to achieve the correct clearance. - What are internal doors standard sizes in the UK?
When learning how to measure a door, it helps to know that most manufacturers work to common door sizes. The typical internal door is 1981 × 762 × 35mm (6’6” × 2’6”), but other standard sizes include:- 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)
Also, check door thickness – fire-rated doors (FD30) can be 44mm thick rather than 35mm. Working with the closest standard size and knowing trimming limits is key before ordering bespoke doors.
- How do I measure if there isn’t a frame?
Without a frame, measuring a door means taking brick-to-brick measurements. Measure the width at the top, middle, and bottom of the opening and note the smallest reading. For height, measure left, centre, and right, again taking the smallest measurement. Oversized openings are safer than undersized ones. Installers can pack and trim around undersized frames if necessary.
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Measure for a doorset with a frame (replacing an internal or external door)
When measuring for a doorset, leave the door in the frame and take internal measurements to ensure a perfect fit.
Let First Class talk you through the next steps:
- Measure width between inside edges at top, middle, and bottom. Use the smallest reading.
- Measure height from the top of the frame to the floor (or threshold) at multiple points. Again, take the smallest.
- Measure frame depth and door rebate for alignment.
- Note door swing (inward/outward) and handedness.
- Check the frame is square by measuring diagonally corner to corner.
Door width (inside the frame)
The vertical parts of a door frame are called jambs.
- Measure the inside width from jamb to jamb at the top, middle, and bottom.
- Record the widest measurement for ordering a door as this reduces the need for trimming and helps achieve a perfect fit.
Door height (finished floor to underside of head)
The best and easiest way to measure a door frame for height:
- Measure from the finished floor (or threshold) to the underside of the frame at the left, centre, and right.
- The tallest reading gives the correct height to stop the door scraping the floor.
The door thickness & frame rebate
When measuring doors, don’t forget to include door thickness and rebate, as this is also important.
- Measure the edge of the door (the part that sits in the frame) from face to face. You can use a ruler.
- Then measure the rebate’s width and depth.
- A replacement door must match the rebate for a flush fit and proper seal.
Before you get started
To avoid a poorly fitting door, you need to prepare properly. Use a metal tape measure, pencil, and paper. Make sure you clear obstructions and remove door stops or trims before measuring doors.
Take all measurements in metric millimetres and record the smallest width, height, and depth readings. We recommend you always double-check measurements before ordering.
Clearances, trimming and floor coverings
A key part of how to measure a door is leaving enough clearance: roughly 3mm at the top and sides and 9mm at the bottom. This allows smooth movement and accounts for expansion or contraction.
Make sure carpets or tiles don’t obstruct the bottom, and follow manufacturer trimming limits. While choosing your replacement door, consider the best front door colours and the best front doors for security UK too.
Map your measurements to UK standard sizes (and when it's best to go bespoke)
Once you’ve double-checked your measurements, compare your recording door measurements with common door sizes: widths 540–838mm, heights 1981 or 2040mm, and thickness 35-44mm. Ensure clearances: 2-3mm top, 2mm sides, 8-10mm bottom. If trimming can achieve these gaps, a standard door will work.
You should go bespoke if the opening exceeds standard sizes, the frame is out-of-square, or a non-standard thickness is required.
If measuring for a door feels tricky, let the First Class team handle it. We’ll visit, inspect the opening, and take exact dimensions so your new door is a perfect fit. Our service makes replacement doors in South Wales easy, precise, and stress-free.
Request your free door quote
Simply fill in your details for your free, no obligation quotation, and click “Get Quote”