How to Get Door Casing Measurements Right
A new door can be perfectly hung and still look unfinished if the trim around it is the wrong size, cut short or out of proportion. Accurate door casing measurements prevent wasted architrave, awkward joints and last-minute compromises with a profile that does not suit the room.
In the UK, the visible moulding around an internal doorway is usually called architrave. “Door casing” can sometimes refer to the complete door lining as well as the decorative trim, so it is worth separating the two before you measure. This guide focuses on measuring for the architrave that fits around the face of the door lining.
Start by identifying what you are measuring
There are three dimensions to establish before ordering: the height of each side, the width across the top and the width of the architrave itself. You also need to know whether the room has its finished floor level in place, as this determines where the vertical lengths start.
Measure the door lining rather than the door leaf. The lining is the timber frame fixed within the wall opening. Architrave sits over the joint between this lining and the wall, leaving a small, even reveal of lining visible around the door.
For a standard internal doorway, you will normally need two vertical lengths and one head piece. A single door opening with trim on both sides requires two complete sets of measurements. Do not assume both sides are identical, particularly in older properties where floors, walls and openings can vary.
Door casing measurements for the reveal line
The reveal is the narrow strip of door lining left visible between the architrave and the inside edge of the lining. A 5mm to 6mm reveal is common, but it should be consistent with the rest of the property. Mark this line with a pencil before taking final measurements.
Start by measuring from the finished floor to the horizontal reveal line at the top of the lining. Take this measurement on both sides of the door. Even where the head appears level, check each side independently. A difference of a few millimetres can matter when fitting a square-edged modern profile or matching a nearby door.
Next, measure between the two vertical reveal marks across the head of the door lining. This is the internal width for your top piece. Keep the tape level and measure at the line where the inner edge of the architrave will sit, rather than measuring the outside of the lining at random.
If flooring has not yet been fitted, allow for its final thickness. Carpet and underlay, engineered wood, laminate and tile all change the finished floor level. Measuring from the subfloor without making this allowance often leaves an unwanted gap below the architrave, or means the bottom must be cut back later.
Allowing for mitred joints
Most architrave is fitted with 45-degree mitres at the top corners. The measurements marked on the lining are not the same as the finished cutting lengths of the pieces.
For a mitred set, use the width of the selected architrave to calculate the outer points of the mitres. If the height from floor to top reveal is H, the width between the vertical reveal lines is W and the architrave width is A, the approximate finished lengths are:
- Each vertical leg: H + A
- Top head piece: W + (2 x A)
For example, with a 2,040mm height, a 760mm head width and 69mm architrave, each leg will need roughly 2,109mm and the head piece roughly 898mm. These are cutting lengths, not just lining dimensions.
This approach assumes standard 45-degree mitres and a square, reasonably level opening. Always offer the first piece up before cutting every length to size, especially when renovating a period property. Walls may not be plumb and the head may not be level, even if the door itself operates correctly.
Measuring for butt joints
Butt-jointed architrave is common with contemporary square, chamfered and pencil-round profiles. In this arrangement, the vertical legs usually run full height and the head sits between them, or the head runs over the legs depending on the preferred detail.
There is no single formula for every butt joint because the cutting order changes the required length. Decide which piece will cover the joint, then measure to the actual meeting point. Marking the reveal lines and holding an offcut of the chosen architrave against the lining makes this much clearer than working from opening sizes alone.
Choosing the right architrave width
Door casing measurements are about proportion as well as length. A narrow 44mm architrave may suit a compact new-build room, while a 69mm or 94mm profile can give a more substantial finish in a larger space or period-style renovation.
The available wall space matters. Check the distance from the edge of the door lining to the nearest corner, wall return, switch or adjoining trim. If the architrave is too wide, it may run into a return or leave a narrow, difficult strip of wall beside it. On tight openings, a slimmer profile often gives a neater result than forcing a wide moulding into the space.
For a coordinated finish, match the architrave style to the skirting board. They do not have to be the same height, but profiles from the same range will generally sit together more naturally. A simple modern architrave can work with deeper skirting, while more detailed traditional profiles are usually best paired with skirting that carries a related moulding detail.
Measure wall thickness when ordering door linings
If “door casing” refers to the structural lining rather than the visible trim, the key measurement is different. You will need the finished wall thickness, including plasterboard, plaster, tiles or panelling where relevant.
Measure the wall in several places across the opening. Older solid walls and newly formed stud walls can both vary. Order or assemble a lining to suit the widest point, then pack, scribe or trim during installation as required. A lining that is too narrow will not cover the wall depth cleanly, while one that is too wide may need planing back.
Also confirm the opening width and height, the door leaf size, handing, hinge positions and whether the lining needs a stop. These are installation measurements, not architrave measurements, but getting them wrong affects every finishing detail that follows.
Work out how much architrave to order
Once each opening has been measured, add the required cutting lengths together. Include both sides of each doorway if both faces will receive architrave. Then add a sensible allowance for mitres, imperfect walls, damaged ends and selecting the best grain on oak or pine.
For straightforward MDF architrave in a regular property, an allowance of around 10% is usually sensible. Increase this where openings are noticeably out of square, the profile has a directional grain effect, or you are fitting several detailed mitres. It is more economical to allow for a small amount of waste than to stop the job for one short length.
Check the stock lengths available before finalising your order. A 2,100mm vertical leg may not be achievable from a 2.1m board once a clean square cut and mitre allowance are included. Ordering longer lengths can reduce joints and make cutting more efficient, particularly on taller doorways.
Where several doors use the same profile, prepare a cutting plan. Group the longest legs first and identify whether offcuts can become heads or shorter legs. This is particularly useful on trade jobs, where careful planning reduces both material waste and fitting time.
Common measuring mistakes to avoid
The most frequent error is measuring the opening only and forgetting the width added by mitres. Another is taking dimensions before the final floor finish is known. Both can leave fitters trying to conceal a gap with filler or an overly large bead of caulk.
Do not measure one doorway and assume the rest match. Nominally identical openings can differ enough to show when fitted with crisp, straight architrave. Measure every opening, label each set clearly and keep a note of whether it is viewed from the hall, bedroom, landing or another room.
It is also worth checking for obstructions before choosing a profile. Radiator pipes, sockets, dado rails, wall panelling and low ceilings can all influence the best width and design. Free samples are useful here, as they let you hold the actual profile against the lining and wall before committing to full lengths.
A practical check before you cut
Before cutting the first length, mark the reveal lines, hold the architrave in position and confirm the profile sits flat against both wall and lining. Check the finished floor level, the direction of any moulded profile and the space around nearby corners.
For made-to-specification architrave, recording door casing measurements carefully at the start makes ordering simpler and fitting more predictable. Take your time with the first opening, use it as the reference for the rest of the project, and keep a little extra material available for the unexpected angles that older homes often reveal.