Architrave and casings are measured per opening and bought by the length, which means two different units meet in the middle and the waste happens where they meet. A door needs roughly five metres of architrave — two legs and a head, plus the mitres at the corners — and a window needs slightly less. Those five-metre openings do not pack neatly into 2.4-metre lengths, and every change of direction is a mitre that costs a piece of material. This adds up the openings, converts them into the lengths you buy, and shows how much of what you buy ends up in the offcut bin.
Openings, lengths and price
Trim is priced per opening, bought by the length, and wasted at the mitres.
Two legs plus a head, mitred. About 5.0 m for a standard 2.0 m door.
Four sides for a casement with a sill; about 4.2 m for a 1.2 x 1.2 m window.
Twelve percent is normal; go higher with many short pieces or a deep profile.
Result
Why trim is measured per opening and bought by the length
Architrave is a frame, and a frame is a fixed number of pieces with a fixed number of corners. A door has two legs and a head, which is four mitres and about five metres. A casement window has four sides, which is four mitres and about four metres. The measurement that governs the job is the opening count, not the wall length — a house with eight doors needs the same trim as a house with four doors and four windows of similar size, and neither figure has anything to do with the floor area.
The purchase, though, is by the length. Two and a half lengths of trim come out of every door, and the half is the problem: the offcut from one door is often just too short for the leg of the next. That is what the cutting allowance is for, and it is why the allowance grows with the number of openings rather than with the total metres.
The mitre is where the material goes
A mitre is a 45-degree cut, and a 45-degree cut across a board removes a length equal to the width of the board. On 70 mm architrave that is 70 millimetres per cut, or 280 millimetres per opening across four corners — over a metre of material on a house with four or five openings, all of it in the bin.
Deep profiles waste more, because the diagonal across a deeper board is longer. A torus or ogee profile at 95 mm loses noticeably more per mitre than a square-edged board at 70, and that difference is exactly the sort of thing that turns a twelve-percent allowance into a twenty-percent one.
The practical consequence is that trim is one of the few materials where buying longer lengths genuinely reduces waste, because it reduces the number of joints inside openings as well as between them. A 3.0 m length covers a door leg plus most of a head; a 2.4 m length does not.
A mitre cut exactly to the measured length will be short, because reveals are not square and reveals are never quite the width on the tape.
Cut two millimetres long, test the fit, and take another shaving. A mitre that is a millimetre proud can be sanded back; one that is a millimetre short is a recut.
Ordering the work
Trim goes on after the plaster is dry, after the floor is down and after the decoration of the walls if the walls are being painted first — though most decorators prefer to fill and prime the trim, paint the walls, then cut in the trim last, because it gives a cleaner line than masking. Either way the sequence matters more than the measurement, and getting it wrong means redoing the cutting in.
Fit the head first, then the legs. The head is cut to the exact opening width plus two mitres, and the legs then fit up under it. Working the other way round means the head has to be scribed to two legs that are already fixed, which is possible and much harder. Pin the mitres rather than relying on adhesive, because mitres open.
| Opening | Pieces | Mitres | Metres | Note |
|---|---|---|---|---|
| Standard door, 2.0 m | 3 | 4 | 5.0 | Two legs plus a head |
| Tall door, 2.4 m | 3 | 4 | 5.8 | Longer legs, same corners |
| French doors, 1.5 m | 3 | 4 | 6.2 | Wide head |
| Casement window 1.2 x 1.2 | 4 | 4 | 4.2 | Four sides, sill included |
| Sash window 1.2 x 1.5 | 4 | 4 | 4.8 | Taller, more on the legs |
| Small window 0.6 x 0.6 | 4 | 4 | 2.4 | Exactly one length, no waste at all |
A worked example
- DoorsSix doors at 5.0 m each is 30 m of casing.
- WindowsFour windows at 4.2 m each is 16.8 m.
- Total to fit46.8 m across ten openings, with forty mitres to cut.
- Convert to lengthsTwelve percent for mitres takes it to 52.4 m, which at 2.4 m is 22 lengths — 52.8 m bought and 6.0 m binned.
- Price itTwenty-two lengths at $12 is $264, or $26.40 per opening.
Twenty-six dollars an opening is the number worth remembering, because it scales linearly and it is almost entirely independent of how big the room is. A house with twenty openings costs twice what a house with ten costs, whatever the floor area.
Fitting and finishing
Use a mitre box or a mitre saw with a positive stop at 45 degrees; freehand mitres on architrave are visible from across the room. Cut with the face up on a chop saw so any breakout happens on the back, and use a fine-toothed blade — a 60-tooth or finer blade on MDF pays for itself in the time saved on sanding.
Fix with pins into the frame and the wall, not just into the plaster, and punch the nails below the surface before filling. Adhesive on the back of the board helps hold the mitre closed while the pins go in, and a dab of glue in the mitre itself is worth it on external corners.
The skirting length figure covers the same job at floor level, and the two are usually done together in the same material, so it is worth running both before ordering — the offcuts from one often suit the other. Where the walls themselves are being finished first, the wall area figure sets the sequence.
Do
- Count the openings first — that is what drives the job.
- Buy longer lengths if the supplier stocks them; it cuts mitre waste.
- Fit the head before the legs.
- Cut long and creep up on the mitre rather than cutting once.
- Pin and glue the mitres, then fill and sand.
Avoid
- Measuring by wall length rather than by opening count.
- Assuming a twelve percent allowance covers a deep profile.
- Mitring freehand, or with a coarse blade on MDF.
- Fitting legs before the head and trying to scribe the head to them.
- Relying on adhesive alone to hold a mitre closed.
Where the numbers stop being true
The per-opening figures assume mitred corners and a standard reveal depth. Deep reveals need wider architrave or a lining, and a reveal deeper than about 150 millimetres usually needs a lining board before any trim goes on, which is a separate material entirely.
Plinth blocks, corner blocks, rosettes and any decorative element break the linear model, because they replace mitres rather than adding to them — which sometimes reduces the material and always changes the cutting. Scribed rather than mitred corners, common on site-painted softwood, waste less and look worse if done badly.
Nothing here accounts for the number of openings that turn out not to be square. Older door openings are frequently out by ten or fifteen millimetres over their height, which means one leg is cut to a slightly different angle than the other, and the head has to be scribed rather than mitred. Budget an extra length or two on an old house.
How the figures are produced
Doors and windows are multiplied by their per-opening trim lengths and added to give the run to fit. The cutting allowance is applied, the result is divided by the length you buy and rounded up to whole lengths, and the cost is lengths times price. Mitres are four per opening, which is the count for a standard three-piece door or four-piece window with mitred corners. The stack splits the bought material into doors, windows and the allowance.
Questions people ask
How much trim does a door really need?
About five metres for a standard 2.0 m door: two legs of about 2.1 m and a head of about 0.95 m, plus four mitres. Taller doors need more on the legs.
Why is the waste allowance higher for trim than for skirting?
Because every opening has four mitres, and a mitre removes a board-width of material. The waste scales with the number of openings, not with the metres.
Can I use the offcuts from one opening on the next?
Sometimes. A door leg offcut of about a metre will fit a small window, which is why it is worth cutting the long pieces first and keeping the offcuts sorted by length.
Do I need a plinth block?
If the skirting is deeper than the architrave, yes — it avoids scribing the architrave over the skirting and looks intentional rather than accidental.
Should trim be painted before or after fitting?
Prime before fitting, and put the topcoat on after the walls are done and the mitres are filled. That gives the cleanest line against the wall.
What about a window with a sill?
Count the sill as a piece. A four-sided casement with a sill is four pieces and four mitres, not three.