Door Frame Trim Calculator

How much trim does a door frame need?

Door casing, also called architrave, is the trim that frames a doorway and hides the joint between the frame and the wall, and ordering it is a small job that is easy to get wrong by a surprisingly large amount. A single side of a door needs two vertical legs and one horizontal header, and the total is not the door's perimeter, because the bottom is open and the header spans the width plus the casing reveal on both sides. The calculator on this page takes door width and height, adds two legs plus one header, and includes ten percent for mitre cuts and mistakes, giving you the length to buy for one side of one door. Double it for both sides, and multiply by the number of doors for the whole project.

The formula is simple once you see it. Two side casings run the full height of the door, from floor to the top of the header, and the header runs across the top, spanning the door width plus the width of one casing on each side, or roughly the door width plus a small allowance. In practice, casing is bought in lengths and cut to fit, so the calculator reports the total linear length needed rather than individual pieces, which matches how trim is sold. For a standard interior door of eighty by two hundred and five centimetres, the two legs plus header total four hundred and ninety centimetres before waste, and five hundred and thirty-nine centimetres, about five point four metres, after the ten percent allowance. That is the number to take to the timber merchant.

Measuring your door correctly

Measure the door opening, not the door slab, because casing follows the frame. Width is measured between the inside faces of the frame jambs, and height from the finished floor to the underside of the head jamb. Measure twice, at both sides for height, because floors are rarely perfectly level and older houses settle, leaving one side a few millimetres longer than the other. If you are replacing existing casing, you can measure the old pieces directly, but check that they were fitted correctly in the first place: a common error is legs that stop short of the floor or a header that does not overhang evenly on both sides. New casing should sit with a consistent reveal, usually five to six millimetres, back from the edge of the jamb all the way around.

Choosing a casing profile

Casing profiles range from plain square-edged boards to ornate Victorian and colonial patterns, and the choice should match the house, not the fashion of the moment. Simple flat or pencil-round profiles suit modern homes and are the easiest to mitre cleanly. Traditional homes suit ogee, torus, or Georgian profiles, which catch light and shadow and look right against panelled doors and skirting of the same period. Width matters too: seventy millimetres is a modest modern width, ninety to one hundred millimetres is a comfortable traditional width, and wider casing makes a grander statement in rooms with high ceilings. Match the casing to the skirting board where possible, because mismatched profiles in the same room look accidental.

Material choice affects cost, finish, and durability. Solid pine and hardwood casings take paint or stain beautifully and can be sanded and repaired, but they move with humidity and need priming on all faces in damp rooms. MDF casing is cheaper, perfectly straight, and ideal for painted finishes, but it swells if it gets wet and does not hold screws as well at joints. Pre-primed MDF is the common choice for painted interiors, while hardwood suits stained finishes and period restorations. Buy casing in the longest lengths available for your longest pieces, because a joint in the middle of a leg is visible and weak, and keep offcuts: a short piece is often exactly what a small cupboard door or a window needs.

Cutting and fitting

Cut the header first, mitring both ends at forty-five degrees, then cut the legs with a mitre at the top and a square cut at the floor, allowing the legs to be scribed to an uneven floor if necessary. Dry-fit all three pieces before fixing anything, checking that mitres close tightly and that the reveal is even around the jamb. Pin or nail the casing to the jamb and to the wall studs, not just to the plaster, and punch nails below the surface for filling. Mitre joints deserve glue as well as pins, because a glued mitre resists seasonal movement far better than a pinned one alone. Fill nail holes and any small gaps with filler, sand lightly when dry, and prime before painting, including the back of the casing if it will be exposed to moisture.

The most common fitting mistakes are legs cut too short, mitres that gap because the wall is not square, and casing fixed only to plasterboard, which eventually works loose. If a mitre gaps, check whether the wall is out of square and adjust the mitre angle slightly rather than forcing the joint closed, which springs open again. For uneven floors, scribe the leg by holding it in place, marking the floor line, and trimming to it, rather than leaving a gap filled with caulk. Caulk has its place for small gaps against uneven walls, but it is not a substitute for a well-cut joint, and paint over caulk only after it has fully cured.

Both sides, every door, and windows

Remember that the calculator gives one side of one door. Most doors need casing on both faces, so double the figure, and a house with ten doors needs ten times the single-side figure, plus a little extra for the inevitable miscut. Window architraves follow the same logic but with four sides: two legs, a header, and a sill, or a full picture-frame of four mitred pieces. Skirting boards are measured separately in linear metres around each room, minus door openings. Buying all trim for a project at once, from the same batch and supplier, keeps profiles and timber colour consistent, and ordering ten percent extra across the project covers mistakes without a second delivery charge.

• Measure the opening, not the door slab, at both sides for height.
• One side = two legs + one header; double for both sides.
• Keep a consistent 5-6 mm reveal around the jamb.
• Glue mitres as well as pinning them.
• Buy the longest lengths available to avoid mid-leg joints.

Door casing is a small line item that makes a large visual difference, and it rewards accurate measuring. Use the calculator above for each door size, double for both sides, and order with the waste allowance included, so every door in the house gets a crisp, even frame.

Painting and finishing new casing

New casing looks unfinished until it is primed, filled, and painted or stained, and the finish deserves the same care as the fitting. Prime all faces, including the back, before installation if possible, because unprimed timber absorbs moisture from plaster and paint unevenly. After fitting, fill nail holes with a filler matched to the finish, sand smooth when dry, and caulk the joint between casing and wall with a paintable flexible caulk, which hides small gaps from uneven walls. Apply two topcoats for durability, sanding lightly between coats, and use a semi-gloss or satin paint on trim, because it resists scuffs and wipes clean far better than flat paint. For stained hardwood, apply the stain evenly with the grain, wipe off excess promptly, and finish with two coats of varnish or oil, allowing full curing time before the room returns to normal use.