How do I cut door casing at 45 degrees?

Cut each piece with the mitre saw set to 45 degrees so the outer (long) edge is longer than the inner edge. Mark the reveal on the door jamb, measure your lengths along the long outer edge, then step in one casing width to the inner edge and join the marks to set each mitre. The free planner does this math for you.

How much casing do I need for one door?

A standard three-sided door casing uses two vertical legs plus one horizontal top piece, each mitred at 45 degrees. Because the mitres add length beyond the opening, you need extra material at each corner. The planner outputs exact piece lengths so you can buy the right amount with minimal waste.

Can Build Theory install trim and doors for me?

Yes. Build Theory is a design-build renovation firm in Mississauga serving the GTA. We handle trim and door installation as well as larger renovations, and we are Licensed and Insured with design by a Registered Designer.

Free Door Casing Cut Planner: Get Your Exact Cut List & 45° Miter Marks

Q: What is the reveal or offset on door trim?

The reveal is the small, consistent setback — commonly around 3 to 6 mm (about 1/8 to 1/4 inch) — between the inner edge of the casing and the face of the door jamb. It gives the trim a clean, intentional shadow line instead of sitting flush.

Trimming out a door looks simple until you're standing at the mitre saw trying to figure out exactly where to mark a 45-degree cut. A three-sided door casing — one top piece and two legs — has four mitres that all have to meet cleanly, and a mark that's a few millimetres off leaves a gap you'll see every time you walk past. Get it wrong and you're back at the lumber yard buying more trim.

So we built a free tool to take the guesswork out of it. Use the free door casing cut planner, enter a few measurements, and it hands you the exact pieces to cut, their lengths, and where to mark each mitre.

Why is marking door casing mitres so fiddly?

The tricky part of a mitred casing is that each piece isn't the same length on both edges. Because the cut runs at 45 degrees, the outer (long) edge is longer than the inner (short) edge by one casing width at each mitred corner. You can't just measure the door opening and cut to that number — you have to account for the reveal on all three sides and the extra length the mitres add.

Do that in your head, twice per piece, across three pieces, and it's easy to transpose a number or mark the wrong edge. That's exactly the kind of repetitive geometry a tool should handle for you.

What does the free door casing cut planner do?

The planner turns a few simple inputs into a ready-to-cut plan. You enter:

The three sides — the top, left, and right of the door jamb opening.
The casing width — how wide your trim stock is.
The reveal (offset) — the small, consistent setback from the edge of the jamb.

It then outputs:

The exact pieces to cut — the two legs and the top.
Their lengths, measured along the long outer edge.
Where to mark the mitres so all four corners meet cleanly.

Try the free tool here — it works whether you're a homeowner tackling a weekend door or a trade carpenter batching out a whole house.

How do I actually mark and cut the mitres?

Here's the method the planner is built around, so the numbers make sense at the saw:

Set your reveal. Pencil a light, consistent reveal line around the top and both legs of the jamb — commonly around 3–6 mm. This is your reference for everything.
Measure on the long (outer) edge. Each piece is longest on its outer edge; that's the number the planner gives you.
Step in one casing width on the inner edge. From each end, come in by exactly one casing width along the inner edge — that's where the 45-degree line starts.
Join the marks. Draw the line from the outer mark to the inner mark; that line is your mitre. Set the saw to 45 degrees and cut to it.

Cut the top piece and both legs the same way, dry-fit the corners, and adjust before you nail. The planner gives you the exact numbers so the first cut is the right cut.

What is the reveal, and why does it matter?

The reveal (also called the offset) is the small, even gap between the inner edge of the casing and the face of the jamb — usually around 3–6 mm. It does two jobs: it gives the trim a crisp shadow line so the casing looks intentional rather than flush, and it leaves room so a slightly out-of-square jamb doesn't throw off your mitres. Keeping the reveal consistent on all three sides is what makes a casing look professionally installed, and it's a required input in the planner for exactly that reason.

How much trim should I buy?

Because each mitre adds length beyond the door opening, a casing always needs more material than the raw opening dimensions suggest. The planner's piece lengths let you buy the right amount with a sensible offcut allowance — no guessing, and no second trip. If you're pricing out a bigger job, this is also handy for estimating trim across multiple doors at once. (For the broader question of what needs a permit on a renovation, see our guide to building permits in Ontario — trim and doors on their own typically don't.)

Prefer to have it done right the first time?

The tool is there to make a fiddly job easier, but not everyone wants to spend a Saturday at the mitre saw. Build Theory can handle the full trim and door install — or a larger renovation entirely. We're a design-build renovation firm in Mississauga serving the GTA, Licensed and Insured, with design by a Registered Designer under the Ontario Building Code. See how we work, and if you'd rather hand it off, talk to Build Theory about your trim, doors, or renovation.

Ready to cut? Use the free door casing cut planner and get your exact cut list and 45-degree mitre marks in seconds.