Short answer
Mark a 3/16 in reveal on the jamb edge all the way around. Cut the head first: its short points sit on the reveal marks, so it is the opening width plus two reveals, mitered 45 degrees at each end. Tack it up. Then miter one end of each leg, fit it to the head, mark the length at the floor, cut, glue the miter and nail. Fill, caulk and paint.
Tools and supplies
Before you cut
- Check the jambs. They should be square to each other and flush with the wall. Plane or shim high spots; a proud drywall edge is the most common cause of a gappy miter (Fine Homebuilding).
- Check the saw. Make sure the blade is square to the table and fence.
- Buy enough. One side of a standard door takes about 17 linear feet; use the casing calculator.
Step by step
Mark the reveal
Set a combination square to 3/16 in and make pencil dashes on the jamb edge every foot or two, all the way around. Make sure the dashes meet at the two top corners: those points are where the miters land.
Measure the head
Measure across the opening between the side jambs and add two reveals. On a 36 in opening with a 3/16 in reveal, that is 36-3/8 in, measured on the short points of the miters.
Cut and tack the head
Cut 45 degree miters at both ends with the thick outer edge against the fence, so the short points are 36-3/8 in apart in that example. Hold it on the reveal marks and tack it with a few nails, leaving the heads slightly proud for adjustment.
Miter the legs
Cut a 45 degree miter on the top of each leg, leaving it long. Hold it in place with its miter tight to the head and its inner edge on the reveal marks, then mark where it meets the floor and cut it square there.
Glue and nail the legs
Spread glue on the miter end grain, align the joint, and nail the leg to the jamb every 12 to 16 in along the reveal line, then along the outer edge into the framing.
Pin the miters
Set the nails left proud on the head. A brad driven across the top of each miter, from the head into the leg, keeps the joint from opening.
Fill, caulk and paint
Fill the nail holes, caulk the gap between casing and wall (not the reveal), and paint. Sand filler flush before painting.
Fixing common problems
| Problem | Likely cause | Fix |
|---|---|---|
| Miter open at the front | Drywall proud of the jamb | Crush or shave the drywall near the corner |
| Miter open at the back | Jamb proud of the wall | Back-cut the miter slightly or shim behind the casing |
| Miter open at the top or bottom | Head jamb out of level | Recut one leg a hair long or short, or adjust the angle |
| Leg bows off the wall | Twisted board or hollow wall | Nail into framing; swap a bad board |
| Split at the end | Nail too close to the end | Pre-drill or move the nail; MDF edges are brittle |
Steps follow Fine Homebuilding and the Home Depot installation guide. Butted craftsman heads are covered in craftsman door trim; corner blocks in rosettes and plinth blocks. Finish with how to paint door trim.
Questions people ask
What is the reveal on door casing?
The small strip of jamb edge left showing between the casing and the inside of the jamb. Fine Homebuilding and the Home Depot installation guide both use 3/16 inch as typical. It hides small irregularities in the jamb and gives a crisp shadow line.
Do you install the head or the legs first?
For mitered casing, many carpenters install the head first, then fit each leg to it. Fine Homebuilding tacks the head with nails left slightly proud, so it can be adjusted while fitting the legs.
What nails do you use for door casing?
Fine Homebuilding tacks casing to the jamb with 4d finish nails. With a nail gun, 18 gauge brads into the jamb edge and longer 15 or 16 gauge finish nails through the outer edge into the framing are a common combination.
How do I fix a miter that will not close?
Check that the jamb and wall are flush. If the drywall stands proud, the miter opens at the front; shave or crush the drywall near the corner. If it opens at the back, back-cut the miter slightly. A thin shim behind one piece can also bring a miter flush.
Sources
Last reviewed October 12, 2026. Profiles, sizes and listings change; check the product details before you buy, and follow the maker’s instructions.