Miter saw

Flat trim: half the corner

Baseboard, casing and chair rail lie flat against the wall, so a corner is closed by two pieces each cut to half of it. A square corner is two 45s. That is the whole rule, and it holds at any angle — a 135° corner takes two 22.5s.

There is no bevel to set for flat trim. The blade stays vertical and only the table swings, which is why this is the easy case and why people are then ambushed by crown.

Crown: two settings, and neither is 45

Crown molding does not lie flat. It sits at an angle between the wall and the ceiling — its spring angle — and that tips the cut into three dimensions. Cutting it flat on the saw table needs a compound setting: the table swings to a miter and the blade tilts to a bevel, and neither number is the one you would guess.

For the common 38° crown in a square corner, the settings are these. They are printed on the side of a great many saws precisely because nobody remembers them.

Or stand it up against the fence

The alternative to a compound cut is to hold the molding in the saw at the same angle it will sit on the wall — upside down, with its ceiling edge on the saw table and its wall edge against the fence. Then the cut is a single miter with no bevel at all, which is why a lot of finish carpenters work this way and never touch the bevel scale.

It needs a stop or a jig to hold the spring angle consistently, and it takes a moment's thought to get the molding the right way up. In exchange, one setting instead of two and no chance of tilting the blade the wrong way on an outside corner.

Corners are not square

The numbers above assume a 90° corner, and an existing wall very rarely gives you one. Drywall mud builds up in the angle, framing moves, and a corner that measures 91.5° will show a gap on a 45° cut that no amount of caulk hides on a stain-grade job.

Measure the corner rather than assuming it. Both the crown and the flat trim calculators take whatever angle you actually measured, which is the entire reason they take an angle at all rather than a checkbox marked "corner".

What this page does not tell you

• —A spring angle is a property of the molding, not of the room. If it came off a job and nobody knows, measure how far it drops down the wall and how far it reaches across the ceiling, and the crown calculator will work it out from those two numbers.
• —Inside and outside corners take the same settings — what changes is which side of the line you keep. Mark the waste before you cut; it is the mistake that costs a length of molding.
• —Cut a test pair in scrap at the same settings before cutting the real molding. It costs two minutes and catches a saw whose scale has drifted.