How to Cut Compound Miter Angles

A compound miter cut combines two angles at once: a miter (across the face) and a bevel (through the thickness). You need compound cuts whenever pieces meet at a corner and are tilted — crown molding is the most common example, but picture frames with sloped profiles, multi-sided planters, and angled joinery all require them.

The math behind compound miters involves trigonometry, but you don't need to do it by hand.

When you need a compound miter

A regular miter (like cutting a 45° angle for a picture frame flat on the table) only involves one angle. You need a compound miter when the workpiece is also tilted — meaning the joint has angle in two planes simultaneously.

Common situations:

• Crown molding — the most common. Crown sits at an angle between the wall and ceiling (the "spring angle"), and two pieces meet at a wall corner.
• Multi-sided boxes or planters — if the sides are sloped (not vertical), each joint needs a compound cut.
• Picture frames with a sloped profile — the frame cross-section isn't flat, so the miter joint needs a bevel too.
• Roof rafters at hip and valley intersections — the rafter meets the ridge at a compound angle.

The two angles explained

Miter angle — the angle you set on your miter gauge (the horizontal rotation of the blade). On most saws, this is the number you read on the scale at the base of the saw.

Blade tilt (bevel) — the angle the blade tilts from vertical. On most saws, this is adjusted by tilting the motor/blade assembly.

Both settings must be correct for the joint to close. Getting the miter right but the bevel wrong (or vice versa) produces a gap that no amount of sanding will fix.

Crown molding spring angles

Crown molding has a specific "spring angle" — the angle between the back of the crown and the wall. The two standard types:

• 52/38 crown (38° spring) — the most common residential crown. The back makes a 38° angle with the wall. Sometimes labeled "52/38" because the ceiling angle is 52°.
• 45/45 crown (45° spring) — equal angles to wall and ceiling. Less common but simpler to cut.

If you're not sure which type you have, hold a piece in position against the wall and measure the angle between the back and the wall. Or check the crown molding angle chart for pre-computed settings.

Setting up your saw

1. Find your angles. Use the compound miter calculator or the crown molding chart.
2. Set the miter gauge to the miter angle value. Note whether your saw reads the angle from the blade or from the fence — if from the fence, use the complementary angle (90° minus the value).
3. Tilt the blade to the bevel value. Same note about complementary — some saws show 0° when vertical, others show 0° when fully tilted.
4. Test on scrap. Cut two pieces of scrap at the same settings, hold them together at your corner. The joint should close tightly with no gap.
5. Mark left and right. For inside corners, the long point of the miter is on the back (wall side). For outside corners, the long point is on the front (visible side). Label your pieces before cutting.

Inside vs. outside corners

The miter saw settings are the same magnitude for inside and outside corners at 90° — but the cut direction reverses. For an inside corner, the long point of the cut is on the back of the molding. For an outside corner, the long point is on the front.

The most common mistake: cutting two pieces for an inside corner and realizing one of them is backwards. Always mark "inside" or "outside" on your workpiece before cutting, and indicate which edge is the long point.

Troubleshooting

• Gap at the top of the joint: Your bevel angle is slightly off. Increase the blade tilt by 0.5° and test again.
• Gap at the bottom: Decrease the blade tilt by 0.5°.
• Joint opens on one side: Your miter angle is off, or your walls aren't exactly 90°. Measure the actual corner angle and recalculate.
• Pieces don't sit flat against wall and ceiling: Your spring angle measurement is wrong. Re-check with a protractor.