Design / Structural
Compound Miter & Bevel Angle Calculator
Get the miter and bevel angles for a compound cut — crown molding, picture framing, sloped soffits.
Calculator
What makes this different
Standard miter calculators assume a 90° corner. This one handles two angles at once: the wall corner and the spring angle of crown molding, or the slope of a rafter meeting a fascia. It outputs both the miter and the bevel that your saw needs.
Who it's for & when to use it
Crown molding, picture frames, sloped rafter tails, octagonal assemblies.
How to use
- Enter the dimensions of your material or the parameters of your project.
- Adjust the optional knobs (waste factor, kerf, planer allowance) to match your shop practice.
- Click Calculate and read the output. Use the per-item breakdown for ordering, the totals for budgeting.
- Compare the result to your lumber yard's quote — the effective $/BF column is the apples-to-apples number.
Formula: Compound miter = atan(sin(spring) / (sin(corner/2) / tan(spring) + cos(corner/2))). Bevel = asin(cos(spring) × sin(corner/2)).
Notes & tips from the shop
- For standard 90° corners, set the miter saw to 31.6°/33.9° (miter/bevel) for a 38° spring — memorize this combo.
- Always test-cut two pieces and check the fit before committing to your good stock.
- Crown molding nested in the saw vs flat on the saw produces different bevel angles for the same wall angle.
Frequently asked questions
Why are my miters open at the wall?
The wall isn't a true 90° — measure both walls, plug the actual angles in. Most walls are 88–92°.
What's the spring angle?
The angle between the back of the molding and the ceiling line. 38° and 45° are the common sizes.
Can I use this for octagons?
Yes — set the wall angle to 135° (interior angle of an octagon) for picture-frame miters.
Limitations
Single compound angle. Splayed crowns (different spring angles on the wall and ceiling sides) need a more involved formula.
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