A compound miter calculator determines the exact miter and bevel angles for your table saw or miter saw when building polygon shapes with sloped sides. Enter the number of sides and splay angle to get precise blade settings for picture frames, planters, crown molding, and hopper joints.
Project Parameters
Saw Settings
Technical Blueprint
Precision Measurement Gear
Compound angles require absolute precision. If your table saw blade is off by even half a degree, the joints on your splayed planter or hopper box will have glaring gaps. We strongly recommend using a magnetic digital angle gauge to set your blade bevel perfectly to the decimal. For setting your miter gauge or chop saw rotation, throw away the cheap plastic protractor and use a high-quality machined aluminum miter protractor.
How to Calculate Compound Miter Angles
Cutting a standard box is easy: four sides, 90-degree corners, and straight vertical walls. You simply set your saw to a 45° miter. But the moment you angle (splay) the walls outwardlike when building a wooden planter, a flared crown molding, or a dust collection hopperthe standard 45° cut no longer works. This creates a "compound angle," requiring you to adjust both the Miter Angle (the rotation of your saw table) and the Bevel Angle (the tilt of your saw blade) simultaneously. AngleForge takes the heavy trigonometry off your shoulders. Simply input the number of sides your project has and the outward tilt (splay/pitch) of the walls, and our engine provides the exact decimal saw settings needed for a flawless, gap-free fit.
Frequently Asked Questions
What is a compound miter cut?
A compound miter cut combines two angles simultaneously: a miter angle (the blade rotates left or right) and a bevel angle (the blade tilts). This is required when joining boards that meet at both a horizontal angle and a vertical slope, such as crown molding, picture frames with sloped sides, or polygon planters.
How do you calculate compound miter angles for a polygon box?
For a regular polygon with N sides and a splay angle (tilt from vertical), the miter angle is arctan(cos(splay) × tan(180°/N)) and the bevel angle is arcsin(sin(splay) × cos(180°/N)). These formulas account for the 3D geometry of joints that are both angled and tilted.
Why don't my compound miter joints close properly?
The most common causes are: (1) using the wrong angle — many calculators give blade angle when you need the complementary angle, (2) saw calibration drift on the miter or bevel scales, and (3) not accounting for blade thickness eating into one side of the joint. Always make test cuts on scrap wood first.