Easy Wood Calc

Compound miter calculator

For a box with sides that lean outward, enter the number of sides and their tilt from vertical to get the miter and bevel for each corner.

Enter your measurements

degrees
0 = straight sides
in
Leave blank if you only need the saw settings.
in
Measured on the outside face, corner to corner.
Blade bevel (tilt)43.1 degrees
Miter (table) angle14.5 degrees
Table-saw miter gauge (reads 90° at square)
75.5 degrees
Top & bottom edge bevel
15 degrees
Side board width (up the slope)
12-7/16 in
Bottom edge of each side (outside)
16 in
Top edge of each side (outside)
22-7/16 in
How we got this

bevel = asin(cos 15° × sin(180° ÷ 4)) = 43.1°

miter = atan(sin 15° × tan(180° ÷ 4)) = 14.5°

Side board width = 12 in ÷ cos 15° = 12-7/16 in

Top edge = 16 in + 2 × 12 in × tan 15° × tan(180° ÷ 4) = 22-7/16 in

  • Angles are measured from square, the way most miter saws are marked: 0° miter and 0° bevel is a plain crosscut.
  • Side boards are cut as trapezoids: the bottom edge is the base length and the top edge is longer, both measured on the outside face from long point to long point.

For this job · affiliate link
Digital angle finderA saw’s bevel scale is hard to read closer than about half a degree. Set the blade to 43.1° with a digital angle finder, then make test cuts in scrap.
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Quick answers for common sizes

Worked examples for the Compound Miter Calculator
ExampleAnswer
Compound miter angles for a 4-sided planter with 15° sides43.1 degrees

Settings by side tilt (same number of sides)

Settings by side tilt (same number of sides)
Side tilt from verticalBlade bevel (tilt)Miter (table) angle
0°45 degrees0 degrees
5°44.8 degrees5 degrees
10°44.1 degrees9.9 degrees
15°43.1 degrees14.5 degrees
20°41.6 degrees18.9 degrees
30°37.8 degrees26.6 degrees
45°30 degrees35.3 degrees
60°20.7 degrees40.9 degrees

Planters, hoppers and tapered bins all have sides that lean outward, so their corner joints need both a miter and a bevel. Add the box height and base size to also get the width of each side board and the length of its top and bottom edges. New to this? Words explained.

The formulas

With the board lying face-down on the saw table and the cut running across its width, the two settings depend only on the number of sides (n) and the tilt of the sides from vertical (t):

blade bevel = asin( cos t × sin(180° ÷ n) )
miter angle = atan( sin t × tan(180° ÷ n) )
top & bottom edges: rip at t so they sit level
Three sketches: a top view of a board against the fence with the blade swung to a miter angle; a front view with the blade tilted to a bevel angle; and a section through a box whose sides lean outward from vertical by the tilt angle.
The miter is the saw table’s swing, across the face of the board. The bevel is the blade’s tilt, through the board’s thickness. The side tilt is how far each side of the box leans out from vertical — the number you enter.

Check the extremes: with straight sides (t = 0) a square box needs a 45° bevel and no miter; laid flat (t = 90°) it becomes a picture frame with a 45° miter and no bevel.

Four-sided box, side tiltBlade bevelMiter
5°44.8°5°
10°44.1°9.9°
15°43.1°14.5°
20°41.6°18.9°
30°37.8°26.6°
45°30°35.3°

Board width and edge lengths

Because the sides lean, each board is wider than the box is tall, and its top edge is longer than its bottom edge. Measure the height straight up and the base along the outside of the bottom; the calculator gives the width to rip each board to and the length of its two edges on the outside face.

side board width = height ÷ cos t
top edge = bottom edge + 2 × height × tan t × tan(180° ÷ n)

For a four-sided box tan(180° ÷ 4) = 1, so each top edge is the bottom edge plus twice the outward lean. A planter 12 in tall with 15° sides leans out about 3-3/16 in on each side, so a 16 in base becomes a top edge of about 22-7/16 in, and the boards are about 12-7/16 in wide.

Cutting tips

  • Make a full set of test pieces in scrap and dry-fit them with a band clamp before touching your good stock.
  • Alternate the orientation of each cut (flip the board end for end) so the angles mirror correctly at each corner.
  • Many saws cannot bevel past 45–48°. For shallow-tilt boxes near 45° bevel, a table saw with a miter gauge is often easier.
  • The top and bottom edges need ripping at the tilt angle, or the box will rock and the rim will not be level.

Worked examples

The most common case, a square planter with a 15° flare, is worked through with board widths and saw setup on Compound miter angles for a 4-sided planter with 15° sides.

TipCrown molding is the same geometry: its “sides” are tilted by the spring angle. The crown molding calculator does it for any wall corner — the everyday case is Crown molding miter angle for a 90° inside corner (38° crown).
CalculatorMiter Angle Calculator →Straight-sided frames only need a simple miter.

What you’ll need

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Frequently asked questions

What are the compound miter angles for a 4-sided planter with 15° sides?

Blade bevel about 43.1° and miter about 14.5°. Board widths and saw setup are on Compound miter angles for a 4-sided planter with 15° sides.

What is the difference between a bevel and a miter?

A miter is an angle across the face of the board (the saw table rotates); a bevel is an angle through the thickness (the blade tilts). A compound cut uses both at once.

Does the board thickness change the angles?

No. The bevel and miter depend only on the number of sides and the tilt. Thickness affects the piece lengths, not the angles.

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