Flat miters, compound crown settings and the five-cut check
Three different questions share the word miter, and this page keeps them in three sections with their conventions stated. Flat work: the saw setting for a corner, from the corner angle or from a polygon’s number of sides. Compound work: the miter and bevel pair for crown molding from its spring angle and the corner. And calibration: the five-cut method turned into a fence correction, for when the saw itself is the problem. Each result names its convention, because a miter figure without one is a number waiting to ruin a board.
Mode 1: flat miter for a corner or frame
Mode 2: crown molding (compound)
Mode 3: five-cut saw check
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Saw work carries real risk: guard in place, eye and ear protection, and a test cut in scrap before any setting on this page touches good material.
What this calculator does
It answers the three miter questions of a workshop with the geometry for each shown. For flat joints it bisects the corner: the cut on each piece is half the corner from the board’s edge, and the saw’s miter setting is that same cut expressed the way saw gauges count it, from square. For polygons it derives the corner from the side count first, which is why a hexagon’s setting (30 degrees) is not half of anything obvious until the 120 degree corner is on the table. For crown it solves the compound geometry of a spring angle meeting a corner and returns the miter and bevel pair for the standard upside-down cutting position, the same pair printed on the charts taped to workshop walls. And for calibration it converts the five-cut method’s two width measurements into an angular error and a physical fence correction at a stated pivot distance, turning “the saw cuts slightly out” into a number of thousandths to move.
How to use it
- Corner angle means the inside of the joint. Measure the corner the pieces will form, not the turn of the path. A frame going around a square table turns through 360 degrees in four 90 degree corners; each corner entered here is 90. For walls, a protractor or angle finder on the actual corner beats assuming 90, because real rooms are routinely a degree or two out, and crown shows every bit of it.
- The saw setting and the cut angle are two readings of one cut. Mode 1 prints both: the angle the cut makes with the board’s edge, and the miter gauge setting counted from a square crosscut. They coincide at 45 and diverge elsewhere; setting a saw by the wrong one is the classic hexagon-frame failure, where a 60 degree cut set as a 60 degree gauge reading produces a frame that will not close.
- Crown settings assume the standard cutting position. Upside down and backwards, spring angle held against the fence and table as it sits against wall and ceiling. The miter/bevel pair is only valid in that position; cutting crown flat uses different, larger settings that this mode does not return, deliberately, so the two methods can never be half-mixed on the same molding.
- Spring angle is a property of the molding, not a preference. It is printed on the profile’s specification and measurable from an offcut against a protractor. Guessing 38 for a 45 crown produces settings wrong by degrees, not thousandths, and no test cut in scrap was ever wasted on crown: cut two short pieces and hold them in the corner before the long stock is committed.
- The five-cut mode wants honest measurements. The method amplifies the fence error across five cuts so a caliper can see it; sloppy measuring spends that amplification. Measure the final piece’s two ends at the same points the cuts define, use the piece’s true length as the span, and expect corrections in hundredths of a degree and thousandths of an inch, which is the scale fence accuracy lives at.
How it is calculated
- Flat: for corner angle A, the cut bisects it: cut angle from the board’s edge = A / 2; miter saw setting (from square) = 90 – A / 2. From n sides, A = (n – 2) x 180 / n, and the saw setting simplifies to 180 / n.
- Crown: with spring angle s and corner C: miter = arctan( sin s / tan(C / 2) ); bevel = arcsin( cos s x sin(C / 2) ). These are the standard compound-miter relations for the upside-down position.
- Five-cut: the final piece’s taper (width difference d over its length L) is the accumulated error of four fence passes, so the fence’s angular error = arctan(d / L) / 4 in the strict method; this page reports both the piece’s taper angle, arctan(d / L), and the per-cut fence error, a quarter of it, and the linear correction at a pivot distance P = tan(per-cut error) x P. Stating which angle is which is the difference between a correction and a new error.
Worked examples
A hexagon frame. Six sides give 120 degree corners. Each cut is 60.00 degrees from the board’s edge, and the miter saw setting is 30.00 degrees. An octagon (8 sides, 135 degree corners) sets the saw to 22.50 degrees. The pattern is the simplification on display: for regular frames the setting is 180 divided by the sides, and the bisection does the rest.
Crown, 38 degree spring, square corner. The standard pair: miter 31.62 degrees, bevel 33.86 degrees. The other common crown, 45 degree spring in the same corner, takes miter 35.26 and bevel 30.00. A 38 crown run into a 135 degree bay corner changes character entirely: miter 14.31, bevel 46.72, which is why bay windows have their own chart column and why guessing from the square-corner pair fails there.
A five-cut reading. The final piece measures 2.005 at one end and 1.985 at the other, over a 24 inch length. The taper is 0.020 over the span: a taper angle of 0.0477 degrees, a per-cut fence error of 0.0119 degrees, and a correction at a 12 inch pivot of 0.0025 inches, about 2.5 thousandths, applied in the direction that narrows the wide end. Numbers that small are the point of the method: four cuts of accumulated error made a fifth of a degree’s worth of taper measurable with a ruler.
Reading your results
For flat work, set the saw by the gauge figure and check the first two offcuts against each other before the frame stock: two pieces cut at the setting should form the corner angle exactly on a flat surface. For crown, the pair is meaningless in isolation; the test is two short pieces held into the actual corner, because the wall’s true angle and the molding’s true spring both vote. For the five-cut, treat a correction under a thousandth as the saw being right and the measuring being human, and repeat the sequence after any adjustment rather than stacking corrections on a single reading.
Limitations
- Real corners are rarely the entered angle. Every mode assumes the corner is what it is told. An angle finder reading is the cheapest accuracy in the whole subject; a degree of wall error becomes a visible crown gap no setting can hide, only caulk can, which is a confession, not a joint.
- Crown cut flat is out of scope. Large crown that cannot stand in the saw is cut flat with different formulas. This page’s compound mode is for the upside-down position only and says so at the field, because returning one method’s numbers to the other’s user is how molding gets wasted.
- Saw scales are approximate instruments. Detents and printed scales carry their own tolerances, commonly half a degree, larger than many corrections computed here. The five-cut mode exists precisely because the scale cannot be trusted at the precision joints want; set by measurement, verify by test piece, in that order.
- Blade deflection and stock movement are not geometry. A wandering blade, a board that is not straight, and seasonal movement across a mitered frame all open joints that were cut perfectly. The calculator governs angles; the rest is stock preparation and the honest knowledge that miters in solid wood move, which is why splines exist.
Frequently asked questions
What angle do I cut for a 90 degree corner?
45 degrees on each piece: the corner is bisected, so each end is cut at half the corner, and at 90 degrees the cut angle from the board’s edge and the saw’s gauge setting happen to be the same 45, which is why this one case teaches a habit that fails everywhere else. For any other corner, halve the corner for the cut and set the saw to 90 minus that: a 120 degree hexagon corner is a 60 degree cut at a 30 degree setting, both printed by Mode 1 so the distinction is never left to memory.
What are the miter and bevel settings for crown molding?
They follow from the crown’s spring angle and the corner: for the common 38 degree spring in a square corner, miter 31.62 and bevel 33.86; for a 45 spring, 35.26 and 30.00. Those pairs are for cutting upside down and backwards in a compound saw. Crown charts exist because the formulas are unmemorable; Mode 2 is the chart with your values in it, and the worked examples show how sharply a bay corner (135 degrees) moves the pair, to miter 14.31 and bevel 46.72 for the 38 crown.
What is the five-cut method?
A calibration ritual for a table saw or sled fence: a panel is cut on all four sides in sequence, each cut against the fence, and the strip from the fifth cut is measured at both ends. Because each pass inherits the fence’s angular error, the strip’s taper is about four times the error that a single cut would show, which brings thousandths-level misalignment into ruler range. Mode 3 turns the two measurements into the fence’s angular error and the physical correction at your fence’s pivot, completing the loop from ritual to adjustment.
Why won’t my frame close?
In the order workshops meet them: the saw setting was the cut angle instead of the gauge angle (fatal on anything but squares); the corner count or corner angle was assumed rather than measured; the saw’s scale is off by a fraction the five-cut mode can expose; or the pieces are not equal in length, which no angle fixes because opposite sides of a frame must match in length as well as angle. The calculator covers the first three with numbers; the fourth is a tape measure and the discipline to cut pairs together.
Related calculators
Method: corner bisection for flat miters with the saw-gauge convention stated, the standard compound relations for crown in the upside-down position, and five-cut taper analysis with the per-cut division stated, exactly as defined on this page. Last reviewed: 13 October 2026.