Knife Sharpening Angle

This calculator turns two things you can measure with a ruler — blade width and how high the spine sits above the stone — into a sharpening angle in degrees.

How the calculation works

With the edge on the stone and the spine raised, the blade forms a right triangle: the blade width W is the hypotenuse and the spine height h is the opposite side. The angle per side is therefore θ = arcsin(h / W). Doubling it gives the inclusive angle, the full included angle of the edge when both sides are sharpened equally.

Enter the blade width in millimetres, from edge to spine at the point you are sharpening — it differs between heel and tip. Spine height is the gap between stone and spine, also in mm. The tool also shows how high to lift the spine for 15° and 20° per side, handy for sizing a stack of coins or a wedge guide.

Based on the angle it suggests a typical use: under 12° per side for very fine edges, 12–17° for Japanese kitchen knives, 17–22° for Western kitchen knives, 22–27° for outdoor and hunting knives, and 27° or more for axes and machetes.

Worked example

The defaults are a 40 mm blade with the spine raised 13.7 mm. sin θ = 13.7 / 40 = 0.3425, so θ ≈ 20.0° per side and the inclusive angle ≈ 40.1° — a typical Western chef's knife edge. For 15° the same knife needs a spine lift of 40 × sin 15° ≈ 10.4 mm; for 20°, 13.7 mm.

Things to keep in mind

• The formula assumes an idealised flat blade. Spine thickness and grind add roughly 1–2° on a typical kitchen knife, so aim slightly lower if you need an exact angle.
• A lower angle cuts more easily but rolls or chips sooner. Knives used on bones, frozen food or glass boards need a sturdier edge.
• Single-bevel tools (chisels, plane irons, some Japanese knives) have the whole bevel on one side, so the per-side angle is the full edge angle.
• Match the existing bevel rather than changing it: re-profiling removes a lot of steel. Colour the bevel with a marker and see where the stone takes it off.