A straight razor’s edge angle is not chosen at the stones — it is built into the blade: honing flat on the spine, the spine thickness and blade width fix the angle forever. This calculator does the trigonometry: enter both measurements and read the angle per side, with guidance against the classic targets.
How this is calculated
Honing on the spine forms a triangle: half the spine thickness is the rise, the blade width is the hypotenuse. Angle per side = arcsin(half the spine ÷ width). The classical proportion — spine about one quarter of blade width — lands at the traditional 7–8° per side; see Straight Razor Geometry for why that ratio rules the design stage.
Using it in the shop
Design first: pick the width, multiply by roughly 0.25, and that is your spine before grinding. Check honing reality second: tape on the spine (a common protective habit) adds thickness and raises the angle slightly — add the tape to the spine number to see by how much. For knives honed on their spine (some traditional single-bevel work), the same math applies; for normal knives, angles come from the bevel-angle chart instead.
Browse more tools in the Calculators section.