Surface Finish Calculator
Estimate the theoretical surface roughness a given feed and nose or corner radius will leave — before you commit to a finishing pass.
What the result roughly corresponds to
| Theoretical Ra (μin) | Roughly comparable to |
|---|---|
| Under 16 | Ground or polished finish |
| 16–32 | Fine finish turning/milling |
| 32–63 | General machined finish |
| 63–125 | Rough finish pass |
| Over 125 | Roughing cut, not a finish surface |
See the full Surface Finish (Ra) Guide for where each range is typically used.
The formula, and what it assumes
Theoretical Ra = feed² ÷ (18√3 × nose radius) models the feed marks left by a rounded cutting edge as a repeating scallop pattern, with no other source of roughness in the picture. It's the same geometry whether you call the input "feed per revolution" (turning) or "feed per tooth" (milling with a round insert or corner radius) — both leave a cusp shaped by the same feed-and-radius relationship.
Why real parts usually finish rougher than this number
This is a purely geometric ideal: it assumes a perfectly sharp, perfectly round edge, zero vibration, no built-up edge, and a material that shears cleanly with no tearing or smearing. Tool wear, chatter, coolant condition, and how a specific material actually shears (gummy materials tend to tear rather than cut cleanly) all push the real, measured finish above this theoretical number — treat it as a best-case floor, not a prediction of exactly what a surface gauge will read.
The two easiest levers if you need a better finish
Since Ra scales with feed squared but only linearly with radius, cutting feed in half drops theoretical roughness to a quarter, while doubling the nose radius only halves it — feed is almost always the more powerful lever if you need a visibly better finish, though a bigger radius also spreads cutting force over a longer edge, which can help or hurt depending on rigidity and chatter tendency.