Surface Roughness Calculator Formula
Understand the math behind the surface roughness calculator. Each variable explained with a worked example.
Formulas Used
Ten-Point Roughness (Rz)
rz_value = ra_value * rz_factorRa in micro-inches
ra_inches = ra_value * 39.37ISO Grade (N)
n_class = (ra_value <= 0.025) ? 1 : ((ra_value <= 0.05) ? 2 : ((ra_value <= 0.1) ? 3 : ((ra_value <= 0.2) ? 4 : ((ra_value <= 0.4) ? 5 : ((ra_value <= 0.8) ? 6 : ((ra_value <= 1.6) ? 7 : ((ra_value <= 3.2) ? 8 : ((ra_value <= 6.3) ? 9 : 10))))))))Variables
| Variable | Description | Default |
|---|---|---|
ra_value | Arithmetic Mean Roughness (Ra)(micrometres) | 1.6 |
rz_factor | Rz/Ra Ratio (typical 4-7) | 5 |
How It Works
Surface Roughness Parameters
Surface roughness describes the fine irregularities on a machined surface.
Key Parameters
Ra (arithmetic average roughness) is the most common measure - the average deviation from the mean line.
Rz (ten-point height) is the average difference between the 5 highest peaks and 5 deepest valleys. Rz is typically 4-7 times Ra depending on the machining process.
ISO Roughness Grades
N1 (Ra 0.025) through N12 (Ra 50), each step doubling the roughness.
Worked Example
A machined surface with Ra = 1.6 micrometres.
ra_value = 1.6rz_factor = 5
- 01Rz = 1.6 x 5 = 8.0 micrometres
- 02Ra in micro-inches = 1.6 x 39.37 = 63 micro-in
- 03ISO grade: Ra 1.6 corresponds to N7
Ready to run the numbers?
Open Surface Roughness Calculator