Surface Roughness Converter

Formula

• Ra (µin) = Ra (µm) × 39.3701. CLA = Ra in µin.
• Rq ≈ RMS ≈ 1.11 × Ra (Gaussian surface screening).
• Rz ≈ k × Ra where k is the process factor (Typical ×6, chart ×5, CNC turning ×7, …).
• Statistical Rz band used here: about 4×Ra to 8×Ra.
• Rt ≥ Rz; this pad uses Rt ≈ max(7.5, 1.25×k) × Ra as a screening estimate.
• ISO 1302 N1–N12 map to fixed Ra maxima (N6 = 0.8 µm, N7 = 1.6 µm, N8 = 3.2 µm).

How it works

Enter Ra, Rz, Rq, Rt, µin, RMS, or an ISO N-grade. Process chips set the Rz/Ra screening factor. Results show metric and imperial equivalents, a statistical Rz band, and the closest N-grade. Ra↔Rz is approximate — measure the drawing parameter for critical work.

Choose Input parameter (Ra / Rz / Rq / Rt / µin / RMS / N-grade). Pick a Process Rz/Ra factor or tap a chip (Typical ×6, chart ×5, CNC ×7, Grind, Lap) — the live profile updates the mean Rz estimate with the factor even before CALCULATE. Enter the value (or whole-number N-grade). CALCULATE fills all equivalents, the Rz band, and dual µm/µin. Editing clears Results. RESET restores Ra 1.6 µm · Typical ×6.

Ra, Rz, RMS — what converts cleanly

Default CALCULATE path: Ra 1.6 µm × 6 → Rz 9.6 µm; closest grade N7; CLA ≈ 63 µin.

Rq / RMS ≈ 1.11 × Ra for a Gaussian screening estimate.

Process Rz/Ra factors — shop chips

Chip examples: ×5 on Ra 0.8 µm → Rz 4 µm · N6; CNC ×7 on Ra 1.6 µm → Rz 11.2 µm.

Pick the process that matches how the surface was made when you only have Ra on the print.

What this pad does not do

Visual scale idea: mirror / ground / machined / rough — lower Ra is smoother.

Standard CNC as-machined often lands near Ra 63 µin (N7).

Worked example

Default Ra 1.6 µm · Typical screening (×6). Reproduce on CALCULATE.

1. Leave Input = Ra (µm), Process = Typical screening (×6), value 1.6.
2. CALCULATE → Rz 9.6 µm · band ≈ 6.4–12.8 µm · Rq ≈ 1.78 µm · N7 · ≈ 63 µin.
3. ×5 example: chip ×5, Ra 0.8 µm → Rz 4 µm · N6.
4. CNC example: chip CNC ×7, Ra 1.6 µm → Rz 11.2 µm · still N7.
5. µin example: Input Ra/CLA (µin) = 63 → Ra ≈ 1.60 µm · N7.

Result: Default: Ra 1.6 µm · Rz 9.6 µm · N7 · ≈ 63 µin CLA.

When to use

• Converting Ra ↔ µin / CLA / RMS for prints and CMM reports
• Estimating Rz from Ra with a process factor
• Mapping measured Ra to the closest ISO N-grade
• Seeing a statistical Rz band instead of a single false precision number
• Quick shop screening before quoting a finish

Limitations

• Ra↔Rz is approximate; process and material change the real ratio.
• Does not compute roughness from a raw profile trace.
• Does not predict Ra from turning feed and corner radius.
• N-grade uses closest log match to ISO 1302 Ra maxima.
• Not a full ISO 1302 drawing-symbol builder.

FAQ

Ra vs Rz?

Ra is the arithmetic average roughness. Rz averages peak-to-valley heights over sampling lengths and is more sensitive to outliers. Typical ratios run about 4–8× Ra; this pad’s process factor sets the mean estimate.

Can I convert Ra to Rz exactly?

No. Conversion references treat it as a statistical estimate. For critical specs, measure the parameter written on the drawing.

What is N7?

ISO 1302 N7 corresponds to a maximum Ra of 1.6 µm (about 63 µin) — a common as-machined CNC finish.

What is CLA or RMS?

CLA is the same as Ra in microinches. RMS (Rq) squares deviations before averaging and is about 1.11 × Ra for many surfaces.

Why pick a process factor?

Ground, lapped, and milled surfaces have different Rz/Ra ratios. Process factors (and flat ×5 charts) give a better mean than one fixed multiplier for every process.