ENGINEERING CALCULATOR
Surface Roughness Calculator
Convert between Ra, Rz, RMS, CLA, and ISO N-grades instantly, and estimate the theoretical surface finish of CNC turning and milling from your cutting parameters. Whether you are specifying surface finish on a drawing, programming a finishing pass, or evaluating a supplier, this free tool gives you accurate conversions with full formula transparency.
✓
Free Online Tool
✓
Formula-Based Results
✓
ISO 1302 N-Grades
Quick Ra Converter
Enter an Ra value in micrometers to see common equivalents.
Ra (µin)
63 µin
Rz ≈ (µm)
8.0 µm
ISO N-Grade
N7
Formula
Ra(µin) = Ra(µm) × 39.37 · Rz ≈ 5 × Ra
ONLINE ENGINEERING TOOL
Surface Roughness Calculator — Unit Converter
Convert between Ra, Rz, RMS, CLA, and ISO 1302 N-grades in real time, or estimate the theoretical surface finish of a CNC turning or milling pass from your cutting parameters.
Theoretical Surface Finish Calculator
Ra (µm)
1.6 µm
Ra / CLA (µin)
63 µin
Rz ≈ (µm)
8.0 µm
Rz ≈ (µin)
315 µin
RMS ≈ (µin)
69.9 µin
ISO N-Grade
N7
Formula Used
Rz ≈ 5 × Ra · RMS ≈ 1.11 × Ra · CLA = Ra · N-grade per ISO 1302 (Approximation)
HOW IT WORKS
How the Surface Roughness Calculator Works
Learn the conversion ratios behind Ra, Rz, and RMS, and the theoretical formula used to predict surface finish from feed rate and tool nose radius.
Theoretical Surface Finish Formula
Ra = f² ÷ (32 × r)
f = Feed per Revolution
r = Tool Nose Radius
Ra to Rz Conversion (Approximation)
Rz ≈ 5 × Ra
RMS ≈ 1.11 × Ra
CLA = Ra · N-grade per ISO 1302
Worked Example
For a CNC finishing pass with a feed of 0.008 in/rev and a tool nose radius of 0.032 in (1/32"), the theoretical roughness is f² divided by 32 × r — a standard N7 CNC finish.
0.008²
Feed Squared
÷
32 × 0.032
32 × Nose Radius
=
62.5 µin
Theoretical Ra
Theoretical Ra
62.5 µin ≈ 1.59 µm
Rz ≈ (Approximation)
8.0 µm
ISO N-Grade
N7
Understanding the Gap Between Theory and Reality
01
Vibration & Tool Wear
Machine tool vibration, chatter, and progressive flank wear degrade the finish well beyond the geometric ideal.
Actual Ra is typically 1.5×–3× worse02
Built-Up Edge (BUE)
Material-specific built-up edge formation on the cutting edge roughens the surface. Stainless steel and titanium suffer most.
Adjusted Ra = Theoretical Ra × BUE factor03
Workpiece & Coolant
Thin-wall deflection, poor chip evacuation, and weak coolant delivery all widen the gap between theory and measurement.
Plan theoretical Ra ≈ half your targetWhat the Results Mean
Ra — Average Roughness
The arithmetic mean of profile deviations. Reliable for general quality, but it can miss a single deep scratch on a sealing surface.
Rz — Roughness Depth
Averages the five highest peak-to-valley distances. Sensitive to isolated defects — preferred for sealing and bearing surfaces. Always larger than Ra.
N-Grade — ISO 1302 Class
Twelve grades (N1–N12) shorthand for maximum Ra. Standard as-machined CNC finish is N7–N8 (Ra 63–125 µin).
SURFACE TEXTURE BASICS
Surface Roughness Parameters Explained
Every surface texture callout references one or more of these parameters. Understanding what each measures — and what it misses — helps you specify the right one.
Ra
Arithmetic Mean Roughness
The most widely used parameter worldwide: the arithmetic average of absolute profile deviations from the mean line. Averaging neutralizes isolated extremes, so Ra can miss a single deep scratch that would leak on a sealing surface. Measured in µin (US) or µm (ISO); 1 µm = 39.37 µin.
Rz
Mean Roughness Depth
Averages the vertical distance between the five highest peaks and five deepest valleys. Highly sensitive to extreme features, Rz is preferred for sealing surfaces, bearing journals, and sliding interfaces. Always larger than Ra — the ratio typically ranges from 4:1 to 7:1.
Rq
RMS — Root Mean Square Roughness
RMS (Rq) squares each deviation before averaging, giving more weight to larger deviations. RMS ≈ 1.11 × Ra for most machined surfaces. Common in optical specifications, tribology research, and bearing industry standards.
CLA
Center Line Average / AA
Legacy terms numerically identical to Ra. CLA appears in microinches on older US drawings — a vintage callout of "CLA 63" is functionally identical to Ra 63 µin.
N
N-Grade — ISO 1302 Classification
ISO 1302 defines twelve grades (N1–N12), each a maximum Ra value. Instead of calling out "Ra 1.6 µm," a drawing can simply specify "N7." Common in European and international engineering standards.
≈
A Note on Conversions
Ra and Rz measure fundamentally different characteristics, so no exact conversion exists. The 5× ratio here is a practical approximation per ISO 4287 and ASME B46.1 — for critical applications, measure the specified parameter directly with a profilometer.
ISO 1302 REFERENCE
ISO Surface Finish Grade Reference Table
Cross-reference Ra, Rz, RMS, and N-grade values. Each N-grade corresponds to a specific maximum Ra value per ISO 1302.
| N-Grade | Ra (µm) | Ra/CLA (µin) | Rz (µm) approx. | RMS (µin) approx. | Typical Process | Cost Tier |
|---|---|---|---|---|---|---|
| N1 | 0.025 | 1 | 0.10–0.15 | 1.1 | Superfinishing, lapping | Precision |
| N2 | 0.05 | 2 | 0.25–0.30 | 2.2 | Superfinishing, lapping | Precision |
| N3 | 0.1 | 4 | 0.50–0.60 | 4.4 | Lapping, honing | Precision |
| N4 | 0.2 | 8 | 1.00–1.20 | 8.8 | Fine grinding, honing | Precision |
| N5 | 0.4 | 16 | 2.00–2.40 | 17.6 | Fine grinding | Premium |
| N6 | 0.8 | 32 | 3.20–4.00 | 35.2 | Grinding, fine turning | Premium |
| N7 | 1.6 | 63 | 8.00 | 69.3 | Turning, milling — standard CNC | Standard |
| N8 | 3.2 | 125 | 16.00 | 137.5 | Turning, milling | Standard |
| N9 | 6.3 | 250 | 32.00 | 275 | Rough machining | Standard |
| N10 | 12.5 | 500 | 63.00 | 550 | Rough machining, saw cut | Economy |
| N11 | 25 | 1,000 | 125.00 | 1,100 | Casting, forging | Economy |
| N12 | 50 | 2,000 | 250.00 | 2,200 | Casting, forging, flame cut | Economy |
Standard Baetro CNC finish is N7–N8 (Ra 63–125 µin). Tighter finishes are available — specify your requirement when uploading your CAD for an accurate quote.
PROCESS CAPABILITY
Typical Surface Finishes by Manufacturing Process
Achievable surface finish depends on the process, material, and production volume. These are realistic Ra ranges for common CNC and secondary processes.
| Process | Ra Range (µin) | Ra Range (µm) | N-Grade | Notes |
|---|---|---|---|---|
| As-Machined CNC (Standard) | 63–125 | 1.6–3.2 | N7–N8 | Default finish unless otherwise specified |
| Fine CNC Machined | 32–63 | 0.8–1.6 | N6–N7 | Reduced feed rate, wiper inserts |
| Precision CNC Machined | 16–32 | 0.4–0.8 | N5–N6 | Specialized tooling, rigid setup required |
| Ground | 8–32 | 0.2–0.8 | N4–N6 | Post-machining cylindrical or surface grinding |
| Honed | 4–16 | 0.1–0.4 | N3–N5 | Cylindrical bores, hydraulic components |
| Lapped | 2–8 | 0.05–0.2 | N2–N4 | Gauge surfaces, optical flats |
| Polished | 1–4 | 0.025–0.1 | N1–N3 | Mirror finish, decorative or sealing surfaces |
| Anodized (post-machining) | Varies | Varies | — | Anodizing adds 0.0001–0.001"; specify pre-anodizing Ra one grade finer |
| Bead Blasted | Varies | Varies | — | Uniform matte texture; masks but does not eliminate machining marks |
| Powder Coated | N/A | N/A | — | Creates a 0.002–0.010" film; substrate roughness is not visually apparent |
For finishes below Ra 16 µin, CNC grinding is typically required as a secondary operation. Baetro offers in-house machining and finishing — reducing supplier handoffs and lead time.
COST DRIVERS
How Surface Finish Affects Machining Cost
Surface finish is one of the biggest cost drivers in CNC machining — and one of the most commonly over-specified. Understanding the cost curve helps you specify the finish each surface actually needs.
Each N-grade improvement from N8 toward N5 roughly doubles finishing time and cost. A part specified at Ra 16 typically costs 4–6× more to machine than the same part at Ra 125.
| Target Ra (µin) | N-Grade | Relative Machining Time | Cost Multiplier | Typical Applications |
|---|---|---|---|---|
| 125 | N8 | 1.0× | 1.0× (Baseline) | Non-critical surfaces, internal features |
| 63 | N7 | 1.5× | 1.3–1.5× | General machined surfaces, mounting faces |
| 32 | N6 | 2.5× | 2.0–3.0× | Bearing seats, sealing surfaces, precision fits |
| 16 | N5 | 5.0× | 4.0–6.0× | Hydraulic spools, gauge surfaces, aerospace |
| 8 | N4 | Requires grinding | 6.0–10.0× | High-precision bearings, optical mounts |
1
Feed Rate & Tooling
Ra scales with the square of feed rate — halving the feed improves finish 4× but doubles cycle time. Fine-finish tooling adds cost on top.
- Feed rate reduction multiplies cycle time
- Wiper inserts cost more and wear faster
2
Secondary Operations
Achieving Ra 16 or better typically requires grinding, honing, or lapping — each adding setup time and process cost.
- Grinding and honing add setups
- Profilometer verification and CMM reports
3
Over-Specification Risk
Drawings often call for Ra 32 or Ra 16 where Ra 63–125 would function perfectly. Ask three questions for every surface:
- Does this surface contact another part?
- Is it a sealing, bearing, or sliding surface?
- Will it be painted, anodized, or coated?
Baetro's engineers review every CAD file for manufacturability before quoting — including surface finish optimization. If a finish spec appears tighter than the application requires, we flag it and suggest alternatives that maintain function while reducing cost. See our engineering materials reference and precision CNC machining service for capability details.
Surface Finish Questions
Frequently Asked Questions
Ra (Arithmetic Mean Roughness) is the arithmetic average of the absolute deviations of the surface profile from the mean line over a defined sampling length. It is measured in microinches (µin) in the US and micrometers (µm) in ISO countries. Standard CNC machining typically produces Ra 63–125 µin (1.6–3.2 µm / N7–N8).
There is no exact conversion — Ra averages all profile deviations while Rz averages the five highest peak-to-valley distances. As a rule of thumb, Rz ≈ 4–7 × Ra (commonly approximated at 5×). For critical applications, measure the specified parameter directly with a calibrated profilometer.
Standard as-machined CNC finish is Ra 63–125 µin (1.6–3.2 µm), corresponding to N7–N8 on the ISO 1302 scale — the default Baetro delivers unless a tighter specification is requested. Ra 63 is roughly the threshold where machining marks become visible to the naked eye.
Cost rises exponentially, not linearly. Each N-grade improvement from N8 to N5 roughly doubles finishing time; Ra 16 typically costs 4–6× more than Ra 125, driven by slower feeds, specialized tooling, secondary grinding, and added inspection. Specify only the finish each surface actually needs.
Standard turning and milling reliably achieve Ra 32–125 µin (0.8–3.2 µm). With reduced feeds, sharp carbide tooling, wiper inserts, and rigid workholding, Ra 16 µin (0.4 µm) is achievable off the machine. Below Ra 8 µin typically requires grinding, honing, or lapping. Free-machining brass and aluminum finish better than stainless steel or titanium at identical parameters.
ISO 1302 defines twelve N-grades (N1–N12), each a specific maximum Ra value: N1 = Ra 0.025 µm (lapped), N6 = Ra 0.8 µm (ground), N7 = Ra 1.6 µm (standard CNC), N12 = Ra 50 µm (as-cast). N-grades replace explicit Ra callouts on many European drawings.
Ra = f²/(32 × r) assumes ideal geometry. In production, measured Ra is typically 1.5×–3× worse due to vibration, tool wear, built-up edge, deflection, and coolant effects. Use it for planning — if your target is Ra 63, plan for a theoretical Ra of about 32, then validate with a profilometer on first-piece inspection.
Roughness covers fine, closely spaced irregularities from the cutting tool, measured over 0.8–25 mm sampling lengths. Waviness covers broader deviations from machine vibration, deflection, or thermal effects over longer wavelengths. Roughness affects friction and sealing; waviness affects contact stiffness and load distribution.
PRECISION MANUFACTURING SUPPORT
Get Precision Parts with Your Specified Surface Finish
Surface finish directly affects part function, wear life, sealing performance, and fatigue resistance. Whether you need standard as-machined Ra 63 or Ra 16 with full inspection reports for aerospace applications, Baetro delivers consistent quality backed by ISO 9001-certified processes — shipping from Qingdao to North America, Europe, and Asia in 3–7 days with no minimum order quantity.
✓
Profilometer & CMM verified finishes
✓
Free DFM review on every quote
✓
Full inspection reports with every order