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Surface Finish (Ra) Converter
Convert between Ra, Rz, RMS, CLA, and N-grade surface finish measurements. Enter a value in any unit and see all equivalent values instantly.
To use this tool, select your input unit from the dropdown (Ra µin, Ra µm, Rz, RMS, N-grade, or CLA), enter your value, and all equivalent measurements update in real time. Use the static reference table below to quickly look up standard process finishes without entering any values.
Quick Answer: How Do You Convert Ra, Rz, RMS and N-Grade Surface Finish?
Surface finish units convert through fixed relationships. RMS equals about 1.11 × Ra, CLA equals Ra and Rz is roughly 4–7 × Ra (this tool uses 5 ×). N-grades follow ISO 1302, where each grade maps to a set Ra value: N6 = Ra 0.8 µm (32 µin) and N7 = Ra 1.6 µm (63 µin). A lower Ra means a smoother surface.
This converter accepts a value in any unit (Ra µin, Ra µm, Rz, RMS, N-grade or CLA) and instantly returns every equivalent value, plus a visual roughness scale. A static reference table lists standard process finishes so you can look up values without entering data.
Common conversions: Ra 32 µin = Ra 0.8 µm = N6 = RMS 35.5 µin = Rz 160 µin | Ra 63 µin = Ra 1.6 µm = N7 (standard as-machined CNC) | Ra 16 µin = N5 (ground) | Ra 125 µin = N8 (rough) | CNC machining range Ra 16–125 µin.
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Common CNC Surface Finishes
| Process | Ra (μin) | Ra (μm) | N-grade |
|---|---|---|---|
| As-machined (standard) | 63–125 | 1.6–3.2 | N7–N8 |
| Fine machined | 32–63 | 0.8–1.6 | N6–N7 |
| Ground | 8–32 | 0.2–0.8 | N4–N6 |
| Lapped | 2–8 | 0.05–0.2 | N2–N4 |
| Polished / Mirror | 1–4 | 0.025–0.1 | N1–N3 |
Converted Values
Ra (μin)
32
Ra (μm)
0.80
Rz (μin)
160
Rz (μm)
4.06
RMS (μin)
35.5
CLA (μin)
32
N-grade (ISO 1302)
N6
Key Relationships:
RMS ≈ 1.11 × Ra • CLA = Ra
Rz ≈ 4–7 × Ra (approx. 5× used here)
RMS ≈ 1.11 × Ra • CLA = Ra
Rz ≈ 4–7 × Ra (approx. 5× used here)
Rz/Ra ratio uses approximate 5x multiplier. Actual ratio varies by process and material.
Surface Finish Conversion Reference Table
Standard surface roughness values across all common measurement units. Rz values use the 5× Ra approximation. RMS values use the 1.11× Ra relationship.
| Ra (µin) | Ra (µm) | Rz (µin) | Rz (µm) | RMS (µin) | N-grade | Process |
|---|---|---|---|---|---|---|
| 1 | 0.025 | 5 | 0.125 | 1.11 | N1 | Superfinished / Lapped |
| 2 | 0.05 | 10 | 0.25 | 2.22 | N2 | Superfinished |
| 4 | 0.10 | 20 | 0.50 | 4.44 | N3 | Precision ground |
| 8 | 0.20 | 40 | 1.00 | 8.88 | N4 | Ground |
| 16 | 0.40 | 80 | 2.00 | 17.8 | N5 | Fine ground / Lapped |
| 32 | 0.80 | 160 | 4.00 | 35.5 | N6 | Fine turned / milled |
| 63 | 1.60 | 315 | 8.00 | 69.9 | N7 | As-machined (standard CNC) ★ |
| 125 | 3.20 | 625 | 16.00 | 138.8 | N8 | As-machined (rough) |
| 250 | 6.30 | 1250 | 31.50 | 277.5 | N9 | Rough machined |
| 500 | 12.50 | 2500 | 62.50 | 555 | N10 | Very rough |
★ Ra 63 µin (N7) is the standard CNC as-machined finish included in base pricing at RivCut. Rz uses 5× Ra approximation; actual ratio varies by process.
Need Parts with Specific Surface Roughness or Post-Processing?
We machine and post-process parts to meet tight Ra surface specifications including electropolishing, bead blasting, and anodizing. Upload your CAD models for a quote.
Upload CAD for Instant QuoteFrequently Asked Questions
Ra (Roughness Average) is the most common surface finish measurement. It is the arithmetic average of the absolute values of the surface height deviations measured from the mean line. A lower Ra number means a smoother surface. CNC machined parts typically range from Ra 16 to Ra 125 microinches.
Ra is the average roughness across the entire measurement length. Rz is the average of the five highest peaks and five deepest valleys within the sampling length. Rz is always larger than Ra for the same surface. Rz is more sensitive to occasional deep scratches or high peaks that Ra might average out. A common approximation is Rz equals 4 to 7 times Ra.
RMS (Root Mean Square) is similar to Ra but uses the square root of the mean of the squared deviations. RMS is always slightly higher than Ra for the same surface, typically by about 11 percent. The relationship is RMS equals approximately 1.11 times Ra. RMS was the older standard and Ra has largely replaced it, but some older drawings still call out RMS.
Standard CNC machining produces Ra 63 to Ra 125 microinches (1.6 to 3.2 micrometers). Fine machining can reach Ra 32 microinches (0.8 micrometers). Grinding achieves Ra 8 to Ra 32 microinches. Lapping and polishing can reach Ra 2 to Ra 8 microinches. The achievable finish depends on the material, tooling, speeds, and feeds.
N-grades are ISO 1302 roughness grade numbers from N1 to N12. Each N-grade corresponds to a specific Ra value. For example, N6 equals Ra 0.8 micrometers (32 microinches) and N7 equals Ra 1.6 micrometers (63 microinches). N-grades provide a simple shorthand for specifying surface finish on drawings.
Ra 32 to 63 microinches (N6 to N7) is the recommended pre-anodize surface finish for best results. Anodizing builds a uniform oxide layer that follows the existing surface profile, so a smoother base surface produces a smoother anodized finish. Surfaces rougher than Ra 125 microinches may show inconsistent coating and dull appearance after anodizing.
Finer surface finishes add 15 to 50 percent to part cost because they require additional machining passes, slower feed rates, or secondary operations like grinding or lapping. Ra 63 microinches (N7) is the standard as-machined finish and is included in base pricing. Ra 32 microinches requires additional light finishing passes, adding roughly 15 to 25 percent. Ra 16 or better typically requires grinding, which can add 30 to 50 percent or more to the part cost.
Surface Finish Reference Series
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