---
type: chart
title: "Surface Finish Chart"
tables: 3
url: https://manufacturingprocesses.org/charts/surface-finish-chart
---

# Surface Finish Chart

ISO 1302 N-grades with Ra, RMS, Rz and Rt equivalents, the conversion factors between roughness parameters, and the Ra range each manufacturing process holds.

Surface roughness is specified against ISO 1302 (N-grades), ISO 4287 (the
Ra and Rz parameters), ASME B46.1 and ASME Y14.36 (drawing symbols). Each
N-grade step doubles Ra: N1 is 0.025 µm, N6 is 0.8 µm, N12 is 50 µm.

The RMS and Rz columns here are **computed, not transcribed** — RMS as
Ra × 1.11 and Rz as the shop-rule Ra × 4. That is deliberate, and the reason
is the most important item on this page (see the notes).

**Ra and Rz measure genuinely different things and cannot be converted
exactly.** Ra averages the whole profile; Rz is a peak-to-valley measure and
is far more sensitive to isolated deep scratches. A surface can pass an Ra
spec and fail an Rz spec badly. For sealing and fatigue-critical surfaces,
specify Rz — or both — rather than Ra alone. ASME Y14.36 prefers an explicit
Ra value over an N-grade; N-grades appear mostly on older or ISO/JIS
drawings.

## ISO 1302 N-grades with Ra, RMS, Rz and Rt equivalents

| N-grade | Ra (µm) | Ra (µin) | RMS (µin) | Rz approx (µin) | Rt (µm) | Cut-off (in) | Cut-off (mm) |
| --- | --- | --- | --- | --- | --- | --- | --- |
| N1 | 0.025 | 1 | 1.1 | 4 | 0.3 | 0.003 | 0.08 |
| N2 | 0.05 | 2 | 2.2 | 8 | 0.5 | 0.01 | 0.25 |
| N3 | 0.1 | 4 | 4.4 | 16 | 0.8 | 0.01 | 0.25 |
| N4 | 0.2 | 8 | 8.9 | 32 | 1.2 | 0.01 | 0.25 |
| N5 | 0.4 | 16 | 17.8 | 64 | 2 | 0.01 | 0.25 |
| N6 | 0.8 | 32 | 35.5 | 128 | 4 | 0.03 | 0.8 |
| N7 | 1.6 | 63 | 69.9 | 252 | 8 | 0.03 | 0.8 |
| N8 | 3.2 | 125 | 138.8 | 500 | 13 | 0.1 | 2.5 |
| N9 | 6.3 | 250 | 277.5 | 1000 | 25 | 0.1 | 2.5 |
| N10 | 12.5 | 500 | 555 | 2000 | 50 | 0.1 | 2.5 |
| N11 | 25 | 1000 | 1110 | 4000 | 100 | 0.3 | 8 |
| N12 | 50 | 2000 | 2220 | 8000 | 200 | 0.3 | 8 |

## Conversions between Ra, RMS, Rz and Rt

| Relationship | Formula | Reliability |
| --- | --- | --- |
| Ra ↔ CLA | Identical — CLA (Center Line Average) is the older term for Ra | Exact |
| µm ↔ µin | Ra (µin) = Ra (µm) × 39.37 (commonly rounded to × 40) | Exact |
| Ra → RMS (Rq) | RMS ≈ Ra × 1.11 | Good for typical machined surfaces; assumes a roughly sinusoidal profile |
| Ra → Rz | Rz ≈ Ra × 4 (shop rule), up to Ra × 7 depending on process | Approximate only — not a conversion |
| Ra → Rz (ISO) | Rz (ISO) ≈ Ra × 7.6 | Approximate |
| Ra → Rt | Rt ≈ Ra × 8.7 | Approximate |
| Ra → Rmax | Rmax ≈ Ra × 8.0 | Approximate |
| Ra → Rp | Rp ≈ Ra × 3.6 | Approximate |

## Typical Ra achievable by manufacturing process

| Process | Ra typical (µin) | Ra typical (µm) | N-grade band | Verified |
| --- | --- | --- | --- | --- |
| Flame / plasma cutting | 500–2000 | 12.5–50 | N10-N12 | Indicative |
| Sand casting | 250–1000 | 6.3–25 | N9-N11 | Indicative |
| Hot rolling | 250–1000 | 6.3–25 | N9-N11 | Indicative |
| Sawing | 250–1000 | 6.3–25 | N9-N11 | Indicative |
| Forging | 125–500 | 3.2–12.5 | N8-N10 | Indicative |
| Rough milling / hogging | 250–500 | 6.3–12.5 | N9-N10 | Two sources |
| Rough turning | 125–250 | 3.2–6.3 | N8-N9 | Two sources |
| Investment casting | 63–250 | 1.6–6.3 | N7-N9 | Indicative |
| Drilling | 63–250 | 1.6–6.3 | N7-N9 | Indicative |
| Laser cutting (cut edge) | 32–250 | 0.8–6.3 | N6-N9 | Two sources |
| Die casting | 32–125 | 0.8–3.2 | N6-N8 | Two sources |
| Cold rolling / drawing | 32–125 | 0.8–3.2 | N6-N8 | Indicative |
| Standard CNC milling (as-machined) | 63–125 | 1.6–3.2 | N7-N8 | Two sources |
| Standard CNC turning | 63–125 | 1.6–3.2 | N7-N8 | Two sources |
| Boring | 32–125 | 0.8–3.2 | N6-N8 | Indicative |
| Reaming | 32–125 | 0.8–3.2 | N6-N8 | Indicative |
| Broaching | 32–125 | 0.8–3.2 | N6-N8 | Indicative |
| EDM (sinker / wire) | 32–250 | 0.8–6.3 | N6-N9 | Indicative |
| Fine milling / fine turning | 16–32 | 0.4–0.8 | N5-N6 | Two sources |
| Surface & cylindrical grinding | 8–63 | 0.2–1.6 | N4-N7 | Two sources |
| Honing | 4–32 | 0.1–0.8 | N3-N6 | Indicative |
| Lapping | 1–16 | 0.025–0.4 | N1-N5 | Two sources |
| Superfinishing / polishing | 1–8 | 0.025–0.2 | N1-N4 | Two sources |

## Notes

**Several well-known chart pages list N6 RMS as 32.5 µin and N7 as 64.3 µin,
contradicting the ×1.11 rule those same pages state.** Applying the rule
gives 32 × 1.11 = **35.5** and 63 × 1.11 = **69.9**. Every other row on those
charts does follow ×1.11 exactly (1→1.1, 2→2.2, 4→4.4, 8→8.8, 125→137.5,
250→275, 500→550, 1000→1100, 2000→2200) — only N6 and N7 break it. The three
charts that print 32.5 and 64.3 are not independent of each other; they
share an original with a transcription error. This chart computes RMS =
Ra × 1.11 for every row, which agrees with the two sources that do not carry
the error.

**The Rz column is labelled approximate on purpose.** It is computed as
Ra × 4, the convention on machining-shop charts, but published factor tables
also give Rz = Ra × 7.2 and Rz(ISO) = Ra × 7.6. These conflict and none of
them is wrong — the true ratio depends on the process. This is the least
deterministic number on the page.

**The process-versus-Ra table is the least tightly verified data here.** The
rows marked verified were independently confirmed by two sources: standard
CNC milling and turning, fine milling and turning, rough milling, grinding,
lapping and superfinishing, laser cut edge and die casting. The remaining
rows — sand casting, hot rolling, forging, sawing, flame cutting,
investment casting, drilling, boring, reaming, broaching, EDM and honing —
come from general references only and should be treated as indicative.

**Cost note:** finishes tighter than Ra 1.6 µm (63 µin) typically add 40–60%
or more to cycle time, and below Ra 0.8 µm (32 µin) usually require a
secondary grinding or polishing operation. Specify the loosest Ra that meets
the function.

## FAQ

### What Ra does standard CNC machining produce?

Ra 63–125 µin (1.6–3.2 µm), which is the N7–N8 band, for both milling and turning. That is the default as-machined finish and costs nothing extra. Fine milling or turning with sharp tools and a light stepover reaches 16–32 µin (0.4–0.8 µm).

### What is N6 in Ra?

N6 is Ra 0.8 µm, which is 32 µin, and an RMS of 35.5 µin. It is achievable by boring, reaming, broaching, cold rolling, die casting or a fine machining pass.

### How do I convert Ra to RMS?

RMS ≈ Ra × 1.11. It is a good approximation for typical machined surfaces because it assumes a roughly sinusoidal profile. Ra 32 µin gives RMS 35.5 µin, not the 32.5 µin printed on several widely copied charts.

### Can I convert Ra to Rz?

Not exactly. Ra averages the whole profile while Rz is a peak-to-valley measure, so the ratio depends on the process. The shop rule is Rz ≈ Ra × 4, but published factor tables also give ×7.2 and ×7.6. If Rz is what matters, specify Rz and measure it.

### What is the difference between Ra and CLA?

Nothing. CLA (Center Line Average) is the older British and ISO term for the same parameter now called Ra.

### When does surface finish start costing real money?

Below Ra 1.6 µm (63 µin) you typically add 40–60% or more to cycle time, and below Ra 0.8 µm (32 µin) you generally need a secondary grinding or polishing operation. Specify the loosest Ra the function tolerates.

### Should I put an N-grade or an Ra value on a drawing?

An explicit Ra value. ASME Y14.36 prefers it, and it is unambiguous. N-grades still appear on older drawings and on ISO/JIS drawings, which is why the mapping is on this page.

## Sources

- [Engineers Edge — Surface Roughness Conversion Chart Tables](https://www.engineersedge.com/manufacturing/surface-roughness-conversion.htm)
- [Engineers Edge — Surface Roughness (Finish) Review and Equations (ASME B46.1 / ISO 4287)](https://www.engineersedge.com/surface_finish.htm)
- [Mactech Inc. — Surface Roughness Conversion Chart (PDF)](https://www.mactechonsite.com/wp-content/uploads/Surface-Roughness-Conversion-Chart.pdf)
- [Rembar — Surface Conversion Chart (PDF)](https://www.rembar.com/wp-content/uploads/2020/01/Rembar-Surface-Conversion-Chart.pdf)
- [PREMSA Industries — Surface Roughness Chart](https://premsaindustries.com/en/resources/surface-finish-chart)
- [RapidDirect — Surface Roughness Chart](https://www.rapiddirect.com/blog/surface-roughness-chart/)

## Related processes

- [CNC Milling](https://manufacturingprocesses.org/processes/cutting/cnc-milling.md)
- [CNC Turning](https://manufacturingprocesses.org/processes/cutting/cnc-turning.md)
- [Grinding, Sanding and Polishing](https://manufacturingprocesses.org/processes/finishing/grinding-sanding-and-polishing.md)
- [Bead Blasting](https://manufacturingprocesses.org/processes/finishing/bead-blasting.md)
- [Electropolishing](https://manufacturingprocesses.org/processes/finishing/electropolishing.md)

## Related charts

- [ISO 286 Fits & Tolerance Chart (Hole Basis)](https://manufacturingprocesses.org/charts/iso-286-fits-tolerances.md)
- [Common Engineering Material Properties](https://manufacturingprocesses.org/charts/material-properties.md)

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/charts/surface-finish-chart)*

*Last updated: August 11, 2026*