Surface Finish Chart

Name: Surface Finish Chart
Creator: Manufacturing Processes

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.

Tables: 3
Rows: 43
Units: µm, µin, in, mm
References: 6
Revised: 2026-08-11

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

12 rows

N-grade	Raµm	Raµin	RMSµin	Rz approxµin	Rtµm	Cut-offin	Cut-offmm
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
No rows match that filter.

12 rows × 8 columnsUnits: µm, µin, in, mm

Conversions between Ra, RMS, Rz and Rt

8 rows

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
No rows match that filter.

8 rows × 3 columns

Basis and references

Read before use

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.

References

Every value on this page was set against at least two of these sources and recomputed rather than transcribed. Where they disagree, the disagreement is recorded in the notes below.

Typical Ra achievable by manufacturing process

23 rows

Process	Ra typicalµin	Ra typicalµm	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
No rows match that filter.

23 rows × 5 columnsUnits: µin, µm

Revision and twin

2026-08-11

Last reviewed 2026-08-11. This page carries 43 rows across 3 tables, all of them in the HTML — nothing is paginated, gated or fetched.

Plain-Markdown twin: /charts/surface-finish-chart.md

Notes to the tables

4 notes

Where this data disagrees with itself, with other published charts, or with what the number looks like it means.

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.

Questions

7 questions

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.

How to cite this page

Manufacturing Processes. “Surface Finish Chart.” manufacturingprocesses.org, last reviewed 2026-08-11. https://manufacturingprocesses.org/charts/surface-finish-chart

If you reproduce a table, cite the standard it derives from as well — the references are the primary sources, this page is a cross-checked transcription of them.