How do I convert Ra to Rz?

Multiply by about 4 for the conventional pairing, but treat it as an estimate rather than a conversion. Rz and Ra measure different things about the same profile — Ra is the mean deviation and Rz the peak-to-valley height — and the real ratio depends on the process: about 4 for turning, 5 to 7 for grinding, and much higher where isolated deep scratches are present.

What is the difference between Ra and CLA?

Nothing — they are the same parameter under different names. CLA stands for centre line average, the original British term; Ra is the ISO designation for the arithmetic mean deviation. Values are numerically identical, and older drawings carrying CLA can be read as Ra without conversion.

What is RMS surface finish?

Root mean square roughness, a genuinely different parameter from Ra and about 11% higher on typical surfaces. It survives mainly in older American drawings and specifications. A 63 RMS surface is roughly a 57 µin Ra — but the relationship varies with the profile, so the conversion is an estimate.

What is N7 surface finish?

Ra 1.6 µm, or about 63 µin. It is the most commonly specified roughness grade, achievable by ordinary turning and milling without a special finishing operation, and fine enough for most running and sliding fits.

Which is finer, N5 or N7?

N5 is finer — a lower N number means a smoother surface. N5 is Ra 0.4 µm and N7 is Ra 1.6 µm, so N5 is four times smoother and needs a grinding or fine-finishing operation to achieve.

Surface Roughness Conversion

The twelve ISO roughness grades with their Ra, Rz and RMS values in micrometres and microinches — and why converting between the parameters is an estimate rather than a conversion.

Data verified 2026-09-29 · based on ISO 4287:1997

Find a grade

ISO Roughness Grades and Parameter Conversion

ISO grade^[3]	Ra µm^[3]	Rz (nominal 4:1) µm^[2]	RMS (nominal 1.11:1) µm^[2]	Ra µin^[2]	Rz µin^[2]	CLA µm^[3]
N12 #	50	200	55.5	1.97e+03	7.87e+03	50
N11 #	25	100	27.75	984	3.94e+03	25
N10 #	12.5	50	13.88	492	1.97e+03	12.5
N9 #	6.3	25.2	6.993	248	992	6.3
N8 #	3.2	12.8	3.552	126	504	3.2
N7 #	1.6	6.4	1.776	63	252	1.6
N6 #	0.8	3.2	0.888	31.5	126	0.8
N5 #	0.4	1.6	0.444	15.7	63	0.4
N4 #	0.2	0.8	0.222	7.87	31.5	0.2
N3 #	0.1	0.4	0.111	3.94	15.7	0.1
N2 #	0.05	0.2	0.0555	1.97	7.87	0.05
N1 #	0.025	0.1	0.02775	0.984	3.94	0.025

The Rz and RMS columns are nominal relationships, not conversions. Rz and Ra are independent measurements of the same profile, and no exact conversion exists. The 4:1 ratio shown is the conventional pairing used in the ISO grade series; on a real turned surface Rz/Ra is nearer 4, on a ground surface 5 to 7, and on a surface with isolated deep scratches it can be far higher.

RMS and CLA are historical names. CLA (centre line average) is the original British term for exactly what Ra measures — the two are numerically identical, and drawings still occasionally carry CLA values. RMS (root mean square) is a genuinely different parameter, about 11% higher than Ra on typical surfaces, and it survives mainly in older American drawings.

The practical rule: specify Ra and inspect Ra. Converting between parameters introduces an error larger than the tolerance you are trying to hold, and customer and supplier frequently disagree about which parameter a bare number refers to.

Which Grade to Specify

The grades below N5 all require a finishing operation beyond ordinary turning or milling, and each step costs disproportionately more than the last.

N9 to N8 (6.3 to 3.2 µm) — rough machining, clearance surfaces, non-critical faces. Achievable by any turning or milling operation.

N7 (1.6 µm) — the workhorse grade, specified on the majority of engineering drawings. Ordinary turning and milling reach it with a reasonable feed, and it is adequate for most running and sliding fits.

N6 to N5 (0.8 to 0.4 µm) — needs a controlled finish pass, fine feed or grinding. Used for bearing seats, sealing surfaces and close sliding fits.

N4 and finer (0.2 µm and below) — honing, lapping or superfinishing. Used for hydraulic cylinder bores, gauge surfaces and rolling-element bearing raceways.

The cost curve is steep: going from N7 to N5 typically multiplies the finishing cost of that surface several times, and going below N3 usually means a separate operation on different equipment. Specify the coarsest grade the function allows, and specify it only on the surfaces that need it — a blanket roughness note on the whole drawing is a reliable way to double a part's price for no benefit.

Frequently Asked Questions

How do I convert Ra to Rz?: Multiply by about 4 for the conventional pairing, but treat it as an estimate rather than a conversion. Rz and Ra measure different things about the same profile — Ra is the mean deviation and Rz the peak-to-valley height — and the real ratio depends on the process: about 4 for turning, 5 to 7 for grinding, and much higher where isolated deep scratches are present.
What is the difference between Ra and CLA?: Nothing — they are the same parameter under different names. CLA stands for centre line average, the original British term; Ra is the ISO designation for the arithmetic mean deviation. Values are numerically identical, and older drawings carrying CLA can be read as Ra without conversion.
What is RMS surface finish?: Root mean square roughness, a genuinely different parameter from Ra and about 11% higher on typical surfaces. It survives mainly in older American drawings and specifications. A 63 RMS surface is roughly a 57 µin Ra — but the relationship varies with the profile, so the conversion is an estimate.
What is N7 surface finish?: Ra 1.6 µm, or about 63 µin. It is the most commonly specified roughness grade, achievable by ordinary turning and milling without a special finishing operation, and fine enough for most running and sliding fits.
Which is finer, N5 or N7?: N5 is finer — a lower N number means a smoother surface. N5 is Ra 0.4 µm and N7 is Ra 1.6 µm, so N5 is four times smoother and needs a grinding or fine-finishing operation to achieve.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B46.1 — Surface Texture (Surface Roughness, Waviness and Lay)	standard	ASME B46.1-2019
[2]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining relationship and verified against every row.
[3]	ISO 4287 — Surface texture: Profile method, terms, definitions and surface texture parameters	standard	ISO 4287:1997

Data Sources

Standard	Revision	What it covers on this page
ISO 4287 — Surface texture: Profile method	ISO 4287:1997	the Ra, Rz and CLA parameter definitions and the N grade series
ASME B46.1 — Surface Texture	ASME B46.1-2019	the RMS parameter and the American practice referenced in the discussion
ISO 1302 — Indication of surface texture in technical product documentation	ISO 1302:2002	how the grade is written on a drawing

Cross-checked against:

The N grade series verified as √10 steps in Ra from N1 to N12
Ra values converted to microinches at build time and compared against the standard 63 µin for N7
CLA verified as numerically identical to Ra rather than as a separate parameter
Six values held as known-value checks spanning the full grade range

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Rz, RMS, and microinch columns	Rz = Ra × 4 and RMS = Ra × 1.11 are the conventional nominal pairings, not exact conversions — the page says so explicitly. Microinches = micrometres × 39.37, which is exact.

Ra is the parameter to specify and to inspect. The Rz and RMS columns exist because drawings and legacy specifications quote those parameters, and they are estimates. Where a surface's peak height matters — sealing, fatigue, coating — specify Rz directly rather than converting from Ra.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Surface Roughness Conversion Chart." https://refmanual.com/charts/surface-roughness-conversion/ (accessed 2026-09-29). Data from ISO 4287 — Surface texture: Profile method, ISO 4287:1997.

BibTeX

@misc{surface-roughness-conversion,
  title        = {Surface Roughness Conversion Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/surface-roughness-conversion/}},
  note         = {Accessed: 2026-09-29},
  source       = {ISO 4287 — Surface texture: Profile method, ISO 4287:1997}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Surface Roughness Conversion Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/surface-roughness-conversion/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ISO 4287 — Surface texture: Profile method, ISO 4287:1997
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.