How do I convert Ra to Rz?

Ra does not uniquely determine Rz. The 4:1 values in this table illustrate an assumed ratio only. For inspection, measure the specified parameter using the drawing’s standard, filter and evaluation length.

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?

RMS roughness is Rq. For a sinusoidal profile, Rq/Ra is π/(2√2), about 1.111. Different profiles have different ratios, so a value labelled RMS cannot be treated as Ra by a universal factor.

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.

Page last changed 2026-10-19 · based on ISO 4287:1997

Find a grade

ISO Roughness Grades and Parameter Conversion

ISO grade^[4]	Ra µm^[4]	Rz (nominal 4:1) µm^[2]	RMS (nominal 1.11:1) µm^[2]	Ra µin^[2]	Rz µin^[2]	CLA µm^[4]
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

Rz and RMS columns illustrate stated ratios, not conversions between measured profile parameters. Rz = 4 × Ra is a scenario; Rq ≈ 1.11 × Ra assumes a sine profile. Ra alone cannot determine either value. Microinch columns are rounded.

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.

Legacy N grades and Ra

N1–N12 is a legacy shorthand for the Ra values below. Each successive value is approximately twice the previous one. Use the published rounded values when reading a legacy N-grade callout.

Legacy roughness grades — Ra values
Grade	Ra (µm)	Ra (µin, rounded)
N1	0.025	0.984
N2	0.05	1.97
N3	0.1	3.94
N4	0.2	7.87
N5	0.4	15.7
N6	0.8	31.5
N7	1.6	63
N8	3.2	126
N9	6.3	248
N10	12.5	492
N11	25	984
N12	50	1.97e+03

Source: MISUMI’s Ra comparison-specimen reference table, checked 2026-10-16. A value below 0.025 µm is finer than N1; a value above 50 µm is coarser than N12. For an intermediate input, the calculator shows the next nominal grade value at or above it, explicitly as a ceiling.

ISO 4287:1997 is a withdrawn legacy parameter definition. ISO’s catalogue identifies ISO 21920-2:2021 as its replacement. Follow the edition and measurement conditions stated on the drawing.

What was checked

2026-10-16: corrected the reversed N-grade calculation and checked all twelve published N/Ra pairs. For example, 12.5–50 µm corresponds to N10–N12, and 0.4–6.3 µm to N5–N9. This check does not establish that a particular machining process will achieve a stated finish. Read the recorded check scope or report a correction.

Frequently Asked Questions

How do I convert Ra to Rz?: Ra does not uniquely determine Rz. The 4:1 values in this table illustrate an assumed ratio only. For inspection, measure the specified parameter using the drawing’s standard, filter and evaluation length.
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?: RMS roughness is Rq. For a sinusoidal profile, Rq/Ra is π/(2√2), about 1.111. Different profiles have different ratios, so a value labelled RMS cannot be treated as Ra by a universal factor.
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

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	ASME B46.1 — Surface Texture (Surface Roughness, Waviness and Lay)	reference	ASME B46.1-2019
[2]	Value computed from the standard's defining relationship	derived	Rz = 4 × Ra is an assumed scenario. Rq = 1.11 × Ra approximates a sine-profile relationship only. Neither ratio is a general conversion. Microinch columns use the rounded factor 39.37 µin/µm.
[3]	ISO 4287 — Surface texture: Profile method, terms, definitions and surface texture parameters	reference	ISO 4287:1997
[4]	MISUMI / Nihon Kinzoku Denchu — Ra comparison specimens, N-grade reference table	reference	JIS B 0659-1:2002 reference table; accessed 2026-10-16 — source

Data Sources

Standard	Revision	What it covers on this page
ISO 4287 — Surface texture: Profile method	ISO 4287:1997	legacy parameter definitions; the N-grade pairs are sourced separately from the linked lookup
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
MISUMI / Nihon Kinzoku Denchu — Ra comparison specimens, N-grade reference table	JIS B 0659-1:2002 reference table; accessed 2026-10-16	the twelve legacy N-grade / Ra pairs only

Verification scope

2026-10-16 — All twelve N-grade / Ra pairs compared with the linked reference table. Unit arithmetic and ordering tested separately; process capability and measured surfaces are not verified by this check. Evidence

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 = 4 × Ra is an assumed scenario. Rq = 1.11 × Ra approximates a sine-profile relationship only. Neither ratio is a general conversion. Microinch columns use the rounded factor 39.37 µin/µm.

The 2026-10-16 check covers the N-grade lookup and unit arithmetic only. Process ranges and Rz/Rq scenarios are reference estimates, not measured results or acceptance limits.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. 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

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "Surface Roughness Conversion Chart." https://refmanual.com/charts/surface-roughness-conversion/ (accessed 2026-10-19). 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: 2026-10-19},
  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  - 2026-10-19
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.