What is Ra in surface finish?

The arithmetic mean roughness — the average absolute deviation of the profile from its mean line over a sampling length, in micrometres. It is the most commonly specified roughness parameter.

What is the difference between Ra and Rz?

Ra is the average deviation from the mean line; Rz is the average peak-to-valley height. Ra is stable and widely used, while Rz is more sensitive to isolated peaks and better at predicting sealing and fatigue performance.

What surface finish can turning achieve?

About 0.4 to 6.3 µm Ra, the widest range of any single process. The finish is dominated by feed rate — halving the feed roughly halves Ra — with tool nose radius, material and rigidity also contributing.

What is a 1.6 surface finish?

Ra 1.6 µm, which is ISO grade N7 — the most commonly specified grade on engineering drawings. It is achievable by ordinary turning and milling and is fine enough for most running and sliding fits.

How do I convert Ra to microinches?

Multiply by 39.37. Ra 1.6 µm is about 63 µin. Microinch values are still common on North American drawings, so it is worth checking which unit a specification uses.

Is a finer surface finish always better?

No. Finer finishes cost disproportionately more, and very smooth surfaces can have higher friction because they adhere over a larger real contact area. Specify the roughness the function requires — sealing, fatigue life, appearance or fit — rather than the finest achievable.

Surface Finish Chart

Achievable surface roughness by machining process — Ra in micrometres and microinches, with the ISO grade number for each range.

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

Quick Answer

Ra is the arithmetic mean roughness, in micrometres. Turning spans the widest range of any process — about 0.4 to 6.3 µm depending on feed and tool — while grinding reaches 0.1 to 1.6 µm and superfinishing goes below 0.1 µm. The finer the finish, the more the process costs.

Find a process

Surface Roughness by Process

Process^[5]	Ra min µm^[4]	Ra max µm^[4]	Rz min (est.) µm^[3]	Rz max (est.) µm^[3]	Ra min µin^[2]	Ra max µin^[2]	ISO grade^[1]	Typical use^[4]
Flame cutting #	12.5	50	50	200	492	1.97e+03	N8–N6	Thermal cut edge; scale and dross dominate
Plasma cutting #	6.3	25	25.2	100	248	984	N9–N7	Cleaner than flame cutting, still a thermal edge
Laser cutting #	3.2	12.5	12.8	50	126	492	N10–N8	Fine kerf; striations on the cut face
Waterjet cutting #	3.2	12.5	12.8	50	126	492	N10–N8	No heat-affected zone; taper on thick sections
Sawing #	12.5	25	50	100	492	984	N8–N7	Coarse stock preparation only
Shearing #	6.3	25	28.35	112.5	248	984	N9–N7	Sheet edges; rollover and a rough fracture face
Sand casting #	12.5	50	62.5	250	492	1.97e+03	N8–N6	As-cast surface; usually machined all over
Investment casting #	3.2	12.5	14.4	56.25	126	492	N10–N8	Much finer than sand; often used near-net
Die casting #	1.6	6.3	6.4	25.2	63	248	N11–N9	Good as-cast finish from a polished die
Hot forging #	12.5	50	62.5	250	492	1.97e+03	N8–N6	Scale and draft; machined where it matters
Cold drawing (bar) #	0.8	3.2	3.6	14.4	31.5	126	N12–N10	Smooth, accurate stock for screw machines
Extrusion #	1.6	6.3	7.2	28.35	63	248	N11–N9	Longitudinal lines along the extrusion direction
Planing / shaping #	3.2	12.5	12.8	50	126	492	N10–N8	Large flat surfaces; interrupted cuts
Drilling #	1.6	6.3	6.4	25.2	63	248	N11–N9	Depends heavily on feed and point geometry
Milling #	0.8	6.3	3.2	25.2	31.5	248	N12–N9	Face and peripheral milling
Turning #	0.4	6.3	1.6	25.2	15.7	248	N12–N9	Widest range of any single process
Boring #	0.4	1.6	1.6	6.4	15.7	63	N12–N11	Improves roundness and finish over drilling
Broaching #	0.4	1.6	1.8	7.2	15.7	63	N12–N11	Keyways, splines, internal profiles
Reaming #	0.4	1.6	1.6	6.4	15.7	63	N12–N11	Sizing and finishing a drilled hole
Gear hobbing #	1.6	6.3	6.4	25.2	63	248	N11–N9	Generates the tooth flanks in one pass
Gear grinding #	0.2	1.6	1	8	7.87	63	N12–N11	Finish after heat treatment; corrects distortion
Surface grinding #	0.2	1.6	1	8	7.87	63	N12–N11	Flat surfaces to tight tolerance
Cylindrical grinding #	0.1	1.6	0.5	8	3.94	63	N12–N11	Shafts and bores; the standard precision finish
Centerless grinding #	0.1	1.6	0.5	8	3.94	63	N12–N11	High-volume production of cylindrical parts
Honing #	0.05	0.4	0.3	2.4	1.97	15.7	N12–N12	Cylinder bores; controlled cross-hatch
Lapping #	0.025	0.4	0.1625	2.6	0.984	15.7	N1–N12	Flatness and finish achieved together
Superfinishing #	0.012	0.1	0.084	0.7	0.472	3.94	N1–N12	Bearing surfaces; removes the amorphous layer
Polishing #	0.012	0.1	0.084	0.7	0.472	3.94	N1–N12	Appearance rather than geometry — no size control
Electropolishing #	0.025	0.4	0.1375	2.2	0.984	15.7	N1–N12	Chemical smoothing; deburrs and passivates together
EDM (die sinking) #	0.4	3.2	2	16	15.7	126	N12–N10	Hard materials; recast layer must be removed
Wire EDM #	0.2	1.6	1	8	7.87	63	N12–N11	Through-features in hardened material
Shot blasting #	1.6	12.5	8	62.5	63	492	N11–N8	Surface preparation; also induces compressive stress
Roller burnishing #	0.05	0.4	0.3	2.4	1.97	15.7	N12–N12	Cold-works the surface; no material removed

Ranges are typical for the process under normal conditions. Feed rate is the dominant control in turning and milling — halving the feed roughly halves Ra. Tool wear, built-up edge, vibration and material all shift the result, so treat these as achievable ranges rather than guarantees.

What Ra Actually Measures

Ra is the arithmetic mean of the absolute deviation of the profile from its mean line, measured over a sampling length. It is the most widely specified roughness parameter because it is stable and easy to measure.

Its weakness is that it says nothing about the shape of the profile. A surface with many small scratches and one with a few deep valleys can have the same Ra while behaving completely differently in sealing, fatigue and wear. Where that matters, specify Rz as well — Rz is the peak-to-valley height and is far more sensitive to isolated features.

Ra is not the same as Rz, and the ratio between them is not a constant — it runs around 4 for turned surfaces and 5 to 7 for ground ones. Use the Ra to Rz calculator for an estimate, but specify the parameter you actually intend to inspect.

The ISO Grade Numbers

ISO 4287 assigns roughness grades N1 to N12, in steps of √10 in Ra:

N1 = 0.025 µm · N5 = 0.4 µm · N7 = 1.6 µm · N9 = 6.3 µm · N12 = 50 µm

N7 (Ra 1.6 µm) is the most commonly specified grade on drawings, because it is achievable by ordinary turning and milling without special measures and is fine enough for most running fits.

Grades finer than N5 usually require a finishing operation — grinding, honing or lapping — and each step down costs disproportionately more. Specify the coarsest grade the function allows.

Frequently Asked Questions

What is Ra in surface finish?: The arithmetic mean roughness — the average absolute deviation of the profile from its mean line over a sampling length, in micrometres. It is the most commonly specified roughness parameter.
What is the difference between Ra and Rz?: Ra is the average deviation from the mean line; Rz is the average peak-to-valley height. Ra is stable and widely used, while Rz is more sensitive to isolated peaks and better at predicting sealing and fatigue performance.
What surface finish can turning achieve?: About 0.4 to 6.3 µm Ra, the widest range of any single process. The finish is dominated by feed rate — halving the feed roughly halves Ra — with tool nose radius, material and rigidity also contributing.
What is a 1.6 surface finish?: Ra 1.6 µm, which is ISO grade N7 — the most commonly specified grade on engineering drawings. It is achievable by ordinary turning and milling and is fine enough for most running and sliding fits.
How do I convert Ra to microinches?: Multiply by 39.37. Ra 1.6 µm is about 63 µin. Microinch values are still common on North American drawings, so it is worth checking which unit a specification uses.
Is a finer surface finish always better?: No. Finer finishes cost disproportionately more, and very smooth surfaces can have higher friction because they adhere over a larger real contact area. Specify the roughness the function requires — sealing, fatigue life, appearance or fit — rather than the finest achievable.

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]	ISO grade number	derived	N grade assigned in steps of √10 in Ra, from N1 = 0.025 µm to N12 = 50 µm.
[2]	Microinch conversion	derived	µm × 39.37.
[3]	Rz estimated from Ra	derived	Rz ≈ Ra × a process-dependent ratio, taken as 4 for turning and milling, 5 for grinding, 6–7 for honing, lapping and polishing. Rz and Ra are independent measurements — this column is an estimate for comparison only, not a substitute for specifying and measuring the parameter you need.
[4]	Achievable roughness ranges from standard machining handbooks	standard	n/a — typical ranges, not a revisioned specification — source
[5]	ISO 4287 — Geometrical product specifications: Surface texture, profile method	standard	ISO 4287:1997 — source

Data Sources

Standard	Revision	What it covers on this page
ISO 4287 — Surface texture, profile method	ISO 4287:1997	the Ra parameter definition and the N grade series

Cross-checked against:

Achievable ranges cross-checked against two machining handbooks
Microinch column and ISO grade computed from Ra on every build

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

Value	How it is derived
Microinch values	µm × 39.37, recomputed at build time for every row.
Rz estimates	Ra × a process-dependent ratio between 4 and 7. Approximate: the real ratio depends on the profile shape, which Ra does not describe.
ISO grade numbers	N grade assigned in steps of √10 in Ra, from N1 = 0.025 µm to N12 = 50 µm.

Ranges are typical for normal conditions. Feed rate, tool wear, vibration and material all shift the achievable finish, and Ra alone does not describe the profile shape.

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 Finish Chart." https://refmanual.com/charts/surface-finish/ (accessed 2026-09-29). Data from ISO 4287 — Surface texture, profile method, ISO 4287:1997.

BibTeX

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

RIS (EndNote, Zotero, Mendeley)

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

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