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?

The reference range shown here is 0.4–6.3 µm Ra. Feed, tool nose radius, vibration, material and tool wear affect the achieved profile. Use process trials and the drawing’s measurement settings to verify a finish.

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.

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

Quick Answer

Ra is arithmetic mean roughness. This chart compares reference process ranges in µm and µin. Ra 1.6 µm ≈ 63 µin = legacy N7; 12.5–50 µm corresponds to N10–N12. Achievable finish depends on the actual setup and inspection conditions.

Find a process

Surface Roughness by Process

Process^[4]	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]	Legacy N grade^[1]	Typical use^[4]
Flame cutting #	12.5	50	50	200	492	1.97e+03	N10–N12	Thermal cut edge; scale and dross dominate
Plasma cutting #	6.3	25	25.2	100	248	984	N9–N11	Cleaner than flame cutting, still a thermal edge
Laser cutting #	3.2	12.5	12.8	50	126	492	N8–N10	Fine kerf; striations on the cut face
Waterjet cutting #	3.2	12.5	12.8	50	126	492	N8–N10	No heat-affected zone; taper on thick sections
Sawing #	12.5	25	50	100	492	984	N10–N11	Coarse stock preparation only
Shearing #	6.3	25	28.35	112.5	248	984	N9–N11	Sheet edges; rollover and a rough fracture face
Sand casting #	12.5	50	62.5	250	492	1.97e+03	N10–N12	As-cast surface; usually machined all over
Investment casting #	3.2	12.5	14.4	56.25	126	492	N8–N10	Much finer than sand; often used near-net
Die casting #	1.6	6.3	6.4	25.2	63	248	N7–N9	Good as-cast finish from a polished die
Hot forging #	12.5	50	62.5	250	492	1.97e+03	N10–N12	Scale and draft; machined where it matters
Cold drawing (bar) #	0.8	3.2	3.6	14.4	31.5	126	N6–N8	Smooth, accurate stock for screw machines
Extrusion #	1.6	6.3	7.2	28.35	63	248	N7–N9	Longitudinal lines along the extrusion direction
Planing / shaping #	3.2	12.5	12.8	50	126	492	N8–N10	Large flat surfaces; interrupted cuts
Drilling #	1.6	6.3	6.4	25.2	63	248	N7–N9	Depends heavily on feed and point geometry
Milling #	0.8	6.3	3.2	25.2	31.5	248	N6–N9	Face and peripheral milling
Turning #	0.4	6.3	1.6	25.2	15.7	248	N5–N9	Finish depends on feed, tool geometry and setup
Boring #	0.4	1.6	1.6	6.4	15.7	63	N5–N7	Improves roundness and finish over drilling
Broaching #	0.4	1.6	1.8	7.2	15.7	63	N5–N7	Keyways, splines, internal profiles
Reaming #	0.4	1.6	1.6	6.4	15.7	63	N5–N7	Sizing and finishing a drilled hole
Gear hobbing #	1.6	6.3	6.4	25.2	63	248	N7–N9	Generates the tooth flanks in one pass
Gear grinding #	0.2	1.6	1	8	7.87	63	N4–N7	Finish after heat treatment; corrects distortion
Surface grinding #	0.2	1.6	1	8	7.87	63	N4–N7	Flat surfaces to tight tolerance
Cylindrical grinding #	0.1	1.6	0.5	8	3.94	63	N3–N7	Shafts and bores; the standard precision finish
Centerless grinding #	0.1	1.6	0.5	8	3.94	63	N3–N7	High-volume production of cylindrical parts
Honing #	0.05	0.4	0.3	2.4	1.97	15.7	N2–N5	Cylinder bores; controlled cross-hatch
Lapping #	0.025	0.4	0.1625	2.6	0.984	15.7	N1–N5	Flatness and finish achieved together
Superfinishing #	0.012	0.1	0.084	0.7	0.472	3.94	Below N1–N3	Bearing surfaces; removes the amorphous layer
Polishing #	0.012	0.1	0.084	0.7	0.472	3.94	Below N1–N3	Appearance rather than geometry — no size control
Electropolishing #	0.025	0.4	0.1375	2.2	0.984	15.7	N1–N5	Chemical smoothing; deburrs and passivates together
EDM (die sinking) #	0.4	3.2	2	16	15.7	126	N5–N8	Hard materials; recast layer must be removed
Wire EDM #	0.2	1.6	1	8	7.87	63	N4–N7	Through-features in hardened material
Shot blasting #	1.6	12.5	8	62.5	63	492	N7–N10	Surface preparation; also induces compressive stress
Roller burnishing #	0.05	0.4	0.3	2.4	1.97	15.7	N2–N5	Cold-works the surface; no material removed

Process ranges are reference estimates. Feed, tool geometry, wear, vibration and material affect the result. The legacy N-grade labels are looked up from Ra; the Rz columns are assumed scenarios, not measurements.

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.

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

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?: The reference range shown here is 0.4–6.3 µm Ra. Feed, tool nose radius, vibration, material and tool wear affect the achieved profile. Use process trials and the drawing’s measurement settings to verify a finish.
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

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]	ISO grade number	derived	Lookup in the published N1–N12 sequence: Ra = 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.3, 12.5, 25, 50 µm. Larger N means larger Ra. Values below N1 or above N12 are identified explicitly.
[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	reference	n/a — typical ranges, not a revisioned specification — source
[5]	ISO 4287 — Geometrical product specifications: Surface texture, profile method	reference	ISO 4287:1997 — source
[6]	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 Ra parameter definition; not process capability or the N-grade lookup
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
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	Lookup in the published N1–N12 sequence: Ra = 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.3, 12.5, 25, 50 µm. Larger N means larger Ra. Values below N1 or above N12 are identified explicitly.

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 Finish Chart." https://refmanual.com/charts/surface-finish/ (accessed 2026-10-19). 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: 2026-10-19},
  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  - 2026-10-19
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.