Surface Roughness Chart: Ra μin to Ra μm ConversionCommon machining surface finish callouts in microinch and micrometer

Use this surface roughness chart to convert Ra values between microinch (μin) and micrometer (μm) for machining drawings, CNC setup notes, inspection planning, and supplier discussions. The core conversion is exact for units: Ra μm = Ra μin × 0.0254. Process bands are approximate references and must be confirmed against the drawing note, material, machining method, applicable standard, inspection method, and supplier capability.

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Ra Unit Conversion

Convert common Ra callouts between μin and μm using the exact 1 μin = 0.0254 μm relationship.

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Drawing Callouts

Compare common values such as 16, 32, 63, 125, and 250 μin with metric equivalents.

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Machining Context

Use approximate process bands without treating them as guaranteed machining results.

Answer first: Ra microinch to micrometer conversion

Ra measures average surface roughness. To convert Ra from microinch to micrometer, multiply by 0.0254. For example, 32 μin Ra is 0.8128 μm Ra, and 63 μin Ra is approximately 1.6002 μm Ra.

This conversion only changes the unit. It does not prove that a machining process, supplier, material, or inspection method will produce that finish.

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Showing 18 Ra rows. On small screens, swipe the table horizontally to see all columns.

Surface roughness Ra conversion chart

Convert common Ra callouts between microinch and micrometer. The unit formula is exact, while process references are approximate planning notes.

Ra μin ↑	Ra μm	Common inch callout	Common metric callout	Relative finish	Approximate process reference	Caution
1	0.0254	Ra 1 μin	Ra 0.025 μm	Ultra fine	lapping / polishing reference only	Confirm standard, filtering, and inspection equipment
2	0.0508	Ra 2 μin	Ra 0.05 μm	Ultra fine	lapping / superfinishing reference only	Confirm standard, filtering, and inspection equipment
3	0.0762	Ra 3 μin	Ra 0.08 μm	Ultra fine	lapping / superfinishing planning reference	Planning reference only; confirm the actual requirement
4	0.1016	Ra 4 μin	Ra 0.1 μm	Very fine	fine grinding / honing / polishing reference only	Confirm drawing note and measurement method
6	0.1524	Ra 6 μin	Ra 0.15 μm	Very fine	fine grinding / honing reference only	Do not infer process capability from this row alone
8	0.2032	Ra 8 μin	Ra 0.2 μm	Very fine	grinding / honing reference only	Confirm material and inspection method
12	0.3048	Ra 12 μin	Ra 0.3 μm	Fine	fine grinding or very fine machining reference	Confirm standard and supplier capability
16	0.4064	Ra 16 μin	Ra 0.4 μm	Fine	grinding or very fine machining reference only	Use as a planning reference, not a guarantee
25	0.6350	Ra 25 μin	Ra 0.6 μm	Fine machined	fine turning, boring, or grinding depending on setup	Confirm tooling, feed, speed, and inspection plan
32	0.8128	Ra 32 μin	Ra 0.8 μm	Fine machined	common turned, milled, bored, or ground finish depending on setup	Verify drawing and inspection method
50	1.2700	Ra 50 μin	Ra 1.25 μm	General machined	machining reference for non-sealing surfaces	Confirm rounding and acceptance criteria
63	1.6002	Ra 63 μin	Ra 1.6 μm	General machined	common machined finish for many non-sealing surfaces	Actual result depends on material and process controls
80	2.0320	Ra 80 μin	Ra 2.0 μm	General machined	turned, milled, bored, or drilled reference range	Do not treat as process capability guarantee
100	2.5400	Ra 100 μin	Ra 2.5 μm	Rougher machined	rough machining reference only	Confirm drawing owner rounding policy
125	3.1750	Ra 125 μin	Ra 3.2 μm	Rougher machined	rough turning, milling, or drilling reference only	Confirm drawing, standard, and supplier capability
160	4.0640	Ra 160 μin	Ra 4.0 μm	Rough	rough machining or cleanup reference only	Planning reference only
200	5.0800	Ra 200 μin	Ra 5.0 μm	Rough	rough machining / casting cleanup reference only	Confirm application and inspection method
250	6.3500	Ra 250 μin	Ra 6.3 μm	Rough	rough machining / casting cleanup reference only	Not suitable as a final requirement without specification review

μin to μm:Ra μm = Ra μin × 0.0254

μm to μin:Ra μin = Ra μm ÷ 0.0254

Last reviewed:2026-07-06

How to use this surface roughness chart

Start with the Ra value shown on the drawing or supplier note.
Choose whether the value is written in microinch (μin) or micrometer (μm).
Convert units using Ra μm = Ra μin × 0.0254 or Ra μin = Ra μm ÷ 0.0254.
Use the process band only as a planning reference, not as a guaranteed capability claim.
Confirm the final requirement with the drawing note, applicable standard, sampling length, inspection method, material, and supplier capability.

Important: Ra, RMS/Rq, Rz, and N grade are not interchangeable

Ra is an arithmetic average. RMS/Rq uses a root-mean-square calculation. Rz is based on peak-to-valley measurements and may vary by standard. N grade references are also standard-dependent. Do not convert these into fixed equivalents unless the applicable standard, measurement method, filtering, sampling length, and inspection equipment are specified.

Common Ra callouts on machining drawings

These examples come from unit conversion and common rounded drawing language. Final rounding and acceptance criteria should follow the drawing owner’s standard.

32 μin

Ra 32 μin

Equivalent to 0.8128 μm Ra; often discussed near Ra 0.8 μm only if the drawing owner accepts that rounding.

Confirm before substitution

63 μin

Ra 63 μin

Equivalent to 1.6002 μm Ra; common general machined reference, not a process guarantee.

Confirm before substitution

125 μin

Ra 125 μin

Equivalent to 3.1750 μm Ra; a rougher machined reference that still needs drawing and inspection confirmation.

Confirm before substitution

0.8 μm

Ra 0.8 μm

Equivalent to about 31.5 μin Ra; close to a 32 μin planning callout but not an automatic substitution.

Confirm before substitution

3.2 μm

Ra 3.2 μm

Equivalent to about 126.0 μin Ra; close to 125 μin for planning, not a guaranteed replacement.

Confirm before substitution

Approximate machining process reference bands

1–4 μin Ra / 0.025–0.10 μm: ultra-fine finishing, lapping, polishing, or superfinishing references.
4–16 μin Ra / 0.10–0.40 μm: fine grinding, honing, or fine finishing references.
16–32 μin Ra / 0.40–0.80 μm: fine machined finish, depending on tool, feed, speed, and material.
32–125 μin Ra / 0.80–3.2 μm: common turned, milled, bored, or drilled finish range.
125–250 μin Ra / 3.2–6.3 μm: rougher machining, casting cleanup, or non-critical surfaces.

These ranges are approximate references only. A process name cannot guarantee a surface finish without setup details and inspection acceptance criteria.

Before using a surface finish value

This chart is for preliminary unit conversion and planning. It does not replace the drawing note, customer specification, ISO/ASME surface texture standard, material requirement, machining process plan, inspection method, sampling length, cutoff/filter settings, or supplier capability data. For sealing surfaces, bearing seats, fatigue-sensitive parts, medical/aerospace/automotive safety parts, and certified inspection, confirm the requirement with engineering and quality teams.

Formula and engineering assumptions

Core formula: Ra μm = Ra μin × 0.0254; reverse formula: Ra μin = Ra μm ÷ 0.0254.
Unit scope: μin ↔ μm conversion is deterministic because 1 inch equals exactly 25.4 millimeters.
Engineering scope: Unit conversion only. Process bands, rounded callouts, RMS/Rq, Rz, and N grade notes are engineering references, not fixed equivalents or supplier capability guarantees.
Inspection scope: Confirm standard, sampling length, cutoff/filter settings, instrument, and acceptance criteria before release.

Surface Roughness FAQ

What is a surface roughness chart?

A surface roughness chart compares common surface finish values, usually Ra, in different units such as microinch (μin) and micrometer (μm). It helps engineers, machinists, inspectors, and buyers read drawing callouts and discuss machining finish requirements.

How do I convert Ra microinch to micrometer?

Multiply the Ra value in microinches by 0.0254. For example, 32 μin Ra × 0.0254 = 0.8128 μm Ra. The conversion changes the unit only; it does not change the inspection requirement.

What is 32 Ra in micrometers?

32 μin Ra is approximately 0.813 μm Ra. It is often discussed near a Ra 0.8 μm metric callout, but rounding and acceptance should follow the drawing note, standard, and inspection plan.

What is 63 Ra in micrometers?

63 μin Ra is approximately 1.600 μm Ra. It is commonly used as a general machined surface reference, but the actual finish depends on material, tool condition, feed, speed, coolant, and inspection method.

Is Ra the same as RMS or Rq?

No. Ra and RMS/Rq use different roughness calculations. There is no universal fixed conversion that is valid for every surface profile, so do not replace one with the other unless the standard or inspection method specifies how.

Is Ra the same as Rz?

No. Ra is an arithmetic average roughness value, while Rz is based on peak-to-valley measurements and can vary by standard. A fixed Ra-to-Rz conversion should not be used without a defined surface profile and measurement method.

Can a machining process guarantee a specific Ra value?

No. A process such as turning, milling, grinding, honing, or polishing can have common roughness ranges, but it does not guarantee a specific Ra value by itself. Final capability depends on machine condition, tooling, parameters, material, and inspection requirements.