Common Surface Finish Values
Common Ra callouts. Tap a microinch value to load it in the converter.
| Ra (µin) | Near (µm) | Legacy |
|---|---|---|
| 0.4 | N5 | |
| 0.8 | N6 | |
| 1.6 | N7 | |
| 3.2 | N8 | |
| 6.3 | N9 |
Metric numbers above are the legacy grade nominal, not exact conversions of the µin callouts. For example, 32 µin is exactly 0.8128 µm—not 0.8 µm.
Surface Finish Converter
Enter Ra in µin or µm. The nearest N grade is reference only, not a tolerance or drawing approval.
Surface Roughness Chart — Ra (µm), µin & N1–N12
The exact unit relationship is 1 µin = 0.0254 µm. To convert micrometers to microinches, divide by 0.0254. Displayed calculated equivalents are rounded to four decimal places. Nominal N-grade roughness and traditional rounded US callouts are shown separately. Use the stated Ra value and units on the drawing for inspection.
| Grade | Ra (µm) | Converted (µin) | Customary (µin) |
|---|---|---|---|
| 0.025 | 0.9843 | 1 | |
| 0.05 | 1.9685 | 2 | |
| 0.1 | 3.937 | 4 | |
| 0.2 | 7.874 | 8 | |
| 0.4 | 15.748 | 16 | |
| 0.8 | 31.4961 | 32 | |
| 1.6 | 62.9921 | 63 | |
| 3.2 | 125.9843 | 125 | |
| 6.3 | 248.0315 | 250 | |
| 12.5 | 492.126 | 500 | |
| 25 | 984.252 | 1,000 | |
| 50 | 1,968.5039 | 2,000 |
No matching grade. Clear the filter to see the full chart.
*N grades are historical shorthand; this is not a current ISO 21920 surface-finish tolerance table. A conventional “125” finish is 125 µin = 3.175 µm exactly, although N8’s metric nominal is 3.2 µm.
Typical Surface Finish by Machining Process
Approximate illustrative Ra ranges commonly reported for each process—not guaranteed outcomes, ISO grades, material-specific limits, or purchase specifications. Your material, feed, geometry, setup, finish pass and inspection method control the actual finish.
| Process | Ra (µm) | Approx. Ra (µin) | What changes the result |
|---|---|---|---|
| Milling | 0.8–6.3 | 32–250 | Toolpath, cutter condition, setup rigidity |
| Turning | 0.4–6.3 | 16–250 | Feed, insert nose radius, vibration |
| Drilling | 1.6–6.3 | 63–250 | Drill geometry, feed, hole condition |
| Reaming | 0.8–3.2 | 32–125 | Allowance, tool wear, alignment |
| Grinding | 0.1–1.6 | 4–63 | Wheel, dressing, passes |
| Honing | 0.05–0.8 | 2–32 | Stone selection, honing setup |
| Lapping | 0.012–0.4 | 0.5–16 | Abrasive, pressure, time |
| Sand casting | 12.5–25 | 500–1,000 | Mold surface, alloy, cleanup |
Ranges are planning context compiled from published machining guides, not manufacturing guarantees. A very low Ra target can increase machining/inspection work; use the least restrictive finish that meets function and the applicable engineering specification.
Surface Finish Symbols on Engineering Drawings
A surface-texture symbol identifies a requirement on the surface it references. To read it correctly, identify the parameter (for example, Ra), numeric limit, units, measurement specification and any material-removal or lay instructions.
General symbol: manufacturing method not specified. Bar addition: material removal required. Circle addition: material removal prohibited. These are conceptual descriptions; current ISO 21920-1 graphical proportions/annotations and drawing defaults must be consulted for a contractual specification.
The former ISO 1302 surface-texture indication standard was replaced by ISO 21920-1:2021; ASME drawings may instead invoke ASME B46.1-2019 (R2026).
Ra vs Rz vs Rq (RMS) vs Rt
Arithmetic mean of absolute profile deviations. This page converts its units exactly.
Root-mean-square profile height. It cannot be determined from Ra alone.
A peak-to-valley style height parameter whose precise definition depends on the governing standard. No exact Ra multiplier exists.
Total roughness-profile height over a specified evaluation length; not computable from Ra alone.
Two parts can have identical Ra but different peak heights, valley shapes, lay or sealing performance. Measurements also depend on filtering/nesting index, evaluation length, tracing direction, calibration and the applicable drawing requirements.
What Does a 32, 63 or 125 Surface Finish Mean?
Near historical N6 (0.8 µm nominal). Often called a fine-machined finish in planning references.
Near historical N7 (1.6 µm nominal). Not a guarantee of turning or milling quality.
Near historical N8 (3.2 µm nominal). A common conventional as-machined callout.
Example: “Ra 0.8 µm” converts to 31.4961 µin; “Ra 32 µin” converts to 0.8128 µm. They are close but not numerically identical. Confirm which value is contractually specified.
Surface Finish Chart FAQ
What is a 32 surface finish?
On a drawing that specifies Ra in microinches, 32 means Ra 32 µin = 0.8128 µm. It is conventionally associated with old N6 (0.8 µm nominal), which is not an exact value match.
Is 125 surface finish the same as 3.2 µm?
Not exactly: 125 µin = 3.175 µm, while 3.2 µm = 125.9843 µin. These are historical customary/nominal equivalents, not a reason to alter drawing tolerances.
Which process can produce a 32 µin Ra finish?
Finish turning, controlled milling, reaming and grinding can sometimes produce it. It depends on material, settings, equipment and verification; a process name alone does not certify the result.
Can I convert Ra to RMS or Rz?
Not exactly. Ra, Rq/RMS and Rz quantify different aspects of the roughness profile. You need appropriate profile measurements or a validated material/process-specific correlation.
Are N1–N12 still current ISO grades?
They are legacy reference notations. Modern drawings should apply the referenced ISO 21920 or ASME B46.1 rules and give the required parameter, limits and relevant evaluation conditions.
Standards, Sources & Scope
The N-grade numeric mapping is historical. Conversions use exactly 1 µin = 0.0254 µm (displayed results are rounded). Process ranges are illustrative, not acceptance limits. This page does not reproduce a complete copyrighted standards table or guarantee surface finishes from a named process.
- ISO 21920-1:2021 — Current published ISO profile-texture indication standard; ISO 1302:2002 withdrawn.
- ISO 21920-2:2021 — Profile-texture definitions and parameters.
- ASME B46.1-2019 (R2026) — US surface-texture standard.
- ASME B46.1 educational poster — Roughness, filtering and measurement context.
- Machining Doctor — Surface Finish Chart — Practical legacy roughness and manufacturing-process context.
This calculator covers Ra unit conversion only. Actual inspection depends on the governing drawing standard, profile measurement conditions and uncertainty. The full grade series represents nominal historical associations, not currently prescribed universal limits.