Exact unit conversion
Convert Ra and Find the Closest Surface Finish Grade
Enter one positive Ra value. Calculations use µm internally.
Without JavaScript, use the default example: 0.8 µm = 31.4961 µin, which is the nominal value for legacy grade N6 and is conventionally shown as 32 µin.
0.8 µm Ra (µin)
31.4961 µin Legacy grade
N6 — exact nominal match Conventional callout
32 µin
Difference from N6: 0 µm (0%).
Conversion boundary
Only µm ↔ µin conversion of the same Ra quantity is exact. Rq/RMS, Rz, and Rt describe different profile properties and cannot be calculated from Ra alone.
Legacy reference
Surface Finish Reference Chart: Legacy N Grades and Ra Values
Exact calculated conversions are separated from conventional historical callouts.
Search by grade or value. On smaller screens, swipe within the table to see exact and conventional microinch values.
No legacy grade matches that search. Try an N grade such as N6 or an Ra value such as 0.8 µm or 32 µin.
Scroll horizontally to view additional columns and vertically to view additional rows. Column headers remain visible while scrolling.
| Identification | Arithmetic Mean Roughness (Ra) | |||
|---|---|---|---|---|
| Status | ||||
| N1 | Legacy | 0.025 | 0.9843 | 1 |
| N2 | Legacy | 0.05 | 1.9685 | 2 |
| N3 | Legacy | 0.1 | 3.9370 | 4 |
| N4 | Legacy | 0.2 | 7.8740 | 8 |
| N5 | Legacy | 0.4 | 15.7480 | 16 |
| N6 | Legacy | 0.8 | 31.4961 | 32 |
| N7 | Legacy | 1.6 | 62.9921 | 63 |
| N8 | Legacy | 3.2 | 125.9843 | 125 |
| N9 | Legacy | 6.3 | 248.0315 | 250 |
| N10 | Legacy | 12.5 | 492.1260 | 500 |
| N11 | Legacy | 25 | 984.2520 | 1000 |
| N12 | Legacy | 50 | 1968.5 | 2000 |
Exact microinch values are calculated with 1 µm = 39.37007874015748 µin. Conventional values are historical rounded callouts, so they may not equal the exact conversion. N grades are shown for legacy drawing interpretation, not as current universal acceptance ranges.
Ra, Rq, RMS, Rz, Rt, and CLA Explained
| Term | Meaning | Can it be derived from Ra? |
|---|---|---|
| Ra | Arithmetic mean of the absolute profile-height deviations over the evaluation length. | It is the entered or measured quantity. |
| Rq / RMS | Root-mean-square profile deviation; larger deviations receive more weight. | No. The actual profile is required. |
| Rz | A height parameter whose exact definition and evaluation depend on the governing standard. | No universal exact conversion. |
| Rt | Total profile height over the applicable evaluation length. | No universal exact conversion. |
| CLA / AA | Older terminology commonly used for an arithmetic-average roughness quantity. | Confirm the drawing convention before treating it as Ra. |
Why Ra Cannot Be Converted Directly to Rz or RMS
Ra describes one average. It does not preserve the shape, spacing, or extremes of the profile. Two profiles can therefore have the same Ra and different Rq and peak-to-valley height.
These are idealized normalized profiles for explanation only—not measured surfaces or standard acceptance profiles.
| Profile | Ra | Rq | Peak-to-valley height |
|---|---|---|---|
| Symmetric square | 1 | 1 | 2 |
| z = (π/2) sin(x) | 1 | π/(2√2) ≈ 1.11072 | π ≈ 3.14159 |
Do not use a universal multiplier
Any Ra-to-Rz or Ra-to-Rq ratio is an assumption about a particular surface and measurement setup. It is not an exact unit conversion and should not replace the specified parameter.
How to Read the Surface Finish Chart
- Identify the parameter on the drawing. Use this converter only when the value is Ra.
- Choose µm or µin and enter the stated value. The converter retains an unrounded canonical µm value.
- Read the exact converted Ra separately from the conventional legacy callout.
- Use the grade result only to interpret legacy N notation. For production acceptance, follow the drawing, contract documents, and governing standard.
Legacy N Grades and Current Drawing Practice
N1–N12 are historical shorthand associated with nominal Ra values. ISO 1302:2002 has been withdrawn; ISO 21920-1:2021 is the current published ISO profile-texture indication standard. For new specifications, state the required parameter and value, unit, and applicable measurement conditions instead of relying on an unexplained N grade.
Current specification check
Confirm whether the drawing invokes an ISO or ASME system. Similar-looking numbers are not enough when the parameter, filter, evaluation length, or acceptance rule differs.
Measurement Conditions That Change the Result
Before comparing a measured value with a requirement, verify that both refer to the same measurement definition and setup:
- Method: profile measurement versus areal measurement.
- Filtering: the specified filter or nesting index and any short- or long-wavelength cutoff.
- Evaluation: sampling and evaluation lengths required by the specification.
- Direction: trace orientation relative to the predominant lay.
- Instrument: stylus geometry, bandwidth, resolution, calibration, and applicable limitations.
- Condition: cleanliness, fixturing, repeated traces, uncertainty, and the stated acceptance rule.
Manufacturing Process Ranges Are Not Acceptance Limits
Turning, milling, grinding, honing, lapping, casting, and coating can produce overlapping finishes. Material, tooling, feed, speed, tool wear, vibration, and post-processing all matter. A generic process range may help with planning, but it cannot guarantee the finish of a particular part.
Worked Surface Finish Examples
Example 1: Convert nominal N6
Assumption: The source quantity is Ra = 0.8 µm. Multiply by 39.37007874015748:
The exact result is 31.4961 µin to four decimal places. The conventional legacy callout is 32 µin. N6 is an exact nominal metric-grade match; 32 µin is a nearby rounded historical value.
Example 2: Interpret a 125 µin callout
Assumption: “125” denotes Ra in microinches. Divide by 39.37007874015748:
The closest legacy grade is N8 at nominal Ra = 3.2 µm, but 125 µin is not mathematically identical to 3.2 µm; the exact conversion of 3.2 µm is 125.9843 µin. The governing drawing still controls acceptance.
Standards, Scope, and Source Notes
Legacy N1–N12 nominal Ra lookup with independently calculated exact unit conversions.
The numerical grade mapping is a limited, reorganized historical reference. Current ISO and ASME sources are used for scope and terminology, not to reproduce protected figures, symbol-placement rules, or full standards tables.
- ISO 21920-1:2021 — Profile surface-texture indicationSupports the current ISO indication framework and the withdrawn status of ISO 1302:2002.
- ISO 21920-2:2021 — Terms, definitions, and parametersSupports current profile-method terminology and separately defined texture parameters.
- ASME B46.1-2019 — Surface TextureSupports the ASME scope for roughness, waviness, lay, filtering, evaluation, and profile parameters.
- ASME B46.1 Basic Terms posterOfficial limited overview of Ra and related measurement concepts.
- ISO 1302:1978 historical preview — Clause 4.1.5 and Table 1Basis for the limited legacy N1–N12 nominal mapping and conventional microinch values.
Dataset and Source-Check Details
- Source-controlled fields
- Legacy grade, nominal Ra in µm, and conventional historical µin callout
- Calculated field
- Exact Ra in µin using 1 µm = 39.37007874015748 µin
- Valid grade scope
- N1–N12; nominal Ra 0.025–50 µm
- Excluded
- Rq/RMS, Rz, Rt, grit, process capability, cutoff tables, and application suitability
- Rights decision
- No CSV download; selected result copying and printing only
- Dataset label
- Legacy ISO roughness grades; source edition 1978
- Source status
- Standards and source status checked August 7, 2026
- Publisher
- Turn2Engineering
Surface Finish Chart FAQs
It commonly means Ra = 32 µin, which equals exactly 0.8128 µm. It is conventionally associated with legacy N6, whose nominal metric value is 0.8 µm.
No. RMS commonly refers to Rq, which weights larger deviations more heavily. The actual profile is needed to determine it.
No exact universal conversion exists. Ra and Rz respond differently to profile shape, filtering, and evaluation conditions.
Treat them as legacy roughness shorthand found on older drawings. Use the governing current ISO or ASME standard for new specifications.
The exact dimensional conversion is 125.9843 µin. The 125 µin value is a conventional rounded historical callout.
Use Exact Ra Values for the Actual Requirement
Convert the unit of Ra exactly, treat N grades and conventional microinch values as legacy references, and verify the parameter plus measurement conditions on the governing drawing. A matching Ra number alone does not establish Rq, Rz, Rt, process capability, or part acceptance.