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Surface Roughness Chart: N Grades, Ra, Rz and RMS

Guide · Updated October 28, 2026

Drawings call surface finish in several ways: Ra in micrometers on metric prints, Ra in microinches on inch prints, old RMS or CLA callouts, Rz on European prints, and the N grade numbers from the older ISO 1302 scale. This chart lines them up. Ra conversions are exact (1 µm = 39.37 µin) and the N grade Ra values are the ISO 1302 preferred series, which double each step. Rz and RMS relationships are approximate because they depend on the profile shape.

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Step by step

  1. Read the parameter: Note whether the print says Ra, Rz, RMS, CLA or an N grade.
  2. Convert units: µm × 39.37 = µin; µin × 0.0254 = µm.
  3. Use the N table: N grade to Ra µm and µin from the chart.
  4. Set the tester cutoff: 0.8 mm cutoff suits most machined finishes from about Ra 0.1 to 2 µm; 2.5 mm for rougher.
  5. Measure across the lay: Trace perpendicular to tool marks unless the print says otherwise.
  6. Compare to the limit: A single Ra number on a print is normally a maximum.

N grade to Ra chart

Microinch values are the rounded numbers used on inch prints and comparator plates (63, 125, 250 and so on). N grades were dropped from the 2002 revision of ISO 1302 but are still common on older prints and comparator sets.

Ra, RMS and CLA

Ra is the arithmetic average deviation from the mean line. CLA (center line average) is the same thing under an older British name, and AA (arithmetic average) is the older American name. RMS (Rq) is the root mean square, which for a sine-like turned profile is about 1.11 × Ra. Most shops treat an old RMS callout as equal to Ra; the difference is about 11% and usually inside the tolerance of the measurement.

Rz versus Ra

Rz is the average peak-to-valley height over five sampling lengths. There is no exact conversion, but Rz is typically 4 to 10 times Ra for machined surfaces, around 4 for smooth ground or honed finishes and 6 to 7 for turned surfaces. If a print says Rz, measure Rz; do not convert from Ra for acceptance.

Typical finishes by process

Ranges are general shop experience, not guarantees; tooling, rigidity and coolant can move a process a full grade either way.

Worked example: metric print on an inch shop

A print calls Ra 1.6 µm. 1.6 × 39.37 = 63.0 µin, so set the tester limit at 63 µin. That is N7. A finish turned with an 0.008 ipr feed and a 1/32 in nose radius has a theoretical peak-to-valley of 0.008² ÷ (8 × 0.03125) = 0.000256 in = 256 µin, and Ra roughly one quarter of that, about 64 µin; a little feed reduction gives margin.

Common problems

Measuring along the lay instead of across it, using the wrong cutoff, trusting a fingernail comparison for a tight callout, and treating Rz and Ra as interchangeable. Clean the part; coolant film and burrs both read as roughness.

FAQ

What is N7 surface finish?

Ra 1.6 µm, about 63 µin.

How do I convert Ra µm to µin?

Multiply by 39.37.

Is RMS the same as Ra?

Not exactly; RMS is about 1.11 × Ra for a typical turned profile.

What is 32 Ra?

32 µin Ra, which is 0.8 µm, grade N6.

How does Rz relate to Ra?

Roughly 4 to 10 times Ra; there is no exact conversion.

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General shop information only — follow your drawing, tool maker's data and your quality system.