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Reference Chart

Surface Finish Chart

Convert between Ra, Rz, and RMS surface roughness values. See which machining process gives the finish you need, from mirror polish to rough mill.

Showing 16 of 16 finishes
Ra (μin) ▲Ra (μm) ▲Rz (μin) ▲RMS (μin) ▲N Grade ▲Machining Process ▲Typical Application ▲
10.02541.1N1Superfinish / LappingOptical mirrors, gauge blocks
20.0582.2N2Lapping / PolishingPrecision bearings, seals
40.1164.4N3Fine grindingHydraulic valve spools, piston rods
80.2328.9N4GrindingBearing journals, crankshaft pins
160.46317.8N5Fine grinding / HoningCylinder bores, sealing surfaces
320.812535.5N6Fine turning / Fine millingMating surfaces, close-fit shafts
631.625069.9N7Standard milling / TurningGeneral machined surfaces, brackets
1253.2500138.8N8Milling / TurningStructural parts, non-critical faces
2506.31000277.5N9Rough milling / Rough turningRough-machined castings, forgings
50012.52000555N10Rough milling / SawingRaw stock surfaces, non-functional faces
160.46317.8N5Fine turningPrecision OD/ID fits, valve stems
631.625069.9N7Standard turningShafts, bushings, spacers
320.812535.5N6Fine wire EDMMold cavities, precision dies
631.625069.9N7Wire EDMStamping dies, extrusion dies
1253.2500138.8N8Sinker EDMDeep ribs, complex mold shapes
2506.31000277.5N9Rough sinker EDMRoughing cavities, electrode burns
No finishes match your search. Try a different term or filter.

Quick Answer: What Is Surface Finish (Ra, Rz and RMS)?

Surface finish is the measure of small peaks and valleys left on a machined part. Ra is the arithmetic average roughness and the most common spec on drawings. Rz is the average peak-to-valley height over a sample length. RMS is the root mean square of the same heights. A lower number means a smoother surface. These values are used to set fit, seal, wear and appearance requirements.

This chart converts between Ra, Rz, RMS and ISO 1302 N grades to show: Ra in microinches (μin), Ra in micrometers (μm), Rz value and RMS value, plus the matching N grade and machining process. Reference data covers superfinishing through rough milling, turning, grinding and EDM. RMS is about Ra × 1.11 for typical machined surfaces.

Typical Ra by process: Grinding/lapping: 1–16 μin (N1–N5) | Fine turning/milling: 32 μin (N6) | Standard milling/turning: 63–125 μin (N7–N8) | Rough milling: 250–500 μin (N9–N10).

Notes on This Chart

  • Ra is arithmetic average roughness. RMS is approximately Ra x 1.11 for typical machined surfaces.
  • Rz measures peak-to-valley height over a sample length. It is more sensitive to deep scratches.
  • N grades follow ISO 1302. N1 is the smoothest. N12 is the roughest.
  • Actual finish depends on material, tooling, speed, feed, and coolant. Always verify with a profilometer.

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Frequently Asked Questions

What is Ra in surface finish?

Ra is the arithmetic average roughness. It measures the average height of peaks and valleys on a surface. A lower Ra means a smoother finish. Ra is the most common way to call out surface finish on drawings.

What is the difference between Ra and RMS?

Ra is the arithmetic average. RMS (root mean square) gives more weight to large peaks and valleys. RMS is about 11% higher than Ra for most machined surfaces. Multiply Ra by 1.11 to estimate RMS.

What surface finish can CNC milling achieve?

Standard CNC milling gives 63 to 125 microinch Ra. Fine milling with sharp tools and light cuts can reach 32 microinch Ra. With grinding or polishing, you can get below 16 microinch Ra.

How do I specify surface finish on a drawing?

Use the surface finish symbol (a check mark with a number). Write the Ra value in microinches or micrometers next to it. Place the symbol on the surface that needs it. Only call out finishes tighter than standard when needed. Tighter finishes cost more.

Surface Finish Reference Series

Related references — jump to the one you need:

Surface Finish ChartRa ConverterFinish Selector

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