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Surface finish chart and converter
A drawing might call out Ra 1.6, N7, 63 µin or two triangles for the same surface. This shows all of them together, alongside a typical Rz and the processes that normally achieve that finish, so a callout in one notation can be checked against a gauge reading in another.
The reference below is free to read — no account, no signup. The tool itself is inside MechCircle, which is free to start.
ISO 1302 roughness grades, with BIS triangles and Rz estimates
| Grade | Triangles (BIS) | Ra (µm) | Ra (µin) | RMS (µin, approx.) | Rz (µm, estimate) | Typically achieved by |
|---|---|---|---|---|---|---|
| N1 | ▽▽▽▽ | 0.025 | 0.98 | 1.09 | — | — |
| N2 | ▽▽▽▽ | 0.05 | 1.97 | 2.19 | 0.55 | Lapping / reaming / grinding |
| N3 | ▽▽▽▽ | 0.1 | 3.94 | 4.37 | 1 | Lapping / reaming / grinding |
| N4 | ▽▽▽ | 0.2 | 7.87 | 8.74 | 1.6 | Lapping / reaming / grinding |
| N5 | ▽▽▽ | 0.4 | 15.75 | 17.48 | 2.5–3 | Turning / milling or Lapping / reaming / grinding |
| N6 | ▽▽▽ | 0.8 | 31.50 | 34.96 | 4–6 | Turning / milling or Lapping / reaming / grinding |
| N7 | ▽▽ | 1.6 | 62.99 | 69.92 | 8–10 | Turning / milling or Lapping / reaming / grinding |
| N8 | ▽▽ | 3.2 | 125.98 | 139.84 | 16 | Turning / milling |
| N9 | ▽▽ | 6.3 | 248.03 | 275.32 | 32 | Turning / milling |
| N10 | ▽ | 12.5 | 492.13 | 546.26 | 50 | Turning / milling |
| N11 | ▽ | 25 | 984.25 | 1092.52 | — | — |
| N12 | none | 50 | 1968.51 | 2185.04 | — | — |
Ra and the grade are exact — each grade is double the one below it, and N9 is 6.3 µm while N10 is 12.5 µm, the two most often misquoted. The Rz column and the process column are estimates read off a comparator specimen set, not conversions.
How to use it
Read the chart if you have a drawing in front of you, or type into whichever field you have and the rest follow. Enter an Ra value and it names the tightest N grade that value satisfies — which is not always the nearest one, because an N grade on a drawing is a maximum permitted roughness rather than a target. Choosing a process narrows the Rz estimate to that process; leaving it on any keeps the range.
What it does and does not cover
- The RMS conversion uses the ×1.11 factor, which assumes an idealised sinusoidal profile. Real machined surfaces run roughly ×1.1 to ×1.3 depending on the process, so treat RMS as indicative rather than a measurement.
- Rz is a correlation, not a conversion. There is no fixed Rz to Ra factor: on the specimens this is built from, the ratio runs from about 4× for turning to 11× for lapping, so a single factor would be wrong by a factor of two at the extremes. Rz is always shown as an estimate, and as a range unless you pick a process.
- Rz and the process list come from which specimens exist at each Ra on a comparator card, not from an independent source. They are guidance, not a specification, and where the card does not reach — N1, N11 and N12 — nothing is shown rather than a guess.
- The comparator groups its specimens rather than holding one per named process, so the tool can tell turning and milling apart from lapping, reaming and grinding, but not grinding from lapping.
- The triangle symbols are the BIS / Indian bands. US practice puts them on materially different Ra ranges — one US source places two triangles at 3.2 to 6.3 µm and three at 0.4 to 1.6 µm — so a drawing from another country may not mean the same thing.
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