Surface roughness converter
Type a Ra value, or tap one of the ISO preferred values below. Everything below updates from that single number.
One caveat worth reading before you use the Rz number. Ra averages every peak and valley over the sampling length, while Rz measures only the tallest peak-to-valley span inside each cut-off. Two surfaces can share identical Ra and carry very different Rz, so the conversion is an estimate that lands within a band, not a fixed ratio. The tool keeps it honest by printing a range rather than a single figure. Where Rz governs the drawing, measure it directly.
Typical Ra range by process
These bands describe what each process generally reaches across normal shop conditions. Actual results move with tool condition, machine rigidity, coolant, and the material itself, so a fine pass on a tired machine can easily fall outside them. Use the table for planning and first-guess drawing calls, then hold the tolerance to what your process can repeat.
| Process | Typical Ra (um) | Typical Ra (uin) | Where it usually lands |
|---|---|---|---|
| Lapping / superfinishing | 0.01 - 0.2 | 0.4 - 8 | Gauge faces, seal faces |
| Polishing | 0.025 - 0.4 | 1 - 16 | Optical and decorative finishes |
| Precision grinding | 0.05 - 0.4 | 2 - 16 | Bearing journals, slideways |
| Honing | 0.1 - 0.8 | 4 - 32 | Hydraulic bores, cylinder liners |
| General grinding | 0.2 - 1.6 | 8 - 63 | Hardened parts, flat surfaces |
| Fine turning / boring | 0.4 - 1.6 | 16 - 63 | Sealing diameters, accurate fits |
| Fine milling | 0.8 - 3.2 | 32 - 125 | Mould cavities, precision pockets |
| Turning, general | 1.6 - 6.3 | 63 - 250 | Most shaft and bushing work |
| Milling, general | 1.6 - 6.3 | 63 - 250 | Housings, brackets, plates |
| Wire EDM | 0.8 - 3.2 | 32 - 125 | Cut-throughs, hardened tool work |
| Reaming / drilling | 1.6 - 6.3 | 63 - 250 | Holes before finishing |
| Rough machining | 6.3 - 25 | 250 - 1000 | Stock removal passes |
| Die casting | 1.6 - 6.3 | 63 - 250 | As-cast net shapes |
| Sand casting | 12.5 - 50 | 500 - 2000 | As-cast rough surfaces |
The preferred Ra series and N grades
Drawings work better when the roughness call falls on a preferred value rather than an arbitrary one, because tooling, comparator samples and surface finish standards are all built around the same series. ISO 1302 assigns a grade number to each step so a drawing can specify either the Ra value or the N number.
| Grade | Ra (um) | Ra (uin) | Grade | Ra (um) | Ra (uin) |
|---|---|---|---|---|---|
| N1 | 0.025 | 1 | N7 | 1.6 | 63 |
| N2 | 0.05 | 2 | N8 | 3.2 | 125 |
| N3 | 0.1 | 4 | N9 | 6.3 | 250 |
| N4 | 0.2 | 8 | N10 | 12.5 | 500 |
| N5 | 0.4 | 16 | N11 | 25 | 1000 |
| N6 | 0.8 | 32 | N12 | 50 | 2000 |
Questions that come up
Is Ra always the right call for a drawing?
No. Ra hides the difference between a surface with regular fine marks and one with occasional deep scratches, because averaging washes the outliers out. For sealing faces, bearing surfaces and anything carrying cyclic load, add Rz or Rmr alongside Ra so the requirement describes the surface shape and not just its average height.
Why does my gauge read a different Ra than the drawing asks for?
Cut-off length is the usual culprit. Ra measured over a 0.8 mm sampling length differs from the same track measured over 2.5 mm, and the two figures are not interchangeable. Check that the gauge and the drawing agree on cut-off, filter type, and whether the result uses one sample or an average over several.
Does a smoother finish always cost more?
It usually does, and the cost curve steepens sharply below about Ra 0.4. Getting there often means an extra grinding or honing pass, finer media, and far more inspection time. Specify the roughest value the function tolerates; calling Ra 0.4 on a bracket face only invites a rejected first article and a slower part.
How do I write it on the drawing?
ISO 1302 symbols carry the Ra value with the grade optionally in brackets, for example Ra 1.6 or N7. Where a process must produce the surface, the indication can bar material removal. Keeping the same convention across drawings avoids arguments later about whether a finish was specified.
Which standard should I cite?
For terminology and parameter definitions, ISO 21920-2 superseded ISO 4287, and drawings using the older ISO 1302 symbol remain readable in most shops. ASME Y14.5 covers the North American symbol set, which looks different from the ISO symbol for the same requirement. Reference the framework your customer actually inspects against.
