Surface Finish Converter — Ra, Rz, RMS & N Grades
Convert between the four ways surface roughness gets specified on drawings — Ra (µm and µin), RMS, Rz and ISO N grades — in whichever direction you need. Enter any one and the rest fall out, with an honest note on which conversions are exact and which are only ever approximate. No sign-up.
Type into any field — the others update live.
Arithmetic mean roughness — the most common on UK drawings.
Microinches — common on US drawings.
≈ 1.11 × Ra. A defined relationship for a sine-like profile.
⚠ Rz has no exact conversion from Ra. Rule of thumb only.
Ra, Rz, RMS and N grades — what they actually mean
Ra is the arithmetic mean deviation of the profile: average the absolute height of every peak and valley from the mean line. It is the most widely specified parameter in the UK and Europe, and it is what most drawings mean when they just say "surface finish". Its weakness is that it averages — a surface with one deep scratch and an otherwise fine finish can show the same Ra as a uniformly rougher one.
Rz addresses that by averaging the five largest peak-to-valley heights, so it is far more sensitive to isolated defects. This is exactly why there is no exact Ra→Rz conversion. The common rule of thumb is Rz ≈ 4–7 × Ra, but the real multiplier depends on the process: a ground surface and a turned surface with identical Ra can have quite different Rz. Treat any Ra↔Rz number, including ours, as a sanity check and nothing more.
RMS (root mean square) is the same idea as Ra but squares the deviations before averaging, which weights peaks more heavily. For typical machined profiles RMS ≈ 1.11 × Ra. It appears mostly on older or US drawings, usually in microinches.
N grades (N1–N12) are ISO 1302 roughness grade numbers, each mapped to a defined Ra value — N5 is Ra 0.4 µm, N6 is 0.8, N7 is 1.6, and so on, doubling each step. These are exact by definition, so N↔Ra conversion is reliable.
For context on what is achievable: our standard as-machined finishes typically land in the N7–N8 range, and electropolishing on 316L and titanium Grade 23 takes components down to Ra ≤0.4 µm (N5) for medical and food-contact work. If a drawing calls for a specific finish, tell us at quotation stage and we will confirm the process needed to hit it.
Surface finish — FAQ
How do you convert Ra to Rz?
There is no exact conversion. Ra averages all deviations while Rz averages the five biggest peak-to-valley heights, so the ratio depends on the surface profile and the machining process. A common rule of thumb is Rz ≈ 4–7 × Ra, but you should never convert an Rz drawing callout into Ra on a critical feature — specify the parameter you actually need.
What is the difference between Ra and RMS?
Both average the profile deviation from the mean line, but RMS squares the deviations first, which gives more weight to peaks. For typical machined surfaces RMS is about 1.11 × Ra. Ra is standard in the UK and Europe; RMS in microinches appears mostly on US or older drawings.
What Ra is N5, N6, N7 or N8?
ISO 1302 N grades map to defined Ra values: N5 = 0.4 µm, N6 = 0.8 µm, N7 = 1.6 µm, N8 = 3.2 µm, N9 = 6.3 µm. Each step roughly doubles. Because they are defined, N↔Ra conversion is exact.
What surface finish can CNC machining achieve?
As-machined finishes from milling and turning typically fall around N7–N8 (Ra 1.6–3.2 µm) depending on material, tooling and parameters. Finer finishes need a secondary process — we provide electropolishing down to Ra ≤0.4 µm (N5) on 316L stainless and titanium Grade 23 for medical and food-contact components.
Is Ra 0.4 µm achievable on stainless steel?
Yes — Ra ≤0.4 µm (N5) is achievable on 316L via electropolishing, which is our standard route for surgical instruments, implantable device components and food-contact parts. It is not achievable straight off the cutter; it needs the finishing step, so flag it at quotation.
Send Your Drawing for a Same-Day Quote
Talk directly to the engineers who will machine your parts — no account managers, no trading-company markups. Complimentary DFM review with every enquiry.