Surface finish converter.
Convert Ra between microinches and micrometers, with the ISO N grade and the processes that reach each finish.
Enter a roughness value
Why there is no Ra to Rz conversion here
Many surface finish charts online carry an Ra to Rz column, and it causes trouble. Ra averages the deviation of a profile from its mean line, while Rz measures peaks against valleys. The relationship between them depends on the shape of the surface, so two parts at identical Ra can have very different Rz.
A turned surface has regular tool marks with a predictable profile, a shot blasted surface is random, and a ground surface sits somewhere in between. Published Ra to Rz ratios run from about four to seven, so a 32 microinch Ra could correspond to an Rz anywhere across a range wide enough to straddle a drawing limit.
Converting a measured Ra into an Rz and then reporting it as though it had been measured is how a shipment gets rejected on a characteristic nobody actually checked. If a drawing calls out Rz, measure Rz.
ISO N grades against Ra
| Grade | Ra, µm | Ra, µin | RMS, µin | Typically produced by |
|---|---|---|---|---|
| N1 | 0.025 | 1.0 | 1.1 | Outside the usual process ranges listed here |
| N2 | 0.05 | 2.0 | 2.2 | Lapping, Superfinishing |
| N3 | 0.1 | 3.9 | 4.4 | Lapping, Superfinishing, Polishing |
| N4 | 0.2 | 7.9 | 8.7 | Lapping, Superfinishing, Polishing |
| N5 | 0.4 | 15.7 | 17.5 | Polishing, Honing, Electropolishing |
| N6 | 0.8 | 31.5 | 35.0 | Honing, Electropolishing, Cylindrical grinding |
| N7 | 1.6 | 63.0 | 69.9 | Cylindrical grinding, Surface grinding, EDM, finishing settings |
| N8 | 3.2 | 126.0 | 139.8 | Investment casting, Drilling, Shot blasting |
| N9 | 6.3 | 248.0 | 275.3 | Investment casting, Drilling, Shot blasting |
| N10 | 12.5 | 492.1 | 546.3 | Shot blasting, Turning, roughing pass, Milling, roughing pass |
| N11 | 25 | 984.3 | 1,092.5 | Sawing, Sand casting, Hot rolling |
| N12 | 50 | 1,968.5 | 2,185.0 | Hot rolling, Flame cutting |
What each process normally achieves
These are the conventional ranges each process reaches in ordinary work. A process can often be pushed further with the right tool and enough time, so treat these as what normally happens. They are most useful for judging whether a drawing callout is plausible for a given process before you quote it.
| Process | Group | Ra, µin | Ra, µm |
|---|---|---|---|
| EDM, finishing settings | Machining | 16 to 63 | 0.41 to 1.60 |
| Broaching | Machining | 32 to 125 | 0.81 to 3.17 |
| Reaming | Machining | 32 to 125 | 0.81 to 3.17 |
| Turning, finishing pass | Machining | 32 to 125 | 0.81 to 3.17 |
| Milling, finishing pass | Machining | 32 to 125 | 0.81 to 3.17 |
| Boring, finishing pass | Machining | 32 to 125 | 0.81 to 3.17 |
| Drilling | Machining | 63 to 250 | 1.60 to 6.35 |
| Turning, roughing pass | Machining | 125 to 500 | 3.17 to 12.70 |
| Milling, roughing pass | Machining | 125 to 500 | 3.17 to 12.70 |
| EDM, roughing settings | Machining | 125 to 500 | 3.17 to 12.70 |
| Sawing | Machining | 250 to 1000 | 6.35 to 25.40 |
| Lapping | Abrasive | 1 to 8 | 0.03 to 0.20 |
| Superfinishing | Abrasive | 1 to 8 | 0.03 to 0.20 |
| Honing | Abrasive | 4 to 32 | 0.10 to 0.81 |
| Cylindrical grinding | Abrasive | 8 to 63 | 0.20 to 1.60 |
| Surface grinding | Abrasive | 8 to 63 | 0.20 to 1.60 |
| Polishing | Finishing | 2 to 16 | 0.05 to 0.41 |
| Electropolishing | Finishing | 4 to 32 | 0.10 to 0.81 |
| Shot blasting | Finishing | 63 to 500 | 1.60 to 12.70 |
| Die casting | Forming and casting | 32 to 125 | 0.81 to 3.17 |
| Cold rolling and drawing | Forming and casting | 32 to 125 | 0.81 to 3.17 |
| Extruding | Forming and casting | 32 to 125 | 0.81 to 3.17 |
| Investment casting | Forming and casting | 63 to 250 | 1.60 to 6.35 |
| Sand casting | Forming and casting | 250 to 1000 | 6.35 to 25.40 |
| Hot rolling | Forming and casting | 500 to 2000 | 12.70 to 50.80 |
| Flame cutting | Forming and casting | 500 to 2000 | 12.70 to 50.80 |
Questions engineers ask about surface finish
How do you convert Ra from microinches to micrometers?
Divide by 39.3701, because there are 39.3701 microinches in a micrometer. A 63 microinch finish is 1.6 micrometers, a 32 microinch finish is 0.8 micrometers, and a 125 microinch finish is 3.2 micrometers. This is a straight unit conversion, so it is exact, which is not true of conversions between different roughness parameters.
What is the difference between Ra and RMS?
Ra is the arithmetic average deviation of the profile from its mean line. RMS, properly called Rq, is the root mean square of the same deviations, so it weights larger excursions more heavily and always comes out higher. The conventional relationship is Rq equals 1.11 times Ra, which is exactly correct only for a pure sinusoidal profile. Real surfaces are never perfectly sinusoidal, so treat the 1.11 as a shop convention and an approximation.
What Ra is a 32 finish?
A 32 finish means an Ra of 32 microinches, which is 0.8 micrometers, or ISO N6. It is typically reached by a finishing pass in turning or milling, by cylindrical or surface grinding, or by fine reaming. On a US drawing, a bare number in a finish symbol is conventionally Ra in microinches.
Why does this page not convert Ra to Rz?
The Ra to Rz conversion is unreliable enough to cause rejected parts, so this page leaves it out. Ra is an averaged measure and Rz is a peak to valley measure, so the ratio between them depends on the shape of the profile as much as on its height. A turned surface, a ground surface, and a shot blasted surface at the same Ra have materially different Rz. Published ratios run from about four to seven times. If a drawing specifies Rz, measure Rz.
What are the ISO N grades?
N grades are defined bands of Ra in micrometers from ISO 1302, a standard ISO has since withdrawn in favor of ISO 21920-1:2021. The grades run from N1 at 0.025 micrometers to N12 at 50. Each step is roughly double the one below. Reading a grade off a value is a lookup against those defined numbers, so it is exact in a way the RMS relationship is not. N6 is 0.8 micrometers, N7 is 1.6 and N8 is 3.2.
Which process gives which surface finish?
The table on this page lists the ranges each process typically reaches in ordinary work. Grinding lands roughly between 8 and 63 microinches, a finishing turn or mill between 32 and 125, lapping between 1 and 8, and sand casting between 250 and 1000. Those are what a process normally produces in ordinary work. They are useful for judging whether a drawing callout is plausible for a process, and a quote should still rest on what your own equipment holds.
Does plating change the surface finish?
Yes, plating can make the finish better or worse, so it is worth thinking about before the callout is agreed. A thin bright deposit can improve apparent finish by filling fine tool marks. A thicker deposit tends to follow and often amplifies the underlying profile, so a rough substrate stays rough. Nodules, pits and edge buildup all raise measured roughness. Where a finish is specified after plating, it has to be measured after plating, and the base material has to start better than the callout.
Who made it, and what it does not do
This tool was written by Shopfloor Agency's lead engineer, who holds a bachelor’s degree in mechanical engineering and builds websites for US manufacturers. It runs entirely in your browser.
The microinch to micrometer conversion is exact, and so is the N grade lookup. The RMS relationship is a convention that holds precisely only for a sinusoidal profile, and Ra to Rz is not converted at all. The process ranges describe typical work, and what your own equipment holds comes from your own capability data.
If you spot an error, tell us, and we will correct the page and note the change.
We build websites for the shops that use tools like this.
We work only with US manufacturers and industrial service companies, so you never have to explain your trade. A website is a fixed $6,000, and the source code is yours at launch. Tell us what you make and who buys it, and we will reply with the scope and a delivery date.