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Roughness converter: Ra, N grades and microinches
Enter an Ra in micrometres, an N grade or a value in microinches: the other two are displayed straight away. The full table shows, for guidance, the processes that achieve each range.
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Ra roughness converter, N grade and microinches
Roughness · Ra 1.6 µm
N7
Ra 1.6 µm corresponds to grade N7 (Ra 1.6 µm).
Ra in micrometres
1.6 µm
Ra in microinches
62.99 µin
Common processes for N7, for guidance
Milling or turning with careful finish, boring
Typical use
Bearing faces, common functional surfaces
The calculation is done in your browser: the values you enter are neither sent nor saved.
Roughness grades N1 to N12
From finest to coarsest: Ra in micrometres and microinches, common processes and typical use. The processes are given for guidance.
| Grade | Ra (µm) | Ra (µin) | Common processes (for guidance) | Typical use |
|---|---|---|---|---|
| N12 | 50 | 1,968.5 | Rough sawing, flame cutting, coarse sand casting | Rough surfaces |
| N11 | 25 | 984.25 | Sawing, plasma cutting, sand casting, heavy roughing | Rough surfaces |
| N10 | 12.5 | 492.13 | Rough milling or turning, drilling, careful sawing | As-machined rough surfaces, not visible |
| N9 | 6.3 | 248.03 | Semi-finish milling or turning, drilling | Non-functional surfaces |
| N8 | 3.2 | 125.98 | Milling or turning with standard finish | Standard machined surface |
| N7 | 1.6 | 62.99 | Milling or turning with careful finish, boring | Bearing faces, common functional surfaces |
| N6 | 0.8 | 31.5 | Fine finishing, fine boring, reaming, grinding | Precision fits, sealing faces |
| N5 | 0.4 | 15.75 | Grinding, roller burnishing, polishing | Dynamic sealing, severe friction |
| N4 | 0.2 | 7.87 | Fine grinding, lapping, polishing | Dynamic sealing, sliding |
| N3 | 0.1 | 3.94 | Lapping, superfinishing, careful polishing | Precision surfaces, severe sliding |
| N2 | 0.05 | 1.97 | Superfinishing, fine lapping, mirror polishing | Precision surfaces |
| N1 | 0.025 | 0.98 | Fine lapping, optical polishing (highly specialised processes) | Measuring surfaces, optics |
Processes and uses: cautious orders of magnitude, for guidance. The Ra obtained depends on the material, the tool, the cutting parameters and the machine. In microinches, drawings often round the values: 125 µin for N8, 63 for N7, 32 for N6, 16 for N5.
How to read the result
Ra is the arithmetical mean deviation of the roughness profile: the average, over the evaluation length, of the profile’s deviations from its mean line. It is expressed in micrometres (µm). The N grades (N1 to N12) form a scale of Ra, from 0.025 µm to 50 µm: each grade is roughly double the previous one. On a drawing, “N8” therefore corresponds to an Ra of 3.2 µm.
The converter also gives the equivalent in microinches (µin), used on North American drawings: 1 µm = 39.37 µin. An Ra of 3.2 µm corresponds to about 126 µin, which drawings often round to 125 µin; likewise, 1.6 µm corresponds to 63 µin and 0.8 µm to 32 µin.
When a value does not fall on a grade, the tool indicates between which grades it lies. On a drawing, an Ra is generally an upper limit: a surface measured at 2 µm meets an N8 requirement (Ra ≤ 3.2 µm), but not an N7 requirement (Ra ≤ 1.6 µm).
Ra and Rz: no exact conversion
There is no exact conversion between Ra and Rz. Ra is an average of the profile’s deviations. Rz is a height: the sum of the highest peak and the deepest valley of the profile, averaged over several sampling lengths. Two surfaces with the same Ra, one with regular lay marks and the other bristling with isolated peaks, have very different Rz values.
The correspondence tables in circulation are only orders of magnitude, valid for a given process and profile shape: do not use them to specify a drawing. If a requirement is expressed in Rz, ask for it as such from the workshop, which will measure it as such.
Pitfalls to avoid
- The processes in the table are indicative. They give cautious orders of magnitude; the Ra obtained depends on the material, the tool, the cutting parameters and the condition of the machine. Ask the workshop what Ra it can actually hold for your part.
- Ra does not tell the whole story. Two surfaces with the same Ra can have different textures (lay direction, peak shape). For sealing or sliding, the function sometimes calls for another parameter or a description of the process.
- Measurement direction. Roughness is generally measured perpendicular to the machining marks, where it is highest. The same surface gives a lower Ra along the lay.
- Before or after treatment. Blasting, anodising, galvanising or painting change the roughness. State whether the Ra applies before or after treatment.
- Finer is not always better. Each step finer adds passes, even grinding or polishing, and therefore cost. A very smooth surface can also hold an oil film less well or give paint less grip.
- Standards evolve. The N grades come from the historical presentation of ISO 1302, and the ISO 21920 series is gradually replacing the roughness standards. N grades remain very common on existing drawings.
In a drawing and in a request for quote
On a drawing, state a general Ra (for example “Ra 3.2 unless otherwise stated”) and mark individually only the functional surfaces: bearing surfaces, fit surfaces, sealing or sliding surfaces. The other surfaces are fine with the normal machined surface finish.
In a request for quote, flag the surfaces finer than Ra 1.6 µm: they often need rework (grinding, polishing) that makes the part more expensive. Also state the planned surface treatment and whether the Ra applies before or after it. The guide on machining tolerances links roughness, tolerances and cost, and the one on surface treatment of metals details the finishes. Looking for a finishing workshop? See the surface treatment capability page.
Frequently asked questions
A question not answered here? Visit the help centre or write to us.
How do Ra values map to N grades?
N1 = 0.025 µm, N2 = 0.05, N3 = 0.1, N4 = 0.2, N5 = 0.4, N6 = 0.8, N7 = 1.6, N8 = 3.2, N9 = 6.3, N10 = 12.5, N11 = 25 and N12 = 50 µm. Each grade is roughly double the previous one.
How do I convert micrometres to microinches?
Multiply by 39.37: 1 µm = 39.37 µin. For example, an Ra of 1.6 µm is about 63 µin, and an Ra of 3.2 µm about 126 µin, often rounded to 125 µin on drawings.
Can Ra be converted to Rz?
Not exactly. Ra and Rz describe the profile differently, and the ratio between them varies with the shape of the surface and the process. Only a measurement gives the Rz of a surface.
What Ra roughness should I choose for an ordinary machined part?
An Ra of 3.2 µm (N8) suits most ordinary machined surfaces. Reserve 1.6 µm (N7) for functional surfaces and 0.8 µm (N6) for precision fits and sealing surfaces. Beyond that, grinding or polishing is often needed.
Does a finer roughness cost more?
Generally yes: slower finishing passes, more carefully chosen tools, grinding or polishing operations, finer inspection. Ask for the Ra that the function requires, not the finest possible.
Related expertise
Workshops to manufacture, treat or inspect the part you are calculating.
Surface treatment
Painting, powder coating, anodising, galvanising
Inspection & metrology
Dimensional inspection, NDT, reports
Guides to go further
Machining tolerances: ISO 2768, fits and dimensioning
ISO 2768 general tolerances, ISO 286 fits, ISO 1101 geometric tolerancing, Ra roughness: read a drawing and dimension sensibly to control the cost of a machined part.
Anodising, powder coating, zinc plating: choosing a surface treatment
Anodising, powder paint, zinc, passivation, nickel, chrome: the strengths and limits of each treatment, their effect on dimensions and how to specify it in a request.
Anodising, powder coating or galvanising: which should you choose?
Depending on the material, use, appearance and durability required, which treatment should you choose between anodising, powder coating and hot-dip galvanising? A comparison table and the pitfalls to avoid.
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