A stainless finish written as "mirror" or "polished" is an opinion; one written as Ra 0.4 µm is a measurement. The Ra number — the arithmetic average surface roughness in microns — is what makes a surface spec enforceable, and it is the single most useful number on a stainless finish drawing. Here is what it means and how to use it.
What Ra actually measures
Ra is the arithmetic average of the absolute deviations of the surface profile from the mean line, over a defined sampling length. In plain terms: it is the average height of the peaks and valleys on the surface, stated in microns (µm). A lower Ra is a smoother surface.
It matters because Ra is a single, comparable number — two suppliers quoting "satin" cannot be compared, but two suppliers quoting Ra 0.6 µm and Ra 0.4 µm can. The number is measured with a profilometer tracing a stylus across the surface, so it is objective in a way that "smooth" never is.
The numbers for the common finishes
| Finish | Typical Ra | Where it is used |
|---|---|---|
| No.1 (hot-rolled) | 3.2 – 6.3 µm | structural, non-cosmetic |
| 2B (cold-rolled bright) | 0.3 – 0.5 µm | general sheet |
| BA (bright annealed) | 0.1 – 0.3 µm | reflective, cosmetic |
| No.4 (brushed satin) | 0.4 – 1.0 µm | architectural, directional |
| 8K (mirror) | 0.05 – 0.1 µm | decorative, reflective |
The finish designation and the Ra value describe the same surface from two angles. 2B, BA, No.4 and 8K name the process that produced the surface; Ra measures the result. For a drawing that has to be accepted or rejected, the Ra value is the part that is checkable.
Where Ra is the requirement
Food and pharmaceutical contact is where Ra is non-negotiable. Product-contact surfaces in sanitary service are typically specified at Ra 0.8 µm or finer, because roughness hides residue and bacteria. The finish is a hygiene decision before it is a cosmetic one.
Architectural work uses Ra differently — No.4 satin for a consistent directional look, 8K for mirror — but here too, writing the Ra or the finish designation prevents a facade where two panels reflect differently because two suppliers interpreted "brushed" two ways.
Why "mirror" is not a requirement
The word "mirror" has no number behind it, so it gets interpreted five ways and then disputed. A finish specified as "8K, Ra 0.1 µm" is a requirement; "mirror" is a hope. The same applies to "polished", "satin" and "smooth" — each one is a conversation until a Ra value or a mill designation pins it down.
What gets specified wrong
The most common mistake is specifying a finish and never the Ra, then rejecting the delivered surface because it does not match an expectation that was never written down. The second is confusing the finish designation with the grade — a 2B 304 sheet and an 8K 316L sheet are different material decisions, and both the grade and the finish belong on the order. The third is ignoring that roughness changes with processing — a No.4 finish is directional, and Ra measured across the grain differs from Ra measured along it.
Bottom line
Ra is the number that turns a stainless finish from an opinion into a specification. Write the finish as a designation and a Ra value — "No.4, Ra 0.6 µm" — not as "brushed", and state it together with the grade. The surface you get is then the one that was quoted, not the one someone hoped you meant.
For the finish designations in full, see stainless steel surface finishes. If you are specifying a food-contact or architectural surface, send the Ra and the grade — we will quote the finish that meets it, not the one that just looks right.