What Ra actually measures
Ra (roughness average) is a numeric measurement of a surface's microscopic texture, expressed in micrometers — a lower Ra number means a smoother surface. It's a precise, measurable spec rather than a subjective description like "mirror" or "brushed."
Where Ra specs actually get used
Food processing equipment, pharmaceutical and medical fabrication commonly specify exact Ra values, since surface texture affects how easily bacteria can adhere to a surface — this is a genuine functional requirement in those industries, not an aesthetic one.
Why architectural work rarely needs it
For railings, cladding and general architectural fabrication, our standard finish categories (see our finish guide) are entirely adequate — specifying an exact Ra value adds cost and complexity without a corresponding functional benefit for these use cases.
When to actually ask about it
If your project genuinely involves food contact surfaces — a commercial kitchen counter, for instance (see our kitchen spec guide) — it's worth raising Ra requirements explicitly rather than assuming standard finish is sufficient.