Ra Surface Finish Chart (Printable)
Roughness average in microinch and micrometer, the process bands behind typical finishes, and the ASME BPE surface-finish designations used on sanitary work. Bands are typical ranges — specify a maximum Ra with the measurement method, not a process name.
Ra conversion
| Ra µin | Ra µm | Common callout |
|---|---|---|
| 4 | 0.10 | UHP / electropolished target class |
| 8 | 0.20 | Fine electropolished |
| 10 | 0.25 | BPE SF6 limit / fine EP |
| 15 | 0.38 | BPE SF4 limit |
| 16 | 0.41 | 320-grit mechanical polish band |
| 20 | 0.51 | BPE SF1 / SF5 limit |
| 25 | 0.64 | BPE SF2 / SF6 limit |
| 30 | 0.76 | BPE SF3 limit |
| 32 | 0.81 | Common machined finish |
| 63 | 1.60 | General machined |
| 125 | 3.18 | Rougher machined |
1 µin = 0.0254 µm — multiply microinch by 0.0254 (32 µin × 0.0254 = 0.813 µm).
Typical process bands
| Surface condition | Typical Ra µin | Typical Ra µm | Note |
|---|---|---|---|
| Hot rolled | over 100 | over 2.5 | Gauge-dependent |
| 2B mill finish | 15–40 | 0.38–1.02 | Standard cold-rolled sheet |
| 180 grit / No. 4 | 18–31 | 0.46–0.79 | Dairy / sanitary mechanical polish |
| 240 grit / No. 6 | 13–27 | 0.33–0.69 | Finer mechanical polish |
| 320 grit / No. 7 | 10–16 | 0.25–0.41 | Pre-EP mechanical polish |
| Electropolished | 4–16 | 0.10–0.41 | Low single digits only as a UHP target |
Grit-to-Ra charts disagree with each other — one chart's 180 grit is another's 220 — so treat any grit column as approximate and buy to a measured Ra. Rougher is not cheaper in service: a rough product-contact surface harbors residue, cleans slower and can fail a hygienic audit.
ASME BPE surface-finish designations
| Designation | Ra maximum µin | Ra maximum µm | Route |
|---|---|---|---|
| SF0 | no requirement | — | As-fabricated |
| SF1 | 20 | 0.51 | Mechanical |
| SF2 | 25 | 0.64 | Mechanical |
| SF3 | 30 | 0.76 | Mechanical |
| SF4 | 15 | 0.38 | Electropolished |
| SF5 | 20 | 0.51 | Electropolished |
| SF6 | 25 | 0.64 | Electropolished |
SF values are maximum Ra limits, reproduced from supplier BPE references — confirm against the licensed ASME BPE edition for contractual work. From our guides:electropolishing vs passivation andsanitary stainless fittings. State Ra with the measurement method on theRFQ.