Last updated 10 August 2026
Yes. The pairs run from Ra 500 microinches and 12.5 micrometers down to Ra 1 and 0.025, and they are pairs rather than conversions.
The third column below is the one no other chart carries: what dividing the microinch value by 39.37 would have produced. It agrees with the standard at 63 and disagrees at almost every other rung, which is the whole argument for using the table rather than a calculator.
The same data is published as JSON at /data/ra-ladder.json, free to use with attribution, generated from the same module the converter runs on.
| Ra, microinches | Ra, micrometers | If divided by 39.37 |
|---|---|---|
| 500 | 12.5 | 12.7 |
| 250 | 6.3 | 6.35 |
| 125 | 3.2 | 3.175 |
| 63 | 1.6 | 1.6 |
| 32 | 0.8 | 0.813 |
| 16 | 0.4 | 0.406 |
| 8 | 0.2 | 0.203 |
| 4 | 0.1 | 0.102 |
| 2 | 0.05 | 0.051 |
| 1 | 0.025 | 0.025 |
Open the tool · the same data as JSON
Surface finish preferred-number ladder, Ra. ISO 4288, ASME B46.1. CC-BY-4.0, attribute to ctrlPlanner.