Last updated 10 August 2026
Ra 125 microinches is Ra 3.2 micrometers. Dividing by 39.37 gives 3.175, which is not a value any finish specification uses.
This is the case that shows why surface finish does not convert by arithmetic. Dividing 125 by 39.37 gives 3.175, and 3.175 is not a standard value: it is not on the ladder, no comparator block is made to it, and a drawing carrying it tells the grinding shop that somebody used a calculator instead of the standard.
The same failure appears at 32 microinches, a normal ground finish, which divides to 0.813 against a standard 0.8. And at 250, which divides to 6.35 against a standard 6.3. The shortcut works at 63 and almost nowhere else.
| Given | Ra 125 microinches |
|---|---|
| On the preferred-number ladder | yes |
| Standard pair | Ra 3.2 micrometers |
Open this case in the calculator, and change the numbers.
Computed by ctrlPlanner. ISO 4288, ASME B46.1.