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.
This converts one value. ctrlPlanner holds both units on the same plan and applies the same inward rounding everywhere, so a whole print comes out in whichever unit the customer asked for.
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