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What is Ra 63 microinches in micrometers?

Last updated 10 August 2026

Ra 63 microinches is Ra 1.6 micrometers.

Both values are entries on the preferred-number series defined by ISO 4288 and ASME B46.1. They are paired, not converted: each is a standard value in its own right, and the ladder is what makes them equivalent.

This particular pair is the reason a bad habit survives. 63 divided by 39.37 is 1.600, so the one value most people check by hand is the one where dividing appears to work. It stops working immediately either side of it, which is why the next question is worth reading.

Worked

Given	Ra 63 microinches
On the preferred-number ladder	yes
Standard pair	Ra 1.6 micrometers

Ra values pair on the ISO 4288 and ASME B46.1 preferred-number series. They are not conversions of one another and dividing does not produce them.

Open this case in the calculator, and change the numbers.

Computed by ctrlPlanner. ISO 4288, ASME B46.1.