Metric Conversion Table

How the Inch to mm Metric Conversion Table Is Built

Every figure in the inch to mm metric conversion table comes from a single exact definition: one inch is 25.4 mm, fixed by international agreement in 1959. Nothing in the table is an approximation — 1/64″ is exactly 0.396875 mm, and the whole-inch columns simply add 25.4 mm at a time. That is why the inch to mm metric conversion table carries four decimal places: the fourth decimal is still exact, not rounded.

Nominal Size Is Not a Converted Dimension

The most common mistake is to run a nominal size through the inch to mm metric conversion table. NPS 2 is not 50.8 mm of anything: a DN 50 pipe has an outside diameter of 60.3 mm, and the “2 inch” in its name is a label, not a measurement. Use the inch to mm metric conversion table for dimensions that were actually measured or drawn — bolt circle diameter, bolt hole size, raised-face diameter, face-to-face length — and use the DN to NPS equivalence chart for size designations.

Rounding, Tolerance and Drawings

Convert first, round last. ASME B16.5 quotes flange dimensions in inches to two or three decimals, so a value taken from the inch to mm metric conversion table should be rounded back to the precision the drawing actually carries; showing 57.1500 mm where the source says 2 1/4″ ± 1/32″ implies an accuracy that is not there. When a tolerance is given as a fraction, run it through the inch to mm metric conversion table as well, not just the nominal value.

For ordering, the practical rule is short: quote the figure in the unit the standard uses, and put the converted value in brackets. The inch to mm metric conversion table above is what makes that second number defensible. Where a drawing mixes both systems, mark which figures came from the inch to mm metric conversion table and which were given in millimetres at source — that one note prevents most of the re-work.