Short answer
One inch is exactly 25.4 millimeters. This length conversion is not an actual diameter match for a nominal hose size or a pipe thread. In an order, keep the numerical conversion separate from the product size definition.
Length conversion only
| Inch | Millimeter |
|---|---|
| 1/4 | 6.35 |
| 1/2 | 12.7 |
| 3/4 | 19.05 |
| 1 | 25.4 |
| 2 | 50.8 |
Source: S023
What does this table not tell you?
The table does not give a hose's catalog dimensional tolerance or a thread's outside diameter. The Parker thread identification guide calls for pipe threads to be compared against the nominal size reference. For this reason, the expression "half inch" does not mean that all dimensions of the connection are 12.7 mm.
Separate the concepts in the dimension record
We store the numerical length as `length_mm`, the nominal definition as `nominal_designation`, and the manufacturer's actual inside diameter as a separate technical property. It does not produce an automatic "fits" result for DN and dash; we do not match without an applicable product/standard table.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Is 1 inch 25 mm or 25.4 mm?
In a pure length conversion the exact value is 25.4 mm; nominal product naming is a separate matter.
Source: S023
Does the converter find the right fitting?
No. This tool only converts length.
Original editorial recommendation; not a clause of a standard.
Source coverage
-
S019
4 Easy Steps to Identify Hydraulic Threads Parker Hannifin · accessed 2026-10-19 PDF page 1; Step 3 Determine the size
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S023
NIST Guide to the SI, Appendix B.8 NIST · accessed 2026-10-19 inch entry