Short answer
For length conversion between inches and millimeters, 1 inch = 25.4 mm is used. However, the inch value in a thread or pipe nominal name is not always a measurable actual diameter; unit conversion and product size matching must be kept separate.
Technical explanation
In NIST conversion data, the international inch is defined as 0.0254 meter. Therefore, for an actual length, the millimeter value is the inch value multiplied by 25.4. For the reverse conversion, millimeters are divided by 25.4. This mathematical equality does not turn nominal thread names such as BSP or NPT directly into an actual outside diameter. For thread identification, the geometry and measurement method of the standard are used separately.
Topic-specific checkpoints
| Item | Description |
|---|---|
| inch → mm | Length × 25.4 |
| mm → inch | Length ÷ 25.4 |
| Example | 0.5 inch = 12.7 mm |
| Limit | A nominal thread name is not an actual diameter |
How to evaluate it in practice
In the tool, first ask the user to distinguish whether the value is an actual length or a nominal product label. The conversion field should calculate only actual length. For example, an actual length of 1/2 inch equals 12.7 mm; an actual length of 3/4 inch equals 19.05 mm. The result screen should state that these are not a hose or fitting equivalence recommendation. Rounding should be done only for display; subsequent calculations should not continue with an unnecessarily rounded value.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Is the outside diameter of a 1/2 BSP thread 12.7 mm?
No. Here 1/2 is the nominal thread name; the actual geometry is determined by the relevant standard.
Source coverage
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S019
4 Easy Steps to Identify Hydraulic Threads Parker Hannifin · accessed 2026-10-19 PDF page 1-2: thread identification
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S023
NIST Guide to the SI, Appendix B.8 NIST · accessed 2026-10-19 Alphabetical entries: bar, inch, pound-force per square inch