Convert fastener dimensions between millimeters and inches
The Metric / Imperial Converter uses the exact relationship 1 inch = 25.4 mm. It is useful when a drawing, caliper, drill chart or repair manual uses a different unit system from the tools or fasteners in front of you.
Conversion is not thread identification
A converted diameter does not make two thread systems interchangeable. For example, a metric fastener can be numerically close to an inch diameter while using a completely different pitch and thread form. Always identify diameter and pitch/TPI together before attempting to mate parts.
Common workshop uses
- Convert a caliper reading before checking a metric or inch fastener chart.
- Compare drill diameters listed in different unit systems.
- Translate drawing dimensions while keeping the original specification visible.
- Check whether two dimensions are merely close or genuinely equivalent.
Do not round too early
Fastener and machining dimensions often sit close together. Keep enough decimal places during the comparison and round only for the final display or to the precision supported by your measuring tool. A rough conversion is fine for orientation; it is not enough for choosing a tap, insert or precision fit.
Metric thread pitch vs threads per inch
This converter handles linear dimensions only. Metric pitch is normally expressed as millimeters per thread, while Unified threads are commonly described in threads per inch. Converting one concept to the other requires a pitch/TPI relationship, not just a diameter conversion. The Thread Identifier and reference guides handle that context separately.