Millimetres describe physical tool diameter. US letters and numbers, and legacy UK numbers, are naming systems layered over those measurements.
Measurements versus names
A 5 mm tool has a physical diameter. H-8 or US 8 is a regional label whose meaning also depends on whether the tool is a hook or needle. Legacy UK needles add another sequence.
Never convert a bare number without identifying the tool and system. Canadian, Australian and New Zealand patterns may also retain terminology from an imported source.
A clear reading sequence
Look for a metric value, identify the pattern’s origin and tool type, convert the regional label, check the physical tool and finally make the gauge swatch. Write the selected millimetre size in the project notes.
How to use this page
- Prefer the millimetre size when a pattern provides it.
- Use a converter when only a regional label is supplied.
- Check the tool marking or measure it with a gauge before swatching.
What to check before you rely on the result
- A regional label is not always an exact equivalence.
- Canadian, Australian and New Zealand patterns may mix imported terminology.