US knitting needle size to mm
American patterns give a needle number while many needles are labeled in millimeters; this tool links the two.
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How to use it
- Enter us size.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
There is no formula; each US number maps to a set diameter in the Craft Yarn Council chart, for example 6 = 4 mm and 8 = 5 mm.
Example
Inputs: US size = 8. Result: Needle diameter: 5 mm; Diameter in inches: 0.197 in.
Limits and caveats
- Some brands label a 12 mm needle as US 17 instead of 12.75 mm.
- This is the US chart; old UK sizes use a completely different numbering.
Questions about this tool
What is a US 8 knitting needle in mm?
It is 5 mm.
Does every metric size have a US number?
No. The Craft Yarn Council chart used here does not list sizes such as 3 mm or 7 mm; the reverse tool shows the nearest US size for them.
Does a small diameter change matter?
Yes. Changing by 0.25 mm alters stitches per 10 cm, so knit a gauge swatch first.