Needle Size Guide & Converter
Convert between US needle sizes and millimetres, and see the conventional needle range for your yarn weight.
Conversions
Two systems, one measurement
A needle's US size and its metric diameter describe the same physical thing two different ways. The millimetre figure is the more precise of the two — two needles both labeled “US 7” by different manufacturers should measure the same 4.5mm, but a needle that’s lost its label can only be identified by measuring it directly.
This tool converts confidently in either direction, and pairs that with a standard yarn-weight-to-needle chart so you have a sensible starting point before you ever swatch.
Frequently Asked Questions
Why do US needle sizes and millimetres both exist?
US sizing is an older numbering convention, while millimetres measure the needle's actual diameter directly. Because US sizes were standardized before metric measurement was common in knitting, patterns and needle sets from different eras or regions can favor one system over the other — this tool moves between them either way.
How do I identify an unmarked needle?
Measure its diameter with a needle gauge (a flat tool with graduated holes) or calipers, then enter that millimetre value here to find the nearest standard US size. A needle gauge is the more reliable method — eyeballing diameter against a ruler is easy to get wrong by half a size.
Is the yarn-weight needle range a strict rule?
No — it's a conventional starting point, not a requirement. The range reflects what typically produces a balanced, not-too-loose or too-dense fabric for that weight, but your actual choice should follow your gauge swatch. Going up or down within (or even slightly outside) the range is completely normal to hit a pattern's stated gauge.
Follows the standard US/metric equivalency chart used across pattern publishers. Individual manufacturers can vary by a fraction of a millimetre.