Convert knitting needle sizes between US, UK and metric (mm).
Craft • US/UK/mm
Choose the size you already know and the equivalents in all the other standards appear together, with your source highlighted — so nothing has to be converted twice to be checked.
Results are recalculated on every keystroke, with no submit step and no page reload, so a value can be adjusted until the answer looks right rather than guessed once.
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Knitting needles are sized differently in the US, the UK and metric (mm), and the old UK scale runs backwards. This chart converts needle sizes across all three systems so you can follow any pattern.
Knitters working from international or vintage patterns use it to match the gauge a pattern calls for. The metric (mm) value is the unambiguous one to rely on when in doubt.
Knitting needle sizing runs on three incompatible scales. Metric quotes the diameter in millimetres, which is unambiguous and increasingly standard. US sizing uses numbers that increase with diameter, while the old UK system ran the opposite way, with larger numbers meaning thinner needles — so a UK 8 is thinner than a UK 4, which catches out anyone working from a vintage pattern. Neither numeric system maps cleanly onto metric, and conversion charts round to the nearest available size rather than converting exactly.
Metric = needle diameter in millimetresUS sizes increase with diameter: US 8 = 5.0 mmOld UK sizes DECREASE with diameter: UK 6 = 5.0 mmNo exact formula — charts round to available sizeswhere:
Assumptions: Gauge, not needle size, determines whether a garment fits. Always knit a tension swatch: individual knitters vary enough that changing needle size to match a pattern's stated gauge is normal and expected.
Place one diameter on all three scales and show the UK reversal.
Result5.0 mm = US 8 = old UK 6
The opposite direction of the two numeric scales is the trap in vintage patterns, and the coincidental agreement at 4.5 mm makes it worse by suggesting the scales align. Work in millimetres wherever the pattern allows, and always swatch.
The metric size is the only one of the three that states a physical dimension: it is the needle's diameter in millimetres. US numbers rise as needles get thicker, the old UK and Canadian numbers fall as needles get thicker, and neither is a measurement of anything — they are positions in a list. This is why a vintage British pattern and a modern American one can specify sizes that sound wildly different and mean the same needle.
Because the two numbered scales run in opposite directions, a misread size is not a small error but an inversion: taking a UK 8 for a US 8 moves from about 4 mm to about 5 mm, which changes the fabric substantially. Working in millimetres wherever the pattern allows removes the ambiguity, and it is the figure most manufacturers now print on the needle itself.
None of this settles the size you should actually use, because the needle is a means rather than an end. Patterns specify a gauge — a stitch and row count over a fixed square — and knitters differ enough in tension that reaching it may require going a size or two either way. A gauge swatch is the real measurement, and the needle number is only the starting guess that gets you to it.