Knitting needle size converter

A knitting needle has two sizes printed on it and only one of them is a measurement. The millimetre figure is the diameter of the shaft, so 5 mm means the same thing whoever made the needle. The U.S. number is a label that the industry agreed on afterwards, and the agreement is loose enough that the same number can sit against two different diameters in the official chart. This converter reads that chart in both directions and, rather than hiding the awkward cases, shows every row a size matches.

How it is worked out

size → row in the Craft Yarn Council chart → the size in the other system

There is no arithmetic here and no formula to apply: needle sizing is a table that the Craft Yarn Council publishes, not a conversion factor. A U.S. number does not scale with diameter — U.S. 10 ½ sits between 10 and 11 in name only, and above U.S. 17 the numbers jump to 19, 35, 50 and 70 while the diameters climb steadily. The only honest way to convert is to look the size up.

Source: Craft Yarn Council — Hooks & Needles conversion chart: knitting needle sizes, 1.50 mm to 35 mm

The whole chart

Every row the Craft Yarn Council publishes, with the yarn weights each size is recommended for. A dash means the standard lists no size there rather than that we left it out.

Questions people ask

Why does one U.S. number match two different millimetre sizes?

Because the chart records what manufacturers actually stamp on their needles, and they do not agree. U.S. 3 covers both 3.125 mm and 3.25 mm, U.S. 6 covers 4 mm and 4.25 mm, U.S. 9 covers 5.25 mm and 5.50 mm, U.S. 10 covers 5.75 mm and 6 mm, and U.S. 17 covers 12.50 mm and 12.75 mm. A quarter of a millimetre is enough to change your gauge, which is why patterns increasingly give the metric size.

Which number should I go by when buying needles?

The millimetre measurement. The Craft Yarn Council says letter and number sizing vary from company to company and recommends relying on the package millimetre sizing, which is an accurate measurement. If a pattern gives only a U.S. number, this converter will show you every diameter that number can mean.

What about UK or Canadian needle sizes?

The Craft Yarn Council chart does not carry them, so neither does this tool. The old UK numbering runs the other way — a larger number means a thinner needle — and it was never standardised alongside the U.S. system. Rather than transcribing a conversion from an unattributable source, we leave it out: measure the needle in millimetres and you have the figure that works everywhere.

Why do 3 mm and 7 mm have no U.S. size?

They are common in Europe and rarely made for the U.S. market, so the chart lists the diameter with a dash where the number would be. The needle is perfectly ordinary; it simply has no U.S. label to convert to, and a pattern written in U.S. sizes will not call for one.

Does the right needle size guarantee the right result?

No, and the standard says so: always follow the gauge stated in your pattern. Needle size is a starting point, not the target. Two people working the same yarn on the same needles can be a full size apart in tension, which is why a pattern names a stitch count over 10 cm rather than trusting the needle alone.

Related tools