US 8, UK 6, 5 mm: Three Needle Scales That Never Agree

Why the British numbers run backwards

The UK scale descends from the old Standard Wire Gauge, a system for drawn metal wire in which the number counts how many times the wire was pulled through a die. More draws means thinner wire, so a high number is a fine needle: 14 is 2 mm, 8 is 4 mm, 0 is 8 mm, and 000 is 10 mm. Once the origin is known the direction stops being perverse, but it does not stop being the opposite of the American one.

The American scale has no such logic. It runs upward and it is not linear: from US 6 to US 7 is half a millimetre, from US 15 to US 17 is 2.75 millimetres, and above 10½ the numbers skip entirely — 11, 13, 15, 17, 19, 35, 50, with nothing in between. Neither scale can be computed from the other; the converter is a lookup table because there is nothing to compute.

The holes in the table are as informative as the rows

Of the twenty-four diameters in the converter, only fourteen carry all three numbers. Three metric sizes have no American equivalent at all — 3, 7 and 7.5 mm — and seven have no British one, including 3.5 mm and everything above 10 mm. Those gaps are not omissions in the table; they are gaps in the national systems, and they are why a translated pattern sometimes gives a needle size that does not exist in your shop.

The practical consequence is that a conversion is sometimes a choice rather than a lookup. A British pattern calling for size 11 wants 3 mm, which no American numbering covers; the American knitter picks 2.75 mm or 3.25 mm and reswatches. This is exactly the case where the gauge decides and the label does not — pick whichever of the two neighbours gives you the pattern's stitch count over ten centimetres.

Why continental Europe never needed a scale

In France, Spain, Portugal, Germany and Italy a needle is sold as 4 mm or 5.5 mm, and the label is the measurement. There is nothing to convert, nothing to memorise and no direction to get backwards — the number is the thing it describes. This is worth stating plainly because English-language patterns are so dominant online that continental knitters often assume they are missing a system rather than using the simplest one.

Two cautions remain even in millimetres. Needle diameters are nominal, and a 4 mm needle from one maker can measure 3.9 or 4.1 — enough to move a gauge by half a stitch over ten centimetres on fine yarn. And the material changes the fabric at the same diameter: bamboo grips and slows, metal slides and loosens, so the same size in two materials gives two gauges. The millimetre is the honest label, not a guarantee.

Frequently asked questions

What is US 8 in millimetres?

5 mm, which is UK 6. The mirror question catches people out: UK 8 is 4 mm, which is US 6. If a pattern says 8 and does not say whose, look at the yarn — a worsted or aran weight points to the American reading and a DK weight to the British one.

Do crochet hooks use the same table?

No, and assuming they do is a common error. Crochet hooks use letters in the American system — B/1, G/6, H/8, K/10½ — and their own descending British numbers, and neither maps to the knitting-needle numbers at the same diameter. A US knitting needle 8 is 5 mm; a US crochet hook H/8 is also 5 mm, but that agreement is a coincidence of one row and does not hold across the table.

My needle has no marking left. How do I identify it?

Use a needle gauge — a flat plate with holes drilled to the standard diameters — and push the needle through the smallest hole it passes cleanly. A calliper works too and is more precise, but measure the shaft rather than the tip, and measure away from the taper. Once you have the millimetre reading, the table gives you both national numbers.