US Versus Metric Needle Sizes, and Why Sock Knitters Need Half Sizes
Needle sizing carries two systems that do not line up neatly, and the mismatch causes more confusion than it should.
Metric is the real measurement
A needle's metric size is its diameter in millimetres, and it means the same thing everywhere. A 2.5 mm needle is 2.5 mm around, whoever made it.
US sizes are labels on a conventional chart rather than measurements. The needle size guide holds the standard equivalency used across pattern publishers, from US 0000 at 1.25 mm up to US 50 at 25 mm.
The scale is not linear
Down at the sock end, US sizes step in quarter-millimetres: US 0 is 2.0 mm, US 1 is 2.25, US 1.5 is 2.5, US 2 is 2.75, US 2.5 is 3.0. Five sizes covering a single millimetre.
Up at the other end the steps get enormous. US 10½ is 6.5 mm and US 11 is 8 mm — a one-and-a-half-millimetre jump with nothing between them. US 17 is 12.75, US 19 is 15, US 35 is 19.
This is why sock knitters live in half-size territory and bulky-weight knitters do not: at 2 mm, a quarter millimetre is more than a tenth of the diameter and changes gauge measurably. At 15 mm it would be invisible.
Half sizes exist because socks need them
US 1.5 and 2.5 are relatively recent additions to the chart, and they exist because fine-gauge knitting needs finer steps than the original scale offered. If a pattern calls for 2.5 mm and your set jumps from 2.25 to 2.75, that gap is a whole stitch per inch on some yarns.
When buying needles for socks, buying by millimetre rather than by US size avoids the whole problem.
Manufacturers vary
Nominally identical needles from different makers can differ by a fraction of a millimetre, and needle material changes the fabric even when the diameter is identical — a slick metal needle generally produces slightly looser stitches than a wooden one for the same knitter.
This is one more reason the swatch has to be knitted on the needles you will use, not on a nominally equivalent pair.
Yarn weight gives a starting point, not an answer
The guide also gives the conventional needle range for each yarn weight — fingering around 2.25 to 3.25 mm, worsted around 4.5 to 5.5. That is where to begin swatching, not where to stop.
Sock knitters routinely go a size or two below the range for the yarn, because socks want a denser fabric than a scarf does. A loose sock fabric wears through quickly and lets the foot slide inside it; going down a needle size is the standard fix and costs nothing but a slightly slower knit.