US 3 Knitting Needle: Metric & UK Size
Convert US 3 and you land on 3.25mm metric, or UK/Canada (old) size 10 on a pre-metric British chart. It marks the top edge of fingering weight's recommended range (US 1-3) and the bottom edge of sport weight's (US 3-5), making it a genuine dual-purpose size for either weight worked at a firmer-than-standard gauge.
What it's typically used for
Sock knitters after a slightly looser, more flexible fabric than US 2 produces often land here; it's also a common choice for sport-weight baby garments and lightweight accessories where a denser fabric than sport's standard gauge gives more structure. Sitting exactly on the boundary between two weight categories makes US 3 a frequent "try this next" size when a fingering-weight swatch comes out too tight, or a sport-weight swatch comes out too loose.
US 2 and US 4 as reference points
US 2 (2.75mm, UK 12) is one size finer; US 4 (3.5mm, with no UK old equivalent published at all) is one size up. UK 11 never appears in the standard chart — the scale steps directly from 12 to 10 across this stretch, one of a few genuine gaps in the historical system rather than a size anyone forgot to publish.
Gauge implications
Where a US 3 needle actually lands you depends on which yarn you're using: fingering weight on US 3 typically knits up looser and more open than fingering's standard range assumes, while sport weight on the identical needle pulls tighter and firmer than sport's own range predicts. Swatch and compare against whichever specific range applies to your actual yarn.
FAQ
Is US 3 a good "in-between" size to own if I only buy one fine needle? It's a reasonable single choice precisely because it straddles two commonly used weight categories, though you'll get more precise gauge matches at each weight's own dedicated recommended sizes.
What UK pattern era commonly used size 10 needles? Vintage British patterns from before metric conversion frequently specify needle sizes by the UK old number alone; "size 10" in an older British pattern refers to this 3.25mm size, not a US 10 (6.0mm) — mixing the two up would produce a dramatically oversized result.
Does a US 3 needle work for a worsted-weight gauge swatch? Not at standard worsted gauge — US 3 sits far below the needle sizes worsted normally calls for; using it with worsted yarn would produce an extremely dense, stiff fabric well outside the yarn's intended behavior.