What US Needle Size Is 5mm?
5mm is US 8, UK/Canada old-size 6. This is one of the most commonly stocked needle sizes, sitting close to the middle of worsted weight's typical published gauge range.
Why this size shows up so often
US 8 (5mm) is frequently the size printed on a worsted-weight yarn's ball band as the "recommended" needle, which makes it one of the most-owned sizes in most knitters' collections — useful to know if you're building a needle set from scratch and want to prioritize the sizes you'll reach for most.
Checking it against your own gauge
A recommended needle size on a label is a starting point, not a guarantee for your own tension — swatch at 5mm first, and if your resulting gauge doesn't match a pattern's stated numbers, size up or down from there rather than assuming 5mm is automatically correct for your hands specifically.
Its neighbors in the size progression
One step down sits US 7 (4.5mm, UK 7); one step up sits US 9 (5.5mm, UK 5). Moving between adjacent sizes shifts gauge by roughly half a stitch per inch in typical worsted-weight yarn — a meaningful jump, which is part of why half-size increments (like US 8 sitting between whole UK numbers) exist in metric terms even where US sizing doesn't offer an equivalent half-step.
FAQ
What crochet hook is the closest match to a US 8 / 5mm needle? H-8, also 5mm — the identical metric measurement on the crochet side, though matching diameters doesn't guarantee matching fabric density between the two crafts at the same yarn.
Is 5mm considered a beginner-friendly needle size? Often recommended as one, yes — a mid-range size like this is large enough to be easy to see and handle while learning, without being so large that stitches feel unwieldy the way a much bulkier needle can for a first project.
Does 5mm ever appear as a UK size other than 6? No — UK old sizing assigns exactly one number per metric size on this chart, so 5mm and UK 6 are a fixed, unambiguous pair regardless of source.
Does a 5mm needle work well for both knit and crochet-style ribbing? 5mm is a knitting needle measurement specifically; a matching-diameter crochet hook (H-8) produces its own genuinely different ribbing structure, covered on the front post and back post stitches page — the two aren't directly interchangeable despite sharing a diameter.