How to Convert Metric Needle Size to US?
This is the easiest direction to convert, precisely because metric is the actual physical measurement and US is the assigned index built around it — look up your millimetre size against the standard equivalence table (or the Needle & Hook Size Converter) and read off the matching US number directly.
When your metric size falls between two US numbers
Not every metric measurement lands on a whole US number — a needle marked 4.25mm, for instance, doesn't correspond to a standard US size at all, since the published equivalence chart jumps from US 6 (4mm) to US 7 (4.5mm) with nothing standard in between. If your needle's metric marking falls in a gap like this, there simply isn't a matching US number to convert to; the metric size itself is still the accurate one to work from.
Why the mapping isn't perfectly even
US needle sizes were assigned historically to a specific set of metric diameters, not derived from an even mathematical formula — this is why the gap between adjacent US numbers isn't constant in millimetres (0.25mm between some pairs, 0.5mm between others, and larger jumps at the biggest sizes). Expecting a clean, evenly spaced conversion pattern is the wrong mental model; the actual chart is the only reliable source.
A few fixed points worth memorizing
4mm = US 6. 5mm = US 8. 6mm = US 10. These three round numbers are commonly referenced enough that many knitters memorize them directly, using them as anchor points to estimate nearby sizes before checking the full chart for anything less common.
Where this matters most in practice
Buying needles for a metric-labeled international pattern when your own needle collection is US-labeled, or vice versa — getting the conversion right the first time avoids an extra return trip once you discover the wrong size mid-swatch.
FAQ
Is there a metric size with no US equivalent at all, not even an approximate one? Yes — above US 15 (10mm), the chart gets sparse fast: US 17 is 12.75mm and US 19 is 15mm, with genuinely nothing standard in between or beyond, since the largest needles are far less standardized across brands than the mid-range sizes are.
Does UK old sizing complicate a metric-to-US conversion? No, if you're going metric-to-US directly — the UK inversion only becomes relevant when UK sizing is one of your two endpoints; see converting UK to US for that specific direction.
Should I trust a printed conversion chart from an old pattern book, or look it up fresh? Cross-check against a current source if precision matters — needle sizing standards have been stable for a long time, but a fresh lookup costs nothing and rules out a printing error or an outdated regional variation in an old book.