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What does this converter do?
Knitting needle converter: US, mm and UK needle sizes. Free knitting needle converter with the formula shown - runs entirely in your browser, no signup.
- US 8 = 5.0 mm; US 10.5 = 6.5 mm; US 15 = 10.0 mm.
- UK old system runs inverse to US: UK 1 is large (7.5 mm), UK 14 is fine (2 mm).
- Yarn bands specify a gauge, not a needle size — the needle that hits the gauge is correct, whatever its label.
How this calculator works
Formula
US needle sizes have no formula — they are a fixed mapping to mm (US 8 = 5.0 mm, US 10 = 6.0 mm, US 13 = 9.0 mm). UK old sizes run inverse (bigger number = thinner needle).
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.
Frequently asked questions
My swatch is off gauge with the recommended needle. What now?
That is the system working: go up a needle size for too-many stitches per inch, down for too few. Gauge is the target; the printed needle size is only a starting suggestion.
Cite this tool
BoringToolsKit. “Knitting Needle Converter.” boringtoolskit.com/knitting-needle-converter/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Conversion runs locally in your browser; review the output before relying on it.
Worked example: converting US 7 step by step
Take US 7, a common worsted-weight size. Enter 7 in the US field and the calculator performs a table lookup, not an arithmetic conversion: the UK scale is a separate named sizing system, so the mapping is stored as pairs. The entry for US 7 is [4.5, 7], meaning 4.5 mm and UK 7. The result hero prints both: US 7 = 4.5 mm, UK 7.
To see why a swatch still matters, compare neighbors. US 6 is 4.0 mm (UK 8) and US 7 is 4.5 mm (UK 7), so a 0.5 mm step changes diameter by 12.5 percent between those two sizes. If your swatch on 4.5 mm yields 19 stitches per 10 cm but the pattern calls for 20, you need a slightly tighter fabric, which usually means dropping to the 4.0 mm needle rather than adjusting row count. A typical worsted swatch of 20 stitches at 5 stitches per inch is 4 inches wide; if it comes out 4.25 inches, you are off by 6 percent, which over a 40-inch sweater chest is 2.5 inches, more than most patterns allow in finished ease.
The same lookup logic explains the half size: US 10.5 maps to 6.5 mm and UK 3, sitting between US 10 (6.0 mm, UK 4) and US 11 (8.0 mm, UK 0). That 1.5 mm jump between US 10.5 and US 11 is the largest gap in the standard range and catches knitters who assume sizes progress evenly.
Boundary cases the converter will reject or refuse
The converter accepts only US sizes 0 through 15, and anything outside that range fails the input check: -1 or 16 returns the input-error message instead of a result. Blank input counts as 0, so an empty field quietly converts to US 0 = 2.0 mm, UK 14; type the size explicitly to avoid a misleading answer.
Not every integer in 0-15 has a standard counterpart. US 12 has no table entry because the conventional US scale skips from 11 to 13, so entering 12 returns the no-match message rather than inventing a diameter. The valid set is exactly: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5, 11, 13, 15, with 10.5 the only decimal accepted.
The UK column is also non-linear in a way worth knowing. As millimetres grow, UK numbers shrink and then pass through zero into double and triple zeros: US 11 (8 mm) is UK 0, US 13 (9 mm) is UK 00, and US 15 (10 mm) is UK 000. Old UK 'war time' sizes above 14 do not exist in this table, and metric labels like 4.25 mm or 4.5 mm sometimes round differently between brands, so treat the mm figure as nominal. Finally, the result is a needle identity, not a tension guarantee: a swap of 0.25 mm at the fine end (US 0 to US 1, 2.0 to 2.25 mm) shifts gauge roughly as much as 0.5 mm does mid-range, so re-swatch after any change.