Metric to US Size

US Size = Lookup Table (mm → US Number)

Knitting needle sizes do not follow a linear formula between systems. Each US number maps to a specific millimeter diameter via an industry-standard lookup table. For example, US 6 = 4.0 mm, US 8 = 5.0 mm, US 10 = 6.0 mm. Some metric sizes (3.0 mm, 7.0 mm) have no US equivalent.

UK Size to Metric (Inverse Scale)

UK Size = Inverse Wire Gauge (higher UK number → smaller mm)

The UK old-number system is based on historical wire gauge standards and runs inversely: UK 14 = 2.0 mm (thinnest) and UK 000 = 10.0 mm (thickest). Modern UK patterns have switched to metric, but vintage patterns still use this numbering.

Gauge Stitches per Inch Relationship

Gauge (sts/inch) ≈ k / Needle Diameter (mm)

Smaller needles produce more stitches per inch (tighter fabric) and larger needles produce fewer stitches per inch (looser fabric). The constant k varies by yarn weight, fiber content, and knitter tension, but needle diameter is the primary controllable variable in achieving target gauge.

Needle Size to Yarn Weight Recommendation

Yarn Weight Category = CYC Standard Range per mm

The Craft Yarn Council assigns recommended needle size ranges to each yarn weight category: #0 Lace (1.5–2.25 mm), #1 Super Fine (2.25–3.25 mm), #2 Fine (3.25–3.75 mm), #3 Light (3.75–4.5 mm), #4 Medium (4.5–5.5 mm), #5 Bulky (5.5–8 mm), #6 Super Bulky (8–12.75 mm), #7 Jumbo (12.75 mm+).