Use the calculator tabs to convert needle sizes (mm/US/UK), convert stitch and row gauge between per inch and per cm, get a typical starting needle range by yarn weight, and estimate cast-on stitches and rows from a gauge swatch. Note: needle size and gauge don’t have a fixed formula relationship—always confirm with a swatch.

Knitting Needle Size Converter
Pick what’s on your needle, then enter the size.
Metric (mm)
US
UK

Related Calculators

Knitting Needle Size Conversion Formula

The following formula is used to convert a knitting needle diameter between inches and millimeters.

N_{mm} = N_{in} \times 25.4

Variables:

  • Nmm is the needle diameter in millimeters (mm)
  • Nin is the needle diameter in inches (in)
  • 25.4 is the number of millimeters in 1 inch

To convert inches to millimeters, multiply by 25.4. To convert millimeters to inches, divide by 25.4.

What is a Knitting Needle Size?

Knitting needle size refers to the diameter of the needle used in knitting. The size of the needle affects the size of the stitches and the overall gauge of the knitted fabric. Larger needles create larger stitches and a looser fabric, while smaller needles create smaller stitches and a tighter fabric. Needle sizes are typically measured in millimeters, and the appropriate size depends on the yarn weight and the desired gauge for the project.

How to Calculate Knitting Needle Size?

Knitting needle size is not something you can calculate exactly from stitch counts alone. Instead, you choose a starting needle size and then adjust based on a gauge swatch.

  1. Pick a starting needle size from your yarn label (or use the Yarn Weight tab for a typical range).
  2. Knit a gauge swatch in the same stitch pattern you’ll use for the project, then wash/block it the same way you’ll treat the finished item.
  3. Measure your stitch gauge (stitches per 4 in / 10 cm) and row gauge (rows per 4 in / 10 cm).
  4. If your swatch has more stitches per inch than the pattern calls for (too tight), go up a needle size; if it has fewer stitches per inch (too loose), go down a needle size.
  5. Repeat until your measured gauge matches the pattern; then use the Gauge Swatch tab above to estimate cast-on stitches and rows.

Example Problem : 

Use the following values as an example to convert a needle size from inches to millimeters.

Needle size in inches (Nin) = 0.25

Needle size in millimeters (Nmm) = 0.25 × 25.4 = 6.35