Convert knitting needle sizes between mm, US and old UK numbers, and estimate cast-on stitches from your swatch gauge and the finished width you want.

Optional settings
For a cast-on estimate, enter both gauge and finished width.

Related Calculators

Knitting Needle Size Conversion Formula

The formula below converts physical diameter between inches and millimeters. US and old UK numbers use chart lookups, not this formula; numbered sizes vary by manufacturer. Check the actual marked diameter.

Nₘₘ = Nᵢₙ × 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. The calculator uses the Craft Yarn Council US chart and a common old UK / Canadian chart (compare Rowan’s manufacturer chart). A US label may correspond to more than one diameter. Unlisted diameters are not rounded to a numbered size.

Match pattern gauge and adjust simple counts

Select Match pattern gauge and enter your washed/blocked swatch stitch gauge and the pattern stitch gauge, using the same stitch pattern. Select each gauge’s actual measurement length: 4 inches is exactly 10.16 cm. A positive gauge difference means more, smaller stitches over the same length; try larger needles. A negative difference suggests smaller needles. The Craft Yarn Council recommends comparing a swatch with the pattern and trying another needle size. Gauge does not determine an exact needle diameter, so swatch again after each change.

Gauge difference = (your normalized gauge / pattern normalized gauge − 1) × 100%. In Optional stitch and row counts, enter a pattern stitch count to estimate the count that preserves its width at your current gauge. Enter both row gauges to compare rows, and optionally a pattern row count to preserve the section’s length. Adjusted count = pattern count × your normalized gauge / pattern normalized gauge. For gauge expressed per 10 cm, size in cm = count / gauge × 10. Results show the unrounded and nearest-whole counts, intended pattern size, size with unchanged counts, and size after rounding.

For example, your 18 stitches per 10 cm against a pattern’s 20 is −10%. A 100-stitch section is intended to be 50 cm wide; the same 100 stitches at your gauge would be 55.556 cm. The proportional estimate is 90 stitches for 50 cm. If your row gauge is 24 versus the pattern’s 30 rows per 10 cm, a 120-row section calls for 96 rows to keep its 40 cm length.

These are proportional estimates for simple sections, not an automatic garment-pattern rewrite. Adjust stitch repeats, edge stitches, shaping and construction separately. Check the fabric and row gauge as well as stitch gauge: changing needle size may not change both gauges in the same proportion.

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. The result includes overlapping Craft Yarn Council needle-range guidelines; these are starting points and do not identify yarn weight from diameter alone.
  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 stitch gauge over a stated length. Select stitches per 10 cm or stitches per 4 inches accurately: 4 inches is exactly 10.16 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. In Optional settings, enter stitch gauge and finished width together to estimate cast-on stitches: stitches per 10 cm × width in cm ÷ 10. The result rounds to the nearest whole stitch; adjust for stitch repeats, edge stitches and construction. For row-gauge comparison and proportional row-count estimates, use Match pattern gauge.

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