What is tension in knitting and why did my jumper come out the wrong size?

A pattern is a set of stitch counts, not measurements. It only produces the stated size if your stitches are the size the designer's were, which is what the tension square exists to check.

What you'll need

• The yarn and needles you intend to use
• A ruler or tension square
• Pins

Step by step

1. Find the tension stated in the pattern.Something like '22 stitches and 30 rows to 10cm over stocking stitch on 4mm needles'. Every number in the pattern depends on it.
2. Knit a square bigger than 10cm.At least 15cm each way, so you can measure in the middle away from the edges. Edge stitches are always a different tension and they distort the measurement.
3. Use the actual yarn and stitch pattern.A square in stocking stitch tells you nothing about a cabled garment. Work it in whatever the pattern specifies the tension over.
4. Wash and block the square first.Exactly as you will treat the garment. Many yarns, particularly wool and cotton, change measurably after their first wash — which is why a garment that fitted on the needles does not fit afterwards.
5. Measure without stretching it.Lay it flat, unpinned and unstretched. Put pins 10cm apart in the middle and count the stitches between them, including partial stitches.
6. Count rows as well as stitches.Stitch count controls the width, row count controls the length. Row tension matters most on anything knitted top down or with shaping worked over a set number of rows.
7. Change needle size, not your knitting.Too many stitches per 10cm means your stitches are too small — use a larger needle. Too few means go smaller. Trying to knit consciously looser never survives past the first few rows.
8. Reknit the square and check again.Half a millimetre of needle size makes a real difference across a garment. It is worth the second square.
9. Work out the consequence if you cannot match it.Multiply the pattern's stitch count by your tension to get the actual finished width. Sometimes making a different size to the numbers gets you the measurement you want.
10. Keep the square.Label it with the yarn, needle size and tension. It is a useful record and a test piece for washing.

Tips

• Change the needle, not how you knit. Deliberately altered tension does not last a whole garment.
• Wash and block the square before measuring. Many yarns change on the first wash.
• Row tension matters as much as stitch tension on top-down and shaped garments.

Common mistakes

• Skipping the tension square — Every measurement in the pattern is derived from the designer's tension. Being ten per cent out over a hundred stitches is a garment several centimetres wrong across the chest.
• Measuring a small square including its edges — Edge stitches are always looser or tighter than the body of the fabric, so including them makes the measurement inaccurate in an unpredictable direction.

If it doesn't work

Questions people ask