You finish a jumper, block it, try it on, and the sleeves hang past your fingers. Nothing is wrong with the pattern. Somewhere in the first few inches the fabric ended up looser than the designer intended, and every stitch since then has been quietly adding width. Tension — gauge, if you learned from American patterns — is the number that decides whether a project fits. It takes twenty minutes to check and it saves weeks of work. Here is how to do it properly.

Tension is a property of the fabric, not a skill level

Tension is simply how many stitches and rows you work in a set measurement, usually 10cm (4in). A pattern that says "22 stitches and 30 rows to 10cm in stocking stitch" is describing the fabric its designer made. Yours may be denser or looser with the same yarn and needle, and that is normal. Needle brand, yarn fibre, how you hold the yarn and even how you feel on the day all nudge the numbers. Matching tension is a mechanical job, not a test of whether you are a good knitter.

Knitting a swatch that tells the truth

A swatch only helps if it behaves like the finished garment. That means working enough of it, in the right stitch pattern, and treating it the way the finished piece will be treated.

1. Cast on plenty of stitches. Aim for a swatch at least 15cm wide and 15cm tall. If the pattern's tension is 22 stitches to 10cm, cast on around 40 stitches. Edges curl, pull in and lie differently, so you need a generous middle section to measure.
2. Work the stitch pattern. If the garment is mostly stocking stitch, swatch in stocking stitch. Cables, lace and ribbing all behave differently, so swatch in those too if they make up a large part of the piece.
3. Work enough rows. Row tension is what governs length, and it only settles after a good few centimetres.
4. Take it off the needles. Cast off loosely, or slip the stitches onto a holder. Measuring with needles still in place stretches the fabric and gives you a false reading.
5. Wash and dry it as you will the finished item. Woolly yarns bloom and fill out, cotton relaxes, and some superwash yarns grow. Press or pin the swatch flat in the way the ball band suggests, then let it dry completely before measuring.

Counting stitches and rows

Lay the dry swatch on a flat, hard surface and let it sit for a few minutes without pulling it. Place two pins exactly 10cm apart, somewhere in the middle of the fabric and well away from the edges. Count the stitches between the pins, counting a half-stitch at each end if one falls there. Do the same in two or three other places and average the results; a single reading can be off by a stitch or more. Then turn the swatch 90 degrees and count rows over the same 10cm.

For lace and cable patterns, measure the fabric in its relaxed, blocked state rather than stretching it out to show the pattern clearly. What matters is how it sits when worn.

Swatching for crochet

The process is nearly identical: work a square at least 15cm across in the stitch pattern, fasten off rather than measuring with the hook attached, then wash, block and dry it. Two things are worth extra care in crochet. First, stitch height varies a great deal between doubles, half trebles and trebles, so row tension in crochet is often the number that decides how long a garment ends up. Second, crochet fabric benefits enormously from blocking, particularly in lace and granny-style patterns, so always measure after blocking.

Because crochet stitches are taller and chunkier, hook sizes usually need bigger jumps than needle sizes when you adjust. Start with a 0.5mm change, and go to a full millimetre if that is not enough.

Adjusting your needles or hook

The rule is short and it works in both crafts:

• More stitches per 10cm than the pattern states? Your fabric is too tight. Go up a needle or hook size.
• Fewer stitches per 10cm? Your fabric is too loose. Go down a size.
• Rows out but stitches correct? Leave the needle size alone and work to lengths in centimetres rather than row counts.

Change one thing at a time, re-swatch, and expect two or three attempts. Small differences add up faster than most people expect. Suppose a pattern asks for 22 stitches to 10cm and you are getting 21. That is about 5% off. On a jumper with a 100cm circumference, where the pattern would have you cast on 220 stitches, your version comes out roughly 5cm wider: a whole size in some ranges, from a single stitch.

Keeping tension steady across a project

Tension wanders. It loosens when you relax into plain stocking stitch after concentrating hard on a cable, tightens when you are tense, and shifts when you switch from bamboo to metal needles. A few habits help:

• Keep the same needles or hook for the whole project; different brands can measure slightly differently even at the same nominal size.
• Check tension again after a long break, or after the first 10cm of a large piece, and compare it with your swatch.
• Store the swatch with your notes. Write down the yarn, needle or hook size, and whether it was washed. It becomes a genuine reference next time.
• If you substitute yarn, always swatch. Fibre content and drape affect how the fabric hangs as much as the numbers do.

Make swatching part of the process

Twenty minutes of swatching is not a delay; it is the step that makes the rest of the project predictable. If a pattern gives tension in both centimetres and inches, pick one and stay with it. If your blocked swatch matches the pattern, you can trust the instructions. If it does not, adjust and swatch again before you cast on the real thing. And when a project does fit well first time, keep the swatch: that small square is your own record of the numbers that worked.