Why gauge decides the final size
Two people following the same pattern, with the same yarn and the same hook, do not get the same fabric. One pulls tight, the other leaves some slack, and every stitch carries the mark of it. Over ten stitches the difference goes unnoticed. Over a sweater of a hundred and forty stitches it adds up to tens of centimetres: the garment comes out too small, or hangs loose.
A swatch is the only way to find out before you cast on the first stitch of the real piece. It gives the number of stitches and rows your hand produces over a reference length, ten centimetres or four inches. The pattern states its own figures: the calculator on this page compares the two and tells you which way to correct.
How to make a swatch
Work a square about fifteen centimetres, or six inches, on each side, in the pattern’s stitch and with the yarn it calls for. It has to be bigger than the length you want to measure: the edges distort, and it is the stitches in the middle that count. If the pattern asks for a complicated stitch or stripes, work it as written, not in plain single crochet.
Then treat the swatch as you will treat the finished piece: wash it and dry it flat if you will wash the piece, block it with steam or dampness if the pattern says to. Some yarns, cotton or wool, change noticeably after a wash, and a swatch measured beforehand says nothing about the washed piece. Let it rest, then lay it flat, without stretching it.
Measure in the middle, over ten centimetres, with a rigid ruler. Count the stitches along a row, then the rows along a column, and enter them in the “My swatch” fields above. If the result falls between two whole numbers, count the fractions of a stitch: this page takes whole numbers, so round to the nearest, or measure over twenty centimetres and halve it.
Reading the “gauge” line of an English pattern
English patterns give the swatch under the name “gauge” or “tension”, often at the start of the pattern, with the materials. A typical line reads “14 sts and 16 rows = 4 in”: fourteen stitches and sixteen rows make four inches, about ten centimetres. The stitch used is sometimes given, “in single crochet” or “in pattern”, and you must follow it.
European patterns tend to write “14 stitches and 16 rows = 10 cm”. Pick the pattern’s unit with the selector above the fields: the reference length switches from ten centimetres to four inches, and the conversions at the bottom follow.
What the calculator does
It compares your stitches with the pattern’s. Too many stitches over the same length: your work is tight, and a bigger hook will loosen it. Too few: it is loose, and a finer hook will tighten it. A difference of five per cent or less calls for no correction. Rows are compared separately, for information: one row more or fewer over ten centimetres is easier to make up, by adding or removing a round, than a width that does not come out right.
If you tell it which hook you used, the page suggests the neighbouring size in the crochet hook size chart, bigger or finer depending on the advice. It is a starting point: half a millimetre clearly changes the size of a garment, but the only judge is a new swatch. Make one with that hook before you start.
The two conversions at the bottom are for once your swatch is right. With your tension, they tell you how many stitches to cast on for a chosen width, or what width a given number of stitches will give.
When the swatch does not match
Changing hook is the first reflex, and the simplest. If the stitches are right but the rows are not, changing hook would throw the stitches off again: adjust the pattern’s number of rows to your own height instead. If you cannot reach the right gauge without a fabric that is too stiff or too floppy, change yarn: a yarn of the same weight but a different fibre behaves differently.
Finally, do not skip this step because the piece is small. A hat or a shawl needs a swatch as much as a sweater does; only an amigurumi or a decorative object forgives a difference. A quarter of an hour of crochet at the start beats a piece to unpick.