Start here
Cast on enough stitches to make a swatch wider than 10 cm. Work in the pattern’s stitch, then wash and dry as you will the finished item. Measure stitches across the central 10 cm, away from the edges.
A useful habit
Too many stitches in 10 cm means your stitches are smaller: try a larger needle. Too few means they are larger: try a smaller needle. Make and measure another swatch.
Put it into practice
For a simple rectangle, required stitches = width in centimeters × stitches per 10 cm ÷ 10. Round to a count that satisfies the stitch repeat. Do not use this calculation alone to resize a shaped garment.