How it works
Stitches across = width × your stitches per 10cm ÷ 10. A pattern that repeats every 3 stitches, plus 2 to balance the edges, only works on 5, 8, 11 and so on, so the tool rounds to the nearest counts that fit and shows the width each gives. Add the turning chain your pattern asks for on top: some patterns count it as the first stitch, so check how yours words it.
Example
Attic24’s Granny Stripe works to a multiple of 3 plus 2 and starts with 200 chains for a 120cm blanket in Stylecraft Special DK, which works out at about 16.7 stitches per 10cm. Put 120cm, 16.7, 3 and 2 into the calculator and the ideal is 200.4 stitches; the nearest count that fits the multiple is 200, about 119.8cm wide.
Your tension matters most
Published DK blankets run from about 13 to 18 treble per 10cm, so a 120cm blanket can need anywhere from 156 to 216 stitches. Crochet a swatch at least 15cm wide in the stitch you’ll use and measure the middle 10cm. Our guide to how many chains for a blanket lists the tensions from 34 published patterns.