Knitting and crochet calculators. Each gives the answer first and shows the math.
Gauge calculator for knitting and crochet: cast on for a width, resize a pattern to your gauge, and increase or decrease evenly, with the math shown.
Yarn calculator for knitting and crochet: the skeins, yards, and grams your project needs, worked out from your own swatch, with the math shown.
Crochet hook sizes in mm and US letters, the yarn weights each hook suits, and steel hooks by mm, from the Craft Yarn Council standard.
Knitting needle sizes in mm and US numbers, the yarn weights each suits and the matching crochet hook where one is listed, from the Craft Yarn Council standard.
The Craft Yarn Council yarn weight chart, 0 Lace to 7 Jumbo: knit and crochet gauge, needle, hook and WPI ranges, with a label matcher and converters.