48I need to do some yarn math

Open Tools → Calculators. Everything sits on one screen with five tabs: Gauge Swatch, Adapt Pattern, Yarn amount, Units and Pooling. Metric or imperial is chosen once and applies to every tab; it starts from your unit setting. Every tab ends with More on the topic: a few pages that explain exactly what that tab works out.

On Gauge Swatch, enter your swatch and the pattern's gauge. As soon as both are there, How big it comes out knits the pattern's stitches and rows for 10 cm (or 4 in) twice: once at the pattern's gauge and once at yours, side by side and to scale, with the real size under each. Then enter the pattern's stitch and row counts and tap Calculate for the numbers to work with your gauge.

Adapt Pattern turns the pattern's stitch and row counts into yours. Under the result, Width compared draws three strips to one scale: the pattern's count at the pattern's gauge, that same count at your gauge (too wide or too narrow), and the adjusted count at your gauge, which comes back to the pattern's width.

Yarn amount answers three questions. Leftover yarn: from the length and weight on the label, how long a weighed leftover is, or how much to weigh off for a length. Skeins to buy: what the pattern needs, minus the full skeins and leftovers you have, rounded up to whole skeins, with an optional allowance. Yarn substitution: how many skeins of another yarn give the same length, held single or double. Both answers are drawn as a band of skeins, with a line where the pattern's need ends, so what you have, what to buy and what will be left over read at a glance.

Units swaps between meters and yards, centimeters and inches, and grams and ounces, with the exact definitions (a yard is 0.9144 m, an inch 2.54 cm). Type on either side and the other side follows.

Pooling starts with measuring the yarn: enter each colour and how many stitches it lasts, then the stitch (stockinette, moss, single or double crochet) and whether you work in rows or rounds. The Preview shows the fabric at one stitch count; move the slider and it follows, drawn as knitted stitches when they are big enough to see. Under it: how far the colours move every row, and after how many rows the fabric repeats. All stitch counts opens a small swatch for every count in a range; tap one to bring it into the preview.

The Stitch Abbreviation Glossary is a pattern dictionary. Look up k2tog, ssk, yo, sc or hdc, with knitting and crochet terms both covered. Search for the abbreviation, or read the whole list when a pattern throws something new at you.