Your swatch already knows how much yarn you need.
Generic yardage tables don't knit at your gauge, in your stitch, with your needles. Your swatch does. Weigh it, measure it, read the ball band, and the yards and skeins fall out — with the extra-skein arithmetic done honestly. Everything stays in your browser.
Your swatch
Blocked, because blocking changes everything. Weigh it on a kitchen scale in grams; measure the width and height you actually got.
The piece
Finished dimensions — for shaped pieces, the bounding size is the honest over-estimate, which is the direction you want to miss in.
The ball band
The numbers here are yours to check. This page does arithmetic on what you type — it cannot see your recipe, your equipment or your batch.
Want it to remember?
In MadeKeeper the swatch math runs inside the Make form, your margin saves once shop-wide, and the yarn a finished piece consumed comes out of inventory at its real cost — so the shawl knows what it cost, and your price stands on it. See for knitters or poke around the live demo. $19/mo for the whole back office, kept for as long as you stay. The shop stays free for cost a piece and one fair — no product cap. The tax packet, Etsy and Square, and more fairs are the $19.
Will the yarn make it? — answered properly
'Will I run out' is the oldest fear in knitting, and it has an honest answer that no generic table gives: your own swatch, weighed. The math below is the same math the calculator runs — worked in the open, so you can check it against your own needle-born numbers.
Why the swatch instead of a yardage table?
Because the table doesn't knit like you. Published sweater-yardage charts assume an average gauge in an average stitch on average needles, and every one of those assumptions moves the answer — cables drink yarn, lace sips it, and your row gauge is yours alone. The swatch method scales YOUR knitting: the piece is so many times the swatch's area, so it weighs that many times as much, and the ball band turns weight into yards. It is the method experienced knitters trust precisely because every number in it came off their own needles.
How much extra yarn should I buy?
Ten percent is the working convention, and one skein of it should be thought of separately: dye lots. Two skeins of the same colorway from different lots can differ visibly in a way you only discover twenty rows in, so the real rule is buy the whole project's yarn at once, in one lot, with the margin included. The margin here is a setting, not a law — a first sweater or a yarn you can't reorder deserves more; a blanket in a workhorse machine-dyed yarn can run leaner.
Why does one extra yard cost me a whole skein?
Because yarn is sold in skeins and runs out in yards. If the piece needs 441 yards and a skein is 220, two skeins leave you one yard short at the bind-off — so the arithmetic is a ceiling, never a rounding. This page rounds up before AND after the margin. It reads pessimistic; it is just honest. The alternative reading — 'two skeins is probably fine' — is the yarn-chicken game, and the bind-off always wins it.
What if I don't have the ball band?
Weigh what you have. The grams answer still works without any band: the piece needs so many grams of THIS yarn, and your kitchen scale can compare that against the stash directly. Yards only enter when you need to buy more — in which case the seller's listing states yards and grams per skein, and those two numbers are all the band was giving you anyway.