Weigh the swatch, do not measure the yarn
The only reliable way to find out how much yarn a square inch of your fabric holds is to knit some and weigh it. Unpicking a swatch and measuring the yarn is slow and inaccurate; a kitchen scale reads the same information in a second, and the ball band converts grams to yards. A 220 yard, 100 gram ball is 2.2 yards per gram, so a nine gram swatch holds 19.8 yards. Over 16 square inches that is 1.24 yards per square inch.
From there everything is division. Five balls is 1,100 yards; hold back ten percent and 990 remain; at 1.24 yards per square inch that covers 800 square inches, which at 20 inches wide is 40 inches long.
Why the swatch has to be the real fabric
Yards per square inch is a property of the fabric, not of the yarn. The same ball worked in cables uses far more per square inch than in stockinette, because a cable crossing pulls the fabric in and packs more yarn into less area. Lace uses less, because a good part of the fabric is hole. Garter sits between. Change needle size and the figure moves again.
So the swatch has to be the stitch pattern you will use, on the needles you will use, blocked the way you will block. A stockinette swatch used to plan a cabled blanket will leave you short, and the shortfall is not small.
Ten percent bigger is twenty one percent more yarn
This is the arithmetic that catches everyone, and it catches them at the worst moment. Area goes as the square of a length. Add ten percent to the width and ten percent to the length of a blanket and you have added twenty one percent to the yarn, not ten. Twenty percent bigger both ways is forty four percent more yarn. Half again in each direction is more than double.
The failure mode is always the same: someone likes a pattern, decides to make it a bit bigger, buys one extra ball to be safe, and runs out three quarters of the way through with the dye lot gone. The page prints the extra yardage and the extra balls for a ten percent increase precisely so this is a number rather than an instinct.
What to hold back
The default reserve is ten percent, and it is not conservatism for its own sake. It covers the swatch you already knitted, seaming, weaving in, and the fact that the last few rows of anything are where a shortage becomes unrecoverable. If the last thing you work is a border in the main colour, or if the piece has to match a second piece, raise it.
The one thing this page will not do is tell you whether the fabric is any good. It counts yarn. Whether the drape suits a blanket, whether the gauge is right for a garment that has to fit somebody, and whether the yarn will survive being washed are all questions the swatch answers by being looked at rather than weighed.
Questions people ask
How do I work out how much yarn a square inch of knitting uses?
Knit a swatch in the stitch pattern you will use, block it, measure it and weigh it. Multiply the weight by the grist from the ball band — ball yardage divided by ball weight — to get the yards in the swatch, then divide by the swatch area. A 4 inch swatch weighing 9 grams in a 220 yard, 100 gram yarn works out at about 1.24 yards per square inch.
How big a blanket can I make with 5 balls of yarn?
It depends entirely on the fabric, which is why the page asks for a swatch rather than a yarn weight class. On the defaults — 1,100 yards, ten percent held back, and 1.24 yards per square inch — the answer is around 800 square inches, or 20 by 40 inches. Work the same yarn in cables and it will cover noticeably less.
Why does adding a few inches use so much more yarn?
Because you are adding area, not length. Ten percent more width and ten percent more length is 1.1 times 1.1, which is 1.21 — twenty one percent more yarn. Twenty percent bigger in both directions is forty four percent more. This is the most common reason a project runs out, and buying one extra ball rarely covers it.
Can I trust a swatch weighed on a cheap kitchen scale?
If the scale reads whole grams, knit a larger swatch. A one gram error on a nine gram swatch is eleven percent, which propagates straight into the answer. A six inch square weighs more than twice what a four inch one does, so the same absolute error matters half as much. A scale reading to a tenth of a gram removes the problem entirely.
Does this work for crochet?
Yes, and it matters more there. Crochet fabric generally uses more yarn per square inch than knitting at a similar thickness, and the difference between stitch patterns is larger. The method does not care which craft made the swatch — it only needs a piece of the real fabric, measured and weighed.