Continuous granny square blanket calculator

In a big centre-out granny square, each round takes more yarn than the one before, so the outer rounds use far more than you expect. Weigh your piece after a few rounds and this calculator predicts the yarn for the next round, the total for your target size, the width you will reach, and the last round your yarn will finish.

How to use the calculator

1. Work at least 5 rounds and weigh the piece itself, not the ball. The starting ring and tail skew smaller samples.
2. Enter the rounds completed, the weight and, optionally, the width.
3. Enter the rounds you want in total and, optionally, all the yarn you have (the blanket so far plus your balls).
4. Before starting a round, enter what is left on your ball to check it will finish.

How the calculator works

Each round has the same number of extra stitches as the one before, so its length grows steadily: if round 1 uses one unit of yarn, round n uses n units, and R rounds use R × (R + 1) ÷ 2 units.

One unit = weight now ÷ (rounds × (rounds + 1) ÷ 2)

Next round = one unit × (rounds + 1)

Total for T rounds = one unit × T × (T + 1) ÷ 2

The width grows by the same amount every round, so width at T rounds = width now ÷ rounds × T. For your yarn, it finds the largest number of rounds whose total fits.

Reading your results

• Next round alone needs: the yarn for the very next round.
• Total for your target: with how much more you need from now.
• Width at your target: if you entered the width.
• Last round your yarn reaches: with the yarn used and left.
• Ball check: whether the yarn on your ball will finish the next round. It warns if the ball is short, or within 5% of what the round needs.
• Yarn per round table: every few rounds, with the running total.

Worked example

After 10 rounds a granny square is 12 in across and weighs 55 g.

• One unit: 55 ÷ (10 × 11 ÷ 2) = 55 ÷ 55 = 1 g.
• Round 11 alone takes 11 g.
• 40 rounds take 40 × 41 ÷ 2 = 820 g in all (765 g more), and make a blanket 12 ÷ 10 × 40 = 48 in across.
• With 500 g of yarn in all, you can finish round 31 (496 g), with 4 g left, at about 37.2 in across.

Accuracy and limits

• The arithmetic is exact and covered by our automated tests, for squares, hexagons and other shapes that grow by the same number of stitches every round.
• Tension drifts over a big blanket, so keep a margin on the last round.
• Colour changes add ends that the model does not count.

Common mistakes

1. Weighing too early. Before about 5 rounds, the starting ring and tail are a big share of the weight and the prediction runs high.
2. Weighing the ball instead of the piece. Enter what the blanket itself weighs.
3. Expecting the outer rounds to take the same as the inner ones. In a 40-round blanket the last quarter of the rounds uses almost half the yarn.

Tips for more accurate results

• Weigh again every 10 rounds and update the numbers; the prediction gets better as the piece grows.
• Turn your work at the end of each round if a big square starts to twist; the stitches then slant alternately and cancel out.
• Planning a border? Leave yarn for it, and work it out with the border yarn calculator.

Frequently asked questions