Scrap yarn blanket size calculator

Have a fixed amount of yarn and want to know how big a blanket or scarf it will make? This calculator uses a weighed swatch to find the largest size for the shape you choose. For corner-to-corner blankets it also gives readings for your kitchen scale, so you know when to stop increasing and when to start decreasing.

How to use the calculator

1. Weigh all the yarn you have.
2. Crochet a swatch in the project's stitch and yarn, and enter its size and weight.
3. Choose what to keep fixed: a length-to-width ratio (1 for a square, about 1.2 to 1.3 for a throw, 6 to 8 for a scarf), a set width, or corner-to-corner checkpoints.
4. Set the reserve to keep back for weaving in ends; 5% is usual.

How the calculator works

The swatch tells you how much fabric each gram makes:

Area per gram = swatch width × swatch height ÷ swatch weight

Largest area = usable yarn × area per gram, where the usable yarn is what you have minus the reserve.

• Ratio: width = √(area ÷ ratio) and length = ratio × width.
• Set width: length = area ÷ width, and with your row gauge, the number of rows.
• Corner to corner: a C2C rectangle grows as a triangle, runs straight through the middle, then shrinks as a triangle. With a length-to-width ratio α, the increase uses 1 ÷ (2α) of the yarn, the straight middle (α − 1) ÷ α, and the decrease 1 ÷ (2α). The calculator turns those shares into the weight your scale should show at each turn.

Reading your results

• Largest width and length: the biggest size for your shape.
• Checkpoints (corner to corner): weigh your remaining yarn, including any ball you have started, and change phase when the scale reaches each reading.

Worked example

You have 400 g of worsted scraps and keep back 5%, leaving 380 g. A 6 × 6 in corner-to-corner swatch weighs 12 g, so each gram makes 3 sq in.

• Largest area: 380 × 3 = 1,140 sq in.
• At a ratio of 1.25: width √(1,140 ÷ 1.25) = 30.2 in, length 37.7 in.
• Stop increasing the short side after 380 ÷ 2.5 = 152 g, when the scale reads 248 g.
• Start decreasing after a further 76 g, when it reads 172 g.
• Finish with the 20 g reserve left.

Accuracy and limits

• The arithmetic is exact and covered by our automated tests. The size is only as good as your weighed swatch.
• It assumes every part of the blanket uses the same yarn per square inch. Scraps of different fibres and thicknesses vary; swatch with a typical mix.
• Borders are not included.
• Several small balls need extra for joins; raise the reserve if you have many.

Common mistakes

1. Turning a rectangular C2C blanket at half the yarn. Stopping increases at 50% makes a square. For a rectangle, turn earlier: at 40% of the usable yarn for a ratio of 1.25, or a third for 1.5.
2. Turning mid-row. Finish the diagonal row you are on before changing phase; the reserve covers it.
3. Weighing only the full balls. Include the ball you are working from in the scale reading.

Tips for more accurate results

• Joining scraps with a Russian join or magic knot leaves fewer ends to weave in, so you may be able to keep back less.
• Mix thin scraps with thick ones by holding the thin ones double; the yarn held together calculator shows what two strands make.
• Once you know the size, plan the blocks and rows with the C2C crochet calculator.

Frequently asked questions