Granny square blanket calculator

How it works

Across the blanket, the squares, the joins between them and the border on each side add up to its width. The calculator finds the number of squares that comes closest to the width you want, then does the same for the length.

squares across = (width − 2 × border + join) ÷ (square + join), rounded to the nearest whole number
finished width = squares × square + (squares − 1) × join + 2 × border

Example: for a 50 × 60 in throw made of 6 in squares, with flat joins and no border, 50 ÷ 6 = 8.3, so 8 squares across make it 48 in wide. 60 ÷ 6 = 10 squares down, exactly 60 in. That's 8 × 10 = 80 squares.

If one square weighs 15 g, 80 squares take 1,200 g of yarn, or 1,320 g with 10% extra: 14 skeins of 100 g.

Tips for a blanket that comes out the right size

• Make and block a few squares before the rest, then measure them. Blocking can change a square's size, and a ¼ in difference over 10 squares adds up to 2½ in.
• To find your join width, join two squares and measure across both. Whatever is more than twice the square size is the join width.
• If every square uses the same colours, weigh each ball before and after making one square to find how much of each colour a square takes, then enter each colour in turn.
• The border needs yarn on top of the squares. The wider the border, the more you need.
• It works for any square motif: knitted squares, or quilt blocks with the finished block size as the square and the sashing width as the join.