How Many Granny Squares Do I Need for a Blanket?

The Maths

The calculator (and the table above) works to one rule: squares wide = blanket width / square size, squares long = blanket length / square size, both rounded up to a whole square, then multiplied together for the total. For the 120 x 150cm throw at 15cm squares: 120 ÷ 15 = 8 wide, 150 ÷ 15 = 10 long, 8 × 10 = 80 squares. At 10cm squares the same throw needs 12 × 15 = 180 squares; at 20cm squares it is 6 wide by 150 ÷ 20 = 7.5 rounded up to 8 long, so 6 × 8 = 48 squares.

Worked Example: Yarn for the 80-Square Throw

A standard 15cm 4-round granny square in DK yarn uses approximately 10–12 metres, so the calculator's default of 12m per square is a reasonable planning figure:

• Yarn for the squares: 80 × 12m = 960m
• Joining yarn: a slip-stitch or whip-stitch join uses roughly 2–3 times the seam length, about 0.025m per cm of edge. 80 squares × 15cm edge × 4 edges × 0.025m = 120m
• Total yarn: 960m + 120m = 1,080m
• At a typical ball meterage of 170m, that is 1,080 ÷ 170 = 6.35, rounded up to 7 balls

Scaling up: a bigger 200 x 220cm project at the same 15cm square size needs 200 ÷ 15 = 13.3 rounded up to 14 wide, and 220 ÷ 15 = 14.7 rounded up to 15 long, so 14 × 15 = 210 squares. Squares yarn is 210 × 12m = 2,520m, joining yarn is 210 × 15cm × 4 × 0.025m = 315m, for a total of 2,835m, which is 2,835 ÷ 170 = 16.7, rounded up to 17 balls.

The single edge of one 15cm square only needs about 0.3–0.5m of joining yarn, and a border round adds roughly 50–150m depending on the blanket's size and how many rounds you work, so build those in as a separate allowance once the squares themselves are done. The granny square calculator runs this whole calculation for your own dimensions, square size and yarn, including an adjustable joining-yarn rate.

Granny Squares vs a Solid Crochet Blanket

Because granny squares are worked in rounds with open spaces between the clusters, they typically use less yarn than a solid crochet fabric of the same finished size: the same 120 x 150cm throw worked as a solid piece in DK yarn uses approximately 1,500–2,000m, against roughly 1,080m worked in 15cm granny squares. The trade-off is the extra time spent joining and weaving in ends from dozens of separate pieces.

Joining the Squares

Popular joining methods include slip stitch, dc join, whip stitch, mattress stitch and join-as-you-go. Slip stitch creates a flat seam, dc join creates a raised ridge, and join-as-you-go attaches each square during its last round, saving separate seaming time later. Whichever method you choose, budget the 0.3–0.5m per edge above into your total.