Granny Square Blanket Calculator
A 20 cm motif joined with a 0.5 cm seam occupies 20.5 cm, and since the last motif has no seam after it, one join is added before dividing. Across 120 cm that is (120 + 0.5) ÷ 20.5 = 5.87 → 5 motifs; down 150 cm it is 7, so 35 squares. Those five make a finished width of 5 × 20 + 4 × 0.5 = 102 cm.
Your numbers
Squares needed
35
Squares across
5
Squares down
7
Width once joined
102cm
Formula
Squares across = ⌊(Finished width + Join width) ÷ (Square size + Join width)⌋, Squares needed = Squares across × Squares down
102 cm is 18 cm short of the 120 cm you asked for, because the division rounds down. You have to choose: add a sixth column and overshoot to 122.5 cm, or crochet a 9 cm border and make up the difference. Hand-made motifs also vary, so block them all to one size before joining. The join width depends on the method — a slip-stitch join is near zero, a chain join 0.5–1 cm, a single-crochet join more than 1 cm.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Finished width (cm)The width when finished, before any seam or fold allowance. | 120 | 1 and up |
| Finished length (cm)The length when finished. | 150 | 1 and up |
| Square size (cm)One side of the finished motif, measured after blocking. | 20 | 1 and up |
| Join width (cm)Width the join adds between motifs; a slip-stitch join is almost zero. | 0.5 | 0 and up |
Step by step
Quick reference table
Results when only Finished width (cm) changes and everything else stays put.
| Finished width (cm) | Squares needed | Squares across | Squares down |
|---|---|---|---|
| 60 | 14 | 2 | 7 |
| 90 | 28 | 4 | 7 |
| 120 | 35 | 5 | 7 |
| 180 | 56 | 8 | 7 |
| 240 | 77 | 11 | 7 |
What each result means
| Result | At default values |
|---|---|
| Squares neededTotal motifs to make — across times down. | 35 |
| Squares acrossMotifs across the width, rounded down so it never overshoots the target. | 5 |
| Squares downMotifs down the length, counted the same way as across. | 7 |
| Width once joined (cm)The width you actually get once joined, which can fall short of the target. | 102 |
Common mistakes
102 cm is 18 cm short of the 120 cm you asked for, because the division rounds down. You have to choose: add a sixth column and overshoot to 122.5 cm, or crochet a 9 cm border and make up the difference. Hand-made motifs also vary, so block them all to one size before joining. The join width depends on the method — a slip-stitch join is near zero, a chain join 0.5–1 cm, a single-crochet join more than 1 cm.
Glossary
- Finished width
- The width when finished, before any seam or fold allowance.
- Finished length
- The length when finished.
- Square size
- One side of the finished motif, measured after blocking.
- Join width
- Width the join adds between motifs; a slip-stitch join is almost zero.
- Squares needed
- Total motifs to make — across times down.
- Squares across
- Motifs across the width, rounded down so it never overshoots the target.
- Squares down
- Motifs down the length, counted the same way as across.
- Width once joined
- The width you actually get once joined, which can fall short of the target.
Frequently asked questions
Q. How is Granny Square Blanket Calculator calculated?
Squares across = ⌊(Finished width + Join width) ÷ (Square size + Join width)⌋, Squares needed = Squares across × Squares down — A 20 cm motif joined with a 0.5 cm seam occupies 20.5 cm, and since the last motif has no seam after it, one join is added before dividing. Across 120 cm that is (120 + 0.5) ÷ 20.5 = 5.87 → 5 motifs; down 150 cm it is 7, so 35 squares. Those five make a finished width of 5 × 20 + 4 × 0.5 = 102 cm.
Q. Can you walk through an example?
With Finished width 120cm, Finished length 150cm, Square size 20cm, Join width 0.5cm, the answer is Squares needed 35.
Q. What do I need to enter?
Enter Finished width, Finished length, Square size, Join width. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Finished width (cm) moves the answer to Finished width (cm) 60 → Squares needed 14 and Finished width (cm) 240 → Squares needed 77. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
102 cm is 18 cm short of the 120 cm you asked for, because the division rounds down. You have to choose: add a sixth column and overshoot to 122.5 cm, or crochet a 9 cm border and make up the difference. Hand-made motifs also vary, so block them all to one size before joining. The join width depends on the method — a slip-stitch join is near zero, a chain join 0.5–1 cm, a single-crochet join more than 1 cm.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.