Crochet triangle shawl and polygon calculator

Triangle shawls grow by a fixed number of stitches each row, and flat polygons by a fixed number each round. This calculator works out the rows, last-row count and total stitches for a shawl, checks that your stitches will give it a good shape, or works out the increases and dimensions for a flat hexagon, pentagon or octagon.

How to use the calculator

1. Choose a top-down triangle shawl, a sideways (asymmetric) shawl or a flat polygon.
2. Enter the wingspan for a shawl, or the width from one flat side to the opposite side for a polygon.
3. For a shawl, enter the stitches in the first row.
4. Enter your stitch and row gauge over 4 in (10 cm).

How the calculator works

Top-down triangle shawl. Each row adds 4 stitches: 1 at each outer edge and 2 at the centre spine. The top edge is made of the ends of the rows, so the rows come from the row gauge: the centre depth is half the wingspan, and

Rows = (wingspan ÷ 2) × row gauge

Last row = first row + 4 × (rows − 1)

Each lower edge is half of the last row, so its length comes from the stitch gauge: lower edge = (last row ÷ 2) ÷ stitch gauge. For a right-angled triangle each lower edge should be the depth × √2. If your stitches make it more than 10% shorter or longer, the calculator warns you and suggests what to change.

Sideways shawl. One stitch is added at the same edge every row, so rows = wingspan × row gauge, and the depth at the widest end comes from the last row and your stitch gauge.

Flat polygon. A flat shape with p sides needs about

Increases per round = 2 × p × tan(180° ÷ p) × (stitch gauge ÷ row gauge)

rounded to a multiple of p, so every corner gets the same. Corner to corner is the flat width ÷ cos(180° ÷ p), and each side is the flat width × tan(180° ÷ p).

Reading your results

• Shawls: rows, stitches in the last row, stitches in the whole shawl, the centre depth and the length of each lower edge, with a shaping line to follow.
• Polygons: rounds, increases each round and per corner, corner-to-corner width and the length of each side.

Worked examples

• Top-down shawl: a 60 in wingspan with 4 stitches in the first row, at 12 stitches and 8 rows per 4 in. The depth is 30 in, so 30 × 2 = 60 rows; the last row has 4 + 4 × 59 = 240 stitches, and the shawl 7,320 in all. Each lower edge is 120 ÷ 3 = 40 in; a right angle would need 42.4 in, within 10%, so blocking will open it to shape.
• Sideways shawl: the same wingspan and gauge give 120 rows, with 123 stitches in the last row and a depth of 41 in at the widest end.
• Hexagon: 10 in from flat side to flat side, at 12 stitches and 8 rows per 4 in. The ideal is 10.4 increases a round, rounded to 12 (2 at each corner); 10 rounds; 11.55 in from corner to corner, with 5.77 in sides.

Accuracy and limits

• The arithmetic is exact and covered by our automated tests. How well the shape lies flat depends on your tension and blocking.
• Wet-blocking usually opens a triangle shawl up; later rows are much longer, so most of the yarn goes into the last third.
• A hexagon cardigan is not a flat hexagon: it uses square corners so the extra fabric folds into sleeves. Follow that pattern's own increases.
• Polygons here have 5, 6 or 8 sides.

Common mistakes

1. Measuring the depth as the wingspan. In a top-down triangle the depth is only half the wingspan.
2. A tight top edge. The top edge is made of row ends; if it pulls in, work the first and last stitch of each row a little looser, or add a few chains at each edge.
3. Increasing in the same place in every round of a polygon. Keep the increases at the corners; that is what makes the points.

Tips for more accurate results

• Swatch in the shawl's stitch and block the swatch, since the shape check relies on your blocked gauge.
• If the calculator warns that your lower edges are short, a taller stitch or a looser hook helps; if they are long, a shorter stitch or a smaller hook.
• To plan yarn, weigh a swatch and use the yarn yardage calculator with the shawl's area (wingspan × depth ÷ 2).

Frequently asked questions