How this calculator works
A regular hexagon fills space in a honeycomb. With the points at the top and bottom, the width across the flats is the side length times the square root of three, about 1.732. Each row below the first steps down by one and a half side lengths, because the slanted sides interlock. Dividing the planned quilt width by the across-flats distance gives the columns, and the row count comes from the vertical step. Whole hexagons fill the grid, and half hexagons square off the ragged left and right edges. The result is a rectangle that is usually a little smaller than the plan, because the columns and rows are whole pieces.
Formula
columns = quilt width / (sqrt(3) x side), rounded down. rows = (quilt length - 2 x side) / (1.5 x side) + 1, rounded down. whole hexagons = columns x rows. half hexagons = rows. actual width = columns x sqrt(3) x side. actual length = (rows - 1) x 1.5 x side + 2 x side.
Worked example
A 150 by 200 cm quilt with 10 cm hexagons has columns of 150 / 17.32 = 8.66, so 8 columns, and rows of (200 - 20) / 15 + 1 = 13 rows. That is 104 whole hexagons plus about 13 halves, finishing at 138.6 by 200 cm.
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Frequently asked questions
Should the hexagon size include seam allowance?
Yes for cutting. A hexagon cut from template plastic is usually the finished size; add a quarter inch all around when cutting fabric. Enter the finished side here, which is the size the hexagon will be after sewing.
Why are there half hexagons at the edges?
A grid of full hexagons has a zigzag edge. Half hexagons fill the gaps at the sides so the quilt has a straight binding edge. The estimate here allows about one half per row; a real layout may need a few more or fewer.
Can I use this for English paper piecing?
Yes. The geometry is the same. English paper piecing adds basting and hand-sewing time, so add a generous allowance for thread and for the papers, which are usually sold by the hundred.
How much extra should I buy for mistakes?
For a large quilt, 10 percent extra fabric is a common allowance. Hexagons use a lot of fabric compared with squares because each piece has six sides and the cutting leaves angled scraps.