Pinwheel Quilt Block: Cutting Chart for 4 to 12 Inch Blocks

Pinwheel block: four half-square triangles with red and navy blades turning around the center on cream — Red and navy blades on cream. Each blade is the print half of one half-square triangle.

How the block is built

Pinwheel block pulled apart into four half-square triangles — Four half-square triangles. Each print triangle has one point at the center of the block.

Each half-square triangle (HST) finishes at half the block: a 10-inch pinwheel uses four 5-inch HSTs. All four diagonal seams run to the center point, so that's where the block succeeds or fails. The blades can all be one print, two prints alternating (as drawn), or four different scraps.

Cutting chart (one block)

Finished block	Finished HST	Squares, exact	Squares, to trim	Trim HSTs to	8 at a time (exact / to trim)
4 in	2 in	2⅞ in	3 in	2½ in	5¾ / 6 in
6 in	3 in	3⅞ in	4 in	3½ in	7¾ / 8 in
8 in	4 in	4⅞ in	5 in	4½ in	9¾ / 10 in
10 in	5 in	5⅞ in	6 in	5½ in	11¾ / 12 in
12 in	6 in	6⅞ in	7 in	6½ in	13¾ / 14 in

For one two-print pinwheel with the two-at-a-time method, cut 2 background squares, 1 square of each print. Each background and print pair makes two HSTs, which is exactly the two blades of that print. The squares are the finished HST + ⅞ inch (exact) or + 1 inch (to trim). Trimming every HST to the same size is what makes the center meet, so the "to trim" column is the one to use.

The eight-at-a-time column is for batches: one pair of squares (background and print) at that size gives eight identical HSTs, enough for two one-print pinwheels or the blades of one print in four two-print pinwheels. All sizes were checked by calculation; for other sizes, use the half-square triangle calculator.

Piecing steps

Draw a diagonal line on the wrong side of each background square. Pair each with a print square, right sides together.
Sew ¼ inch on both sides of the line, cut on the line, and press the seams toward the print.
Trim each HST to the size in the chart. Put the ruler's 45-degree line on the seam and trim two sides, then turn the HST and trim the other two.
Lay out the four HSTs. Start with the top-left one: print triangle in its lower-left half, background in its upper-right half. Turn the next one a quarter turn clockwise for the top right, and so on around the block. Every print triangle should have one point at the center.
Sew the top pair and the bottom pair. Press the top pair's seam to the right and the bottom pair's seam to the left, so they nest.
Pin the pairs right sides together at the center, pushing a pin straight through the point where the diagonal seams meet on both layers. Sew, crossing the center exactly at the X the seams make on the back.
Spin the center: on the back, unpick the two or three stitches of the last seam that are inside the seam allowances, then press all four seams in the same direction around the center. A tiny four-patch of seam allowance appears in the middle and the block lies flat. Pressing the last seam open works too.
Measure: 4½, 6½, 8½, 10½ or 12½ inches.

Variations

Turn every HST so the print is in the opposite half and the pinwheel spins the other way; mixing both directions in one quilt looks like a mistake unless you do it on purpose in alternate rows. Make the blades from a single directional print and they all point outward like a whirligig. A pinwheel made from four different scraps is a good way to use 5-inch charm squares: a 5-inch square cut to trim gives a 4-inch HST and an 8-inch block, which is the size in our pinwheel baby quilt pattern.

Big pinwheels go fast. Four 12-inch HSTs make a 24-inch block, and twelve of those (3 across, 4 down) make a 72 × 96-inch top with no border.

Common mistakes and fixes

Problem	Cause	Fix
Center points don't meet	HSTs trimmed to slightly different sizes	Trim every HST to the exact unfinished size before sewing
Blades cut off at the center	Last seam sewn wide of the X on the back	Sew through the crossing point of the two seams
Block won't lie flat	Four seam allowances stacked in the middle	Spin the center seams or press open
One blade faces backward	An HST turned 180 degrees	Lay out all four and compare to the diagram first
Wavy edges	HSTs cut from squares cut on both diagonals, putting bias on the edge	Use the two-at-a-time or eight-at-a-time method, which keeps straight grain on the outer edges

Quilts made from pinwheels

Pinwheel quilt: 18 pinwheels in four color pairs alternating with 17 plain cream blocks, 5 across and 7 down, with a navy border — Eighteen 8-inch pinwheels alternating with 17 plain 8½-inch cream squares, 5 across and 7 down: 40 × 56 inches, or 44 × 60 with a 2-inch border.

Alternating pinwheels with plain blocks gives each one room to spin and halves the piecing. Set side by side with no plain blocks, the background triangles of neighboring pinwheels join into their own pinwheels, and the quilt looks busier. Pastels make a classic baby quilt; pinwheels in red and navy on white make a cheerful Fourth of July throw or charity quilt.

Related blocks

The Dutchman's puzzle spins the same way with flying geese, and the friendship star puts four HSTs around a center square. The hourglass uses two HSTs sewn and recut. To make the block a different size, the block resizer covers the grid math, and the quilt block library shows every block on the site.

Pinwheel quilt block