Coconut Robot · Tools

Magic 8 HST calculator

I use the Magic 8 method for half-square triangles because two squares, sewn and cut once, give you eight matching units. Enter your finished block size and I will do the math: the exact size, the size I actually tell you to cut, and the size to trim each unit to once it is sewn.

Finished HST block size

inches, finished

= 4″

Exact size

9.75″

9 3/4″ square

Recommended cut

10″

10″ square

Trim to

4.5″

4 1/2″ square

For a 4″ finished HST block, cut two contrasting squares 10″ x 10″ — exact math is 9 3/4″ x 9 3/4″. Sew, cut apart, and trim each of the eight units down to 4 1/2″ x 4 1/2″ before you assemble the block.

Step-by-step instructions

Step 1 of 6

Mark the X

On the wrong side of one square, mark a line corner to corner in each direction with chalk or a disappearing pen. That gives you a big X to sew and cut along later.

Step 2 of 6

Layer and pin

Place your two squares right sides together, marked square on top. Pin around the perimeter to hold the layers in place before you sew.

Step 3 of 6

Sew the seam

Sew a scant 1/4″ seam on both sides of each diagonal line — four lines of stitching in total, flanking the X.

Step 4 of 6

Cut into quarters

Cut the square in half vertically and in half horizontally through the center, making four smaller squares.

Step 5 of 6

Cut into 8 triangles

Now cut along the diagonal lines you marked in step 1. Two squares become eight matching half-square triangles.

Step 6 of 6

Press, trim, and square up

Press each triangle open toward the darker fabric and trim off the little “dog ears” left from the seam allowances. Then trim and square each unit down to your trim size. Eight finished HSTs, ready to use.

Quick reference

Common Magic 8 HST sizes
Finished HST	Recommended cut	Trim to

Frequently asked questions

How big of a fabric square do I need for a 4 inch finished HST block using the Magic 8 method?

For a 4″ finished HST block, the exact Magic 8 square size is 9-3/4″. The recommended cutting size, which leaves room to trim, is 10″ by 10″. After sewing and cutting, trim each half-square triangle down to 4-1/2″ by 4-1/2″.

What is the Magic 8 HST technique?

The Magic 8 technique is a half-square triangle construction method where two contrasting fabric squares are layered, marked, sewn, and cut apart to produce eight identical HST units at once — less fabric and less cutting time than making HSTs one at a time.

Why cut Magic 8 squares larger than the exact measurement?

Cutting 1/4″ larger than the exact measurement leaves a small margin to square up and trim your half-square triangles after sewing, which corrects small piecing inaccuracies and keeps the finished block accurately sized.

How many half-square triangles does the Magic 8 method make?

Two fabric squares, sewn and cut using the Magic 8 technique, yield eight half-square triangle units in one pass.

All measurements assume a standard 1/4″ seam allowance.

Exact is the true no-margin math. Recommended cut adds a small margin so you can square up your blocks after sewing.

Part of the Coconut Robot quilting tools · coconutrobot.com

Magic 8 HST Calculator: Efficient Fabric Sizing