πΊ Half Square Triangle Calc
Finished size to cut size, unit counts, and fabric math.
Cut square size
How to Use This Calculator
Enter the finished size of your half square triangle, the size it will be in the completed quilt after seaming. The calculator returns the exact cut size (finished plus 7/8 inch), the rounded-up cut size for easy rotary cutting, and the unfinished block size for squaring up. Optionally enter your quilt dimensions to get the total HST count and how many starting squares to cut. Two starting squares make two HST units in the standard method, so the square count is half the unit count, rounded up.
Enter the finished HST size from your pattern, in inches.
Read the exact cut size. Round up to the next eighth for your ruler.
Optionally enter quilt dimensions to count total units and starting squares.
Cut a test pair first, measure the result, then cut the batch.
The Method: Finished Plus 7/8
The 7/8 inch is the standard allowance for the traditional two-at-a-time method: it covers two quarter-inch seam allowances plus the extra the diagonal seam consumes on the bias. The math behind it: the diagonal of the cut square becomes the HST's outer edge after seaming, and working backward from the finished size through the seam allowances lands at finished plus 0.875. The round-up-to-an-inch variant trades a sliver of fabric for ruler-friendly numbers and a trimming step that guarantees accuracy; many modern quilters prefer it because squaring up corrects every small error at once. The unfinished size (finished plus half inch) is your trimming target and the size you measure before the block goes into the quilt top.
Worked Example
A pattern calls for 3-inch finished HSTs in a 48 x 60 inch quilt. Cut size = 3 + 0.875 = 3-7/8 inches; round-up version = 4 inches. Unfinished target = 3.5 inches. Units needed: 48/3 = 16 across, 60/3 = 20 down, total 320 HSTs. Starting squares: 320/2 = 160 squares, cut as 80 light and 80 dark for two-color HSTs. From 44-inch-wide fabric, a strip of 3-7/8 inches yields 11 squares per strip width (44/3.875 = 11.35, floor 11), so 160 squares need 15 strips (160/11 = 14.5, round up). Enter 3, 48, 60 above and the calculator reproduces the unit and square counts.
Cutting and Sewing Tips
Cut squares accurately; every sixteenth of error doubles in the finished quilt. Draw the diagonal on the wrong side of the lighter square, sew a quarter inch on both sides of the line, then cut on the line. Press before trimming, and press toward the dark fabric (or open for bulky seams) without pushing the iron, which stretches bias edges. Square up every unit to the unfinished size with the diagonal line on your ruler aligned to the HST's diagonal seam; this single habit fixes more quilts than any other. Chain-piece pairs assembly-line style to save thread and time.
Common HST Sizes Reference
Finished 2 in: cut 2-7/8, unfinished 2-1/2. Finished 3 in: cut 3-7/8, unfinished 3-1/2. Finished 4 in: cut 4-7/8, unfinished 4-1/2. Finished 5 in: cut 5-7/8, unfinished 5-1/2. Finished 6 in: cut 6-7/8, unfinished 6-1/2. Finished 8 in: cut 8-7/8, unfinished 8-1/2. The pattern is obvious once you see it: add 7/8 to cut, add 1/2 for unfinished. Pin this progression near your cutting table and most HST math disappears; use the calculator for the odd sizes and quilt-wide counts.
Trimming HSTs for Perfect Points
The two-at-a-time HST method produces slightly oversized units you trim to exact size, and trimming is where crisp points come from. Use a square ruler with a diagonal line: align the diagonal with the seam, trim two sides, rotate, and trim the remaining two to the target size. Always trim to the unfinished size (finished size plus half an inch for seam allowances): a 3-inch finished HST trims to 3.5 inches. Press seams before trimming; unpressed units shift under the ruler. Chain-piece generously and trim in batches; the rhythm is fast once set up. Dog-ears (the little triangle tips) get clipped in the trim pass, which is why trimmed HSTs have sharper points than untrimmed ones. Sloppy trimming compounds across a quilt: a sixteenth off per unit becomes inches across a row.
HST Quilt Math and Layouts
HSTs are the atoms of quilt design: pinwheels, chevrons, zigzags, and sawtooth stars all reduce to arranged HSTs. The quilt math runs: units per row times rows equals total HSTs; each HST needs two squares the finished size plus 7/8 inch (the standard oversize formula). A 60x72 throw in 4-inch finished HSTs needs 15x18 = 270 units, or 540 squares cut at 4-7/8 inches. Layouts to try: alternating light/dark for zigzag, grouped fours for pinwheels, ombre gradients for modern quilts. Photograph the layout before sewing; the camera's small preview exposes value mistakes the eye misses. This calculator sizes the units; graph paper (or a quilt app) arranges them. Sew a test block first: one perfect block predicts the whole quilt.
The Magic 8 Method: Eight at Once
When a quilt needs dozens of identical HSTs, sewing them two at a time is a slow death. The Magic 8 method starts with two large squares and yields eight units. The cut formula: (finished size + 7/8 inch) x 2. For 3-inch finished HSTs, that is (3 + 0.875) x 2 = 7.75-inch squares. In practice most quilters round to the friendlier version, (finished + 1) x 2, which gives 8-inch squares for the same units and a little extra to trim away. The trim step is the point: slightly oversized units squared up after sewing are more accurate than exact-cut units sewn perfectly.
The process: layer the two squares right sides together, draw an X corner to corner on the back of one, sew a quarter inch on both sides of both diagonal lines, then cut on all four drawn lines, vertical, horizontal, and both diagonals. Press, trim each to the unfinished size (finished + 1/2 inch), and you have eight identical units. A quilt needing 64 HSTs is eight rounds of this instead of 32 rounds of the two-at-a-time method. The math scales; your patience does not have to.
Bias Edges: The Stretch You Cannot See
Every HST has two bias edges, the diagonal cuts, and bias stretches. That is why HST units sometimes come out wavy or oversized even when the cutting was exact: the bias edge grew while you sewed or pressed it. The defenses are simple. Starch the fabric before cutting; it temporarily glues the weave in place. Sew with the bias edge on the bottom against the feed dogs when you can. And press, do not iron: lift and set the iron down instead of sliding it, because sliding drags the bias out of shape.
This is also why trimming after sewing beats cutting exactly. An HST cut to the mathematically perfect size and then stretched a sixteenth in pressing is now wrong forever. An HST cut a quarter inch generous and trimmed to size after pressing is right regardless. The extra fabric costs pennies. The accuracy is the whole quilt.
Frequently Asked Questions
What size do I cut for half square triangles?
Add 7/8 inch to the finished size. For a 4-inch finished HST, cut two 4-7/8 inch squares. This is the standard rule for the traditional two-at-a-time method. Enter your finished size above and the calculator gives the exact cut size plus the strip-friendly rounded version.
Why 7/8 inch for HST?
The 7/8 inch covers the seam allowances and the diagonal geometry. When two squares are sewn on the diagonal and cut apart, each triangle's bias edge consumes slightly more than the straight-grain math suggests. Generations of quilters tested this: 7/8 inch yields accurate blocks with minimal trimming, while a full inch wastes fabric.
How many HSTs do I get from one square?
Two from the standard method: sew two squares right sides together on both sides of the diagonal, cut apart, and each square yields two HST units. The 4-at-a-time method uses two larger squares to make four units, and the 8-at-a-time magic method uses two squares to make eight. The calculator covers the standard 2-at-a-time math.
Should I round up the cut size?
Many quilters round the 7/8 up to a full inch for easier rotary cutting, then trim the finished HST to size. A 4-inch finished HST cut at 5 inches gives a slightly oversized unit you square up to 4-1/2 inches unfinished. The calculator shows both the exact 7/8 size and the round-up size so you can choose your method.
What is the unfinished size of an HST?
The finished size plus 1/2 inch for seam allowances: a 4-inch finished HST measures 4-1/2 inches unfinished. "Finished" means in the completed quilt after seams; "unfinished" means the block before it is sewn in. Mix these up and every block comes out half an inch small.
How do I make 4 HSTs at a time?
Cut two squares at the finished size plus 1-3/4 inches, sew all four sides at a quarter inch, then cut on both diagonals. A 4-inch finished HST needs 5-3/4 inch squares for this method. It is faster for multiples of four but wastes slightly more fabric than the 2-at-a-time method.
Why are my HSTs coming out the wrong size?
Three usual suspects: a scant versus full quarter-inch seam, stretching the bias edge while pressing, or cutting the starting squares inaccurately. Test with two squares first and measure the result before cutting a whole quilt's worth. Press seams to the dark side (or open) without dragging the iron, which distorts bias edges.
How many HSTs do I need for a quilt?
Divide the quilt dimensions by the finished HST size and multiply: a 60 x 72 inch quilt of 4-inch HSTs needs 15 x 18 = 270 units. Enter your quilt size and HST size above and the calculator counts the units plus the starting squares required. Buy 10 percent extra fabric for miscuts and test blocks.