Half-Square Triangle Calculator
Enter your desired finished HST size and how many you need, then choose your preferred construction method. The calculator returns the cut size for each fabric square, the unfinished trimmed size, how many pairs of squares to cut, and an estimate of the fabric yardage required per colour. Switch between inches and centimetres at any time.
What is a half-square triangle (HST)?
A half-square triangle is a square quilt block made from two right triangles joined along their longest edge (the hypotenuse). When sewn together, the two triangles form a square with a diagonal seam running corner to corner. HSTs are one of the most versatile units in patchwork: rotated and combined they make pinwheels, flying geese, star points, and dozens of other blocks. They come in three main sizes that quilters track: the finished size (the dimension once the block is sewn into a quilt), the unfinished or trimmed size (finished + 1/2" for the two 1/4" seam allowances on either side), and the cut size (the size of the starting square before any sewing, which varies by method).
The three construction methods and when to use each
The 2-at-a-time method draws one diagonal pencil line on a lighter fabric square, stacks it right-sides-together with a contrasting square, sews a 1/4" seam on each side of the line, then cuts on the line. Each pair produces exactly 2 HSTs. The straight grain of the fabric sits on the outside edges of the finished block, which makes the blocks easier to handle. Cut size = finished + 7/8". The 4-at-a-time method starts with two same-size squares. Sew all the way around the perimeter with a 1/4" seam, then cut corner-to-corner in both directions (an X). Each pair yields 4 HSTs. The bias grain lands on the outside edge, so press carefully. Cut size = finished + 1 1/4". The 8-at-a-time method is the fastest for large quantities. Draw two diagonal lines on one square, layer with a second square right-sides-together, sew 1/4" on each side of both lines, then cut on all the drawn lines and cut the square in half vertically and horizontally. Eight HSTs come from one pair of squares. Cut size = (finished x 2) + 1". All methods call for trimming each sewn block to the exact unfinished size before joining.
Calculating yardage for your project
Once you know the cut square size and how many pairs you need, estimating yardage is straightforward. Divide the usable fabric width (usually 40"-42" for quilting cotton after removing selvedges) by the cut size to find how many squares fit across one strip. Divide the total squares per colour by that number and round up to find the number of strips. Multiply by the cut size to get the total fabric length in inches, then divide by 36 and round up to the nearest 1/8 yard. This calculator does all of that automatically when you enter your fabric width. Always add at least 10-15% extra to your estimate for squaring up, cutting errors, and waste at the end of each strip.
Tips for accurate half-square triangles
Accuracy in HSTs starts at the cutting table. Use a rotary cutter and a reliable quilting ruler to cut your squares as precisely as possible - even a 1/16" error multiplies across dozens of blocks. Mark the diagonal with a sharp pencil or a Hera marker rather than a pen, so the line stays thin. Sew a scant 1/4" seam (just a thread or two inside the true quarter inch) to compensate for the fabric folding over when pressed. Press seams to one side or open depending on your block design, but never drag the iron across bias edges. Finally, square up every block using a dedicated square ruler with a 45-degree angle line aligned to the diagonal seam before joining blocks.
Common finished sizes - cut square at a glance
| Finished size | Unfinished size | 2-at-a-time cut | 4-at-a-time cut | 8-at-a-time cut |
|---|---|---|---|---|
| 1" | 1 1/2" | 1 7/8" | 2 1/4" | 3" |
| 1 1/2" | 2" | 2 3/8" | 2 3/4" | 4" |
| 2" | 2 1/2" | 2 7/8" | 3 1/4" | 5" |
| 2 1/2" | 3" | 3 3/8" | 3 3/4" | 6" |
| 3" | 3 1/2" | 3 7/8" | 4 1/4" | 7" |
| 3 1/2" | 4" | 4 3/8" | 4 3/4" | 8" |
| 4" | 4 1/2" | 4 7/8" | 5 1/4" | 9" |
| 4 1/2" | 5" | 5 3/8" | 5 3/4" | 10" |
| 5" | 5 1/2" | 5 7/8" | 6 1/4" | 11" |
| 6" | 6 1/2" | 6 7/8" | 7 1/4" | 13" |
Cut sizes for the three standard HST methods. All measurements in inches. Unfinished size = finished + 1/2".
Frequently asked questions
Why add 7/8" to the finished size for the 2-at-a-time method?
The 7/8" (0.875") accounts for the two 1/4" seam allowances plus a small margin for trimming the block square after sewing. When you sew 1/4" on each side of the diagonal line, then cut and press, each triangle loses exactly 1/4" to the seam, so the two triangles together lose 1/2". The extra 3/8" gives a little room to square up the finished block without ending up undersized.
What is the difference between finished and unfinished size?
The finished size is how large the block measures once it is sewn into the quilt top, with 1/4" seam allowances on all four sides consumed by the surrounding seams. The unfinished size is the block after trimming but before it is joined to anything - it equals finished + 1/2", because 1/4" will be taken by a seam on each of the two sewn sides.
Which method is most efficient for large quilts?
The 8-at-a-time method produces the most HSTs per cut - eight blocks from just one pair of squares. It is the best choice when you need dozens of identical blocks and want to minimise cutting time. The trade-off is that the outside edges of all eight HSTs sit on the bias of the fabric, which can stretch during sewing and handling. Use starch or sizing spray before cutting to stabilise the fabric. For smaller quantities or when you prefer straight-grain edges, the 2-at-a-time method is more forgiving.
How do I convert these measurements to centimetres?
Switch the unit selector at the top of the calculator to Centimetres. All cut sizes, unfinished sizes, and yardage estimates update automatically. If you are converting by hand, 1 inch = 2.54 centimetres. The same formula structure applies: for the 2-at-a-time method in centimetres, add 2.22 cm (7/8 x 2.54) to the finished size in cm.
Why do I get a few extra HSTs?
Because you can only cut whole pairs of squares, the calculator rounds up to the next complete pair. For example, if you need 10 HSTs using the 4-at-a-time method (which yields 4 per pair), you need 3 pairs (12 HSTs). The 2 extra are a bonus - keep them as insurance against blocks that do not trim up perfectly or that you set aside for a future project.
What is a fat quarter and how does it affect yardage?
A fat quarter is a piece of fabric measuring approximately 18" x 22" (half a yard cut in half lengthwise rather than across the width). Enter 18 in the fabric width field if you are working from fat quarters, and the calculator will base the strip calculation on that 18" width. For regular yardage on the bolt, use 40"-42" to reflect the usable width after removing selvedges.
Do I need to add any extra fabric for squaring up?
Yes. The cut sizes in this calculator already include a small trimming margin, but you should still add 10-15% to your total yardage estimate to account for waste at the end of strips, any blocks that need to be re-cut, and the squaring up process itself. For high-contrast or directional fabrics where matching matters, add a little more.