Half Square Triangle Calculator
The classic HST question: how big should the starting squares be, and how many do I need? Here's the math.
Tip: use URL parameters, e.g. ?finishedSize=4&hstCount=40.
How to calculate HST cutting sizes
A half square triangle is simply a square cut in half diagonally. The two most common construction methods are:
Method 1 — two at a time. Place two squares right sides together, draw a diagonal line, sew ¼" on each side of the line, then cut along the line. You get two HSTs. The starting square is the finished size plus 7/8".
Method 2 — four at a time. Draw both diagonals on one square, sew ¼" from each line, cut along both diagonals. You get four HSTs. The starting square is the finished size plus about 1".
The formula. For two-at-a-time, cut squares at finished size + 7/8" (round up to the nearest eighth for ease). Each pair of squares makes two HSTs, so divide your total count by two to find how many square pairs to cut.
Why the extra allowance? The ¼" seam around the diagonal consumes fabric, and the seam allowance on the two straight sides needs trimming. Many quilters cut a full 1" larger and trim down for accuracy.
What each input means
- Finished HST size (inches) — the size of the triangle unit after it's sewn into the quilt. If your pattern calls for 4" finished HSTs, that's the number you enter — not the 4.5" cut square.
- Number of HSTs needed — the total number of triangle units in your layout. For a 5 × 6 grid of HST blocks, that's 30.
The calculator uses the two-at-a-time method: each pair of starting squares produces two HSTs. If you prefer the four-at-a-time method, keep in mind the starting square formula differs (about 1" larger than finished), and you'll still want to trim each unit to the exact finished size plus seam allowance.
When to use it. Use this calculator whenever your pattern calls for a specific finished HST size and you want the two-at-a-time cutting numbers — the most common method in beginner and intermediate quilts. It also helps when you're designing your own layout: pick a finished size, see how many units your grid needs, and get the cutting size in a single step instead of doing the math per block.
Common mistakes to avoid
- Entering the cut size instead of the finished size. The 7/8" allowance is added on top of the finished size; doubling it throws off every unit.
- Forgetting that a pair of squares makes two HSTs. If you need an odd count, one square pair will produce one spare unit — that's normal, not an error.
- Cutting exactly at finished + 7/8" without trimming. Seam pressure and thread thickness make exact seams unlikely. Cutting 1" larger and trimming to size is the most reliable approach.
- Relying on four-at-a-time for precision blocks. The bias edges of the outer triangles stretch easily; press gently and trim every unit.
- Not squaring up units. Even a 1/16" variance per unit multiplies across a quilt, so trim each HST to its exact finished size plus seam allowance.
Frequently asked questions
What size squares do I need for 4-inch finished HSTs?
Cut 4-7/8" squares (or 5" and trim) for the two-at-a-time method. Each pair makes two 4" finished HSTs.
How many HSTs do I need for a quilt?
Multiply the HSTs across by the HSTs down in your layout. A 5 × 6 grid of 4" HSTs needs 30 HSTs.
Two-at-a-time or four-at-a-time — which is better?
Two-at-a-time is easier to get accurate. Four-at-a-time is faster but the outer edges have bias stretch, so trim carefully.
Why are my HSTs coming out too small?
Your seam allowance is likely wider than ¼", or you're cutting the starting squares too small. Cut slightly larger and trim to exact size.