Quilt Half Square Triangle Calculator

This calculator works out the starting square size you need to cut for a half square triangle (HST) unit, based on your desired finished size and seam allowance. You enter the finished HST size (the size it will be after sewing into your quilt) and your seam allowance, and it returns the size of the fabric squares to cut, plus the resulting unfinished HST size. It uses the standard formula: starting square = finished size + 7/8 inch (or 1 inch if you plan to trim), with the seam allowance added correctly.

Formula

The standard formula for a half square triangle made from two squares sewn diagonally is:

Cut square size = Finished HST size + Seam allowance × 3.5

Where:

• Finished HST size (in inches or centimeters) is the size of the HST after it is sewn into the quilt, including all seam allowances.
• Seam allowance (in inches or centimeters) is the distance from the stitch line to the fabric edge, typically ¼ inch (0.25 in) in quilting.
• Cut square size is the side length of each of the two fabric squares you start with.

Why 3.5? When you sew two squares together diagonally, the seam allowance is consumed on both sides of the diagonal seam. The math works out to adding 7/8 inch (0.875 in) for a ¼-inch seam allowance, which is 3.5 times the seam allowance. If you plan to trim your HSTs to size (common for accuracy), many quilters add a full 1 inch instead—this calculator uses the precise 7/8-inch addition by default.

Worked Example

Using the defaults from the calculator: finished HST size = 4 inches, seam allowance = 0.25 inches.

1. Multiply the seam allowance by 3.5: 0.25 × 3.5 = 0.875 inches.
2. Add that to the finished size: 4 + 0.875 = 4.875 inches.
3. So you cut two squares that are 4â…� inches (4.875 in) on each side.
4. After sewing and pressing, the unfinished HST will measure 4.5 inches (4 + 0.25 × 2 = 4.5), which includes the seam allowances on two sides.

The calculator rounds to two decimals, showing 4.88 inches for the cut size and 4.50 inches for the unfinished size.

How to Use the Half Square Triangle Calculator

Half square triangles are one of the most common quilt blocks, appearing in countless patterns. To use this calculator:

• Determine the finished size you need—this is the size the HST will be after all seams are sewn, as stated in your quilt pattern.
• Enter your seam allowance—most quilting uses ¼ inch, but if you use a different allowance (e.g., â…œ inch for some mini quilts), adjust it.
• Read the cut square size—this is the side length of the two fabric squares you cut.
• Cut two squares that size, place them right sides together, draw a diagonal line, sew ¼ inch on each side of the line, then cut along the line to get two HSTs.

The calculator also shows the unfinished HST size—that’s the size after sewing but before you trim or sew into the quilt. This is useful if you want to trim your HSTs to a consistent size before assembling.

Choosing the Right Method

There are several ways to make half square triangles, and the calculator works for the most common two-at-a-time method. But you might use other methods:

• Two-at-a-time: The classic method this calculator assumes. You get two HSTs from two squares.
• Four-at-a-time: You start with two larger squares, sew around all four edges, and cut diagonally both ways to get four HSTs. The formula is different: cut size = finished size + 1.5 × seam allowance + a bit extra. This calculator doesn’t apply to that method.
• Strip piecing: For many identical HSTs, you can sew strips together and cut triangles. Again, the starting strip width differs.

If your pattern says “make 4 HSTs from two squares,â€� don’t use this calculator—use a four-at-a-time formula instead. Always check which method your pattern assumes.

Common Mistakes and How to Avoid Them

Even with a calculator, HSTs can go wrong. Here are the most frequent pitfalls:

• Using the unfinished size instead of finished. If you need a 4-inch HST in your quilt, that’s the finished size. The unfinished size (before sewing in) is larger. Enter the finished size, not the one you measure after sewing.
• Forgetting to add seam allowance. The formula already includes it, but if you use a different seam allowance than ¼ inch, you must adjust. The calculator does that for you.
• Not trimming. Even with the correct cut size, your HSTs may come out slightly off due to fabric stretch or sewing inaccuracies. Trimming to the unfinished size (shown in the calculator) ensures your blocks match.
• Mixing units. If you use metric, stick to centimeters for everything. Don’t mix inches and centimeters.
• Rounding too early. If you round 4.875 up to 5, you’ll have extra fabric to trim, which is fine. But if you round down to 4.75, your HSTs will be too small.

Why the 7/8-Inch Addition Works

Many quilters memorize “add 7/8 inchâ€� for a ¼-inch seam allowance, but understanding why helps you adapt. When you sew two squares diagonally, the seam allowance is taken up on both sides of the diagonal seam. The diagonal of the finished HST is the square root of 2 times the finished size. The seam allowance adds 2 × seam allowance to the diagonal, and then you solve for the side length. The math simplifies to adding 3.5 × seam allowance. For ¼ inch, that’s 0.875 inch, which is 7/8 inch.

If you prefer to trim your HSTs (recommended for accuracy), you can add a full 1 inch instead of 7/8 inch. That gives you a little extra fabric to square up. The calculator lets you adjust the seam allowance, but if you want to add 1 inch, you can enter a seam allowance of 1/3.5 ≈ 0.2857, or simply use the formula: cut size = finished + 1 inch. This calculator’s default uses the precise 7/8-inch addition, but you can override by changing the seam allowance.

Limitations and When Not to Use This Calculator

This calculator is for the two-at-a-time method only. It also assumes you are making standard right-angle HSTs, not equilateral triangles or other shapes. If you are making HSTs from a triangle paper or using a specialty ruler, the starting size may differ. Also, if you are using a very thick fabric or multiple layers, you might want to add a bit more for trimming. The formula is mathematically exact for the seam allowance you enter, but fabric behavior can vary.

Frequently Asked Questions

What does “finished� and “unfinished� mean in HSTs?

Finished size is the measurement of the HST after it is sewn into the quilt, with all seam allowances folded under. Unfinished size is the measurement before you sew it into the quilt, including the seam allowances. For example, a 4-inch finished HST will measure 4.5 inches unfinished (with ¼-inch seams). The calculator shows both.

Should I add 7/8 inch or 1 inch?

Add 7/8 inch if you are confident in your sewing accuracy and don’t plan to trim. Add 1 inch if you want extra fabric to trim the HST to a perfect size. Many quilters prefer the 1-inch addition because trimming gives more accurate results. This calculator uses 7/8 by default, but you can adjust the seam allowance to get 1 inch (enter 0.2857 as seam allowance).

Can I use this calculator for metric measurements?

Yes, as long as you use centimeters consistently. The formula is unit-agnostic. For a 10 cm finished HST with a 0.75 cm seam allowance, the cut size would be 10 + 0.75 × 3.5 = 12.625 cm. The calculator works with any unit as long as you enter both in the same unit.

How do I make HSTs without a calculator?

Use the classic rule: finished size + 7/8 inch (for ¼-inch seams). For example, for a 3-inch finished HST, cut two 3�-inch squares. If you want to trim, use finished + 1 inch (4-inch squares for a 3-inch finished HST). This rule is easy to remember and works for any size.

What if my HSTs are coming out too small?

This usually means you entered the unfinished size instead of the finished size, or your seam allowance is wider than ¼ inch. Check your pattern to confirm the finished size. Also, measure your seam allowance—many machines have a ¼-inch foot, but it’s worth testing. If you’re still small, add a bit more to your cut size (e.g., use 1 inch instead of 7/8).

Can I use this for half rectangle triangles?

No, half rectangle triangles (HRTs) use a different formula because the angles are not 45 degrees. This calculator is specifically for half square triangles, where the diagonal cuts the square into two equal right triangles. For HRTs, you’ll need a different method or template.

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in t | Finished HST size | in | 4 | 0.5
in s | Seam allowance | in | 0.25 | 0.125
out cut | Cut square size | in | t + s * 3.5 | 2
out unfin | Unfinished HST size | in | t + s * 2 | 2