A half square triangle (HST) calculator is an essential tool for quilters and fabric crafters using Finished size + 7/8 inch = Size of squares to cut formula.
It helps determine the size of fabric squares needed to create half square triangles, which are fundamental building blocks in many quilt designs.
The hst calculator takes the guesswork out of fabric cutting by providing precise measurements for creating HSTs of various sizes.
This ensures accuracy in your quilting projects and reduces fabric waste.
Sample conversions:
- To create a 4-inch finished HST:
- Input: 4 inches (finished size)
- Output: Cut two 4 7/8-inch squares
- To create a 2.5-inch finished HST:
- Input: 2.5 inches (finished size)
- Output: Cut two 3 3/8-inch squares
Half Square Triangle Calculator
| Desired Finished Size | Calculation | Cut Square Size | Number of HSTs |
|---|---|---|---|
| 3 inches | 3 + 7/8 = 3 7/8 | 3 7/8 inches | 2 |
| 4.5 inches | 4.5 + 7/8 = 5 3/8 | 5 3/8 inches | 2 |
| 2.25 inches | 2.25 + 7/8 = 3 1/8 | 3 1/8 inches | 2 |
| 6 inches | 6 + 7/8 = 6 7/8 | 6 7/8 inches | 2 |
| 3.75 inches | 3.75 + 7/8 = 4 5/8 | 4 5/8 inches | 2 |
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Half Square Triangle Formula
The formula for calculating the size of squares needed to make HSTs is:
Finished size + 7/8 inch = Size of squares to cut
This formula adds 7/8 inch to the desired finished size of your HST.
The extra 7/8 inch accounts for the 1/4 inch seam allowance on each side and the fabric lost in the diagonal cut.
Examples:
- For a 6-inch finished HST: 6 + 7/8 = 6 7/8 inches Cut two 6 7/8-inch squares
- For a 3.5-inch finished HST: 3.5 + 7/8 = 4 3/8 inches Cut two 4 3/8-inch squares
How do you calculate half square triangles?
Calculating half square triangles involves several steps:
- Determine the finished size of your desired HST.
- Add 7/8 inch to the finished size.
- Cut two squares of fabric to the calculated size.
- Place squares right sides together.
- Draw a diagonal line from corner to corner on the wrong side of one square.
- Sew 1/4 inch away from both sides of the drawn line.
- Cut along the drawn line to create two HSTs.
- Press open and trim to the desired size if necessary.
This method ensures accurate HSTs with proper seam allowances.
What size squares make two half square triangles?
The size of squares needed depends on the desired finished size of your HSTs. Here’s a quick reference:
- For 2-inch finished HSTs, cut 2 7/8-inch squares
- For 3-inch finished HSTs, cut 3 7/8-inch squares
- For 4-inch finished HSTs, cut 4 7/8-inch squares
- For 5-inch finished HSTs, cut 5 7/8-inch squares
Remember, these measurements create two HSTs from two squares. Always cut two squares of the calculated size to create a pair of HSTs.
What size half square triangles do charm packs make?
Charm packs typically contain 5-inch square fabric pieces. To calculate the size of HSTs that can be made from charm packs:
5 inches – 7/8 inch = 4 1/8 inches
Charm packs can create HSTs with a finished size of 4 1/8 inches. Many quilters prefer to trim these down to 4 inches for easier math and more consistent results in quilt patterns.
What is a triangle half of a square?
A triangle that is half of a square is literally one half of a square that has been cut diagonally from corner to corner. In quilting terms, this refers to a Half Square Triangle (HST).
Key characteristics of an HST:
- It has one 90-degree angle and two 45-degree angles.
- The longest side (hypotenuse) forms the diagonal of the original square.
- When two HSTs are sewn together along their longest sides, they reform a square.
Half square triangles 4 at a time chart
| Finished HST Size | Cut Square Size |
|---|---|
| 2 inches | 5 1/4 inches |
| 2.5 inches | 6 1/4 inches |
| 3 inches | 7 1/4 inches |
| 3.5 inches | 8 1/4 inches |
| 4 inches | 9 1/4 inches |
To use this method:
- Cut two squares to the size indicated.
- Place right sides together.
- Draw two diagonal lines corner to corner, forming an X.
- Sew 1/4 inch away from both sides of both lines.
- Cut horizontally, vertically, and on both drawn lines.
- Press open to reveal four HSTs.
Half square triangle size chart
| Finished HST Size | Cut Square Size |
|---|---|
| 1.5 inches | 2 3/8 inches |
| 2 inches | 2 7/8 inches |
| 2.5 inches | 3 3/8 inches |
| 3 inches | 3 7/8 inches |
| 3.5 inches | 4 3/8 inches |
| 4 inches | 4 7/8 inches |
| 4.5 inches | 5 3/8 inches |
| 5 inches | 5 7/8 inches |
