Half Square Triangle Calculator

The Half Square Triangle Calculator is a practical and time-saving tool designed to take the guesswork out of measuring and cutting square or rectangular materials. Whether you're cutting tiles, trimming plywood, quilting fabric, framing artwork, or working on a carpentry project, this calculator ensures precision from edge to edge and corner to corner.

The tool is especially helpful for DIYers, woodworkers, flooring installers, quilters, interior designers, architects, construction workers, and even digital designers. It solves the everyday problem of needing to calculate dimensions quickly and accurately—without having to manually apply geometry formulas like the Pythagorean theorem.

Let’s say you want to cut a square piece of glass that fits perfectly into a diagonal space measuring 56 cm across. Just plug that value into the diagonal field, and the calculator gives you the corresponding side length (approximately 39.6 cm in this case). Or perhaps you have a raw square board that’s 30 cm by 30 cm and want to trim it to a smaller square with a specific finished size or diagonal—this tool handles that too.

Real-world uses include:

• Laying tiles in diagonal or straight layouts
• Cutting accurate patches for sewing projects
• Framing photos or prints
• Preparing square panels for doors, shelving, or custom displays
• Cropping digital images to exact square or diagonal measurements

The calculator works for both square and rectangular cuts. It's perfect for minimizing waste, improving fit, and avoiding costly recuts or reorders.

To help you cut perfect half-square triangles (HSTs), this calculator starts with a single input: the finished size of the square unit you want in your quilt.

1. Once you enter that value in centimeters, the calculator automatically determines:
2. The raw square size you need to cut — this includes extra room for seam allowances and trimming.

The diagonal measurement — this tells you the exact distance across the square for marking and cutting along the bias.

What the Calculator Does:

• It adds 2.2 cm to your finished size to account for:

Two seam allowances (¼ inch or ~0.6 cm each)

A little extra for trimming after sewing and pressing

• Then, it multiplies the raw square size by 1.4142 (the square root of 2) to calculate the diagonal line — the one you’ll sew across or cut along.

Practical Application:

• Use the raw square size to cut two identical fabric squares.
• Mark and sew 0.6 cm on either side of the diagonal.
• Cut along the diagonal to produce two neatly matched HSTs.
• Press, trim, and your block is ready!

This calculator removes the guesswork and helps you work faster and more accurately, whether you’re piecing a lap quilt in Abuja or preparing for a market sale in Lagos.

If you encounter any issues or have suggestions for improvements, please report them using the form below.