How We Check the Math

Every calculator on this site goes through the same checks before it is published, and again whenever it changes.

1. One formula, written down

Each calculator starts as a written formula, which is printed on its page under "The math". If you want to check a result by hand, you can.

2. Checked against published sources

• Quilting: yardage charts published by fabric manufacturers and quilt shops, and widely used tutorials for HSTs and flying geese. We use 42 inches as the usable width of quilting cotton, which matches most published charts, and you can change it.
• Knitting and crochet: the Craft Yarn Council's Standard Yarn Weight System for yarn categories and typical gauges.
• Cross-stitch: fabric counts as sold by major manufacturers, with evenweave and linen stitched over two threads.
• Units: the international yard (exactly 0.9144 m).

Every calculator page links to its sources at the bottom.

3. Rounding the way shops cut

Fabric amounts are rounded up to the next 1/8 yard, the smallest cut most shops sell. Cutting sizes are rounded up to the next 1/8 inch, the smallest mark on a standard rotary ruler. We never round down, because being slightly over is cheap and being short is expensive.

4. Automated tests

All the math lives in one library, separate from the page design, and it is covered by automated tests with known answers taken from published charts (for example, a 60 × 72 inch quilt with 4 inches of backing margin needs 3⅞ yards of 42 inch fabric). The tests run before every update goes live.

5. Where judgment comes in

Some numbers are conventions rather than facts: blanket sizes, how much extra to add for binding joins, the margin around a cross-stitch. We use the most common value, label it as a convention, and let you change it.

Found something that doesn't add up? Email [email protected].