How We Verify Our Calculators
A sewing calculator is only as useful as the allowances and conversion rates behind it. Every figure on this site is checked against a real source: manufacturer thread and zip data, the sizing and seam-allowance charts published by the major pattern companies, and long-established curtain-making and dressmaking conventions. This page explains how we check them and what happens when something is wrong.
The process
- ✓Every rate and allowance is traced to a named source category: thread consumption figures to manufacturer guides such as the Coats Thread Consumption Guide, zip lengths to maker specifications such as YKK, embroidery thread data to brands like DMC, Anchor and Madeira, and sizing to the published charts of Vogue, Butterick, McCall’s, Simplicity and Burda.
- ✓Unit conversions (metres to yards, gsm to oz/yd², fabric width equivalents) are checked as exact arithmetic, not rounded rules of thumb.
- ✓Every calculator is tested against hand-worked examples: we work the sum on paper from the source figures and compare it to what the calculator returns.
- ✓Calculators are then adversarially re-checked by re-deriving each result from scratch and comparing it back, which is how inverted formulas and misread conventions get caught.
- ✓Errors reported through the feedback buttons on each page are investigated, fixed and re-verified against the source before the fix goes live.
- ✓When a source changes, for example a pattern company revising its sizing chart or a manufacturer updating a datasheet, the affected pages are reviewed and updated.
Verified, not guaranteed
These calculators give estimates to plan from, not guarantees. Fabric behaviour varies by weave, finish and dye lot, so always test a swatch, check the bolt width and repeat before cutting, and confirm quantities with your fabric shop before buying. For made-to-measure curtains, upholstery or garment alterations you are unsure about, ask a professional curtain maker, upholsterer or tailor.
Spotted an error?
Every calculator page has feedback buttons. If a figure looks wrong, tell us what you entered and what you expected: we investigate against the original source, fix anything that fails the check, and re-verify before the correction goes live. When a report shows a genuine error, the fix lands on the affected page itself, with a dated correction note where it matters.