Sewing Maths Calculators

Sewing involves more geometry and arithmetic than many people expect. Circle skirts, gored skirts and flounces all require calculating radii from circumference measurements. A full circle skirt uses the formula: radius = waist ÷ (2 × π), then adds the skirt length to get the outer cutting radius.

Stitch length is expressed differently depending on your machine and country. UK and European machines typically show stitch length in millimetres (e.g., 2.5mm), while some older patterns reference stitches per inch (SPI). At 2.5mm stitch length, you get approximately 10 stitches per inch. Thread consumption depends on stitch length, seam length, and thread tension — a lockstitch uses approximately 2.5× the seam length in total thread (top and bobbin combined).

Pattern piece area calculations help with efficient fabric layout. Knowing the total area of all pattern pieces versus the available fabric area tells you how tightly you need to lay out pieces and whether you have enough fabric. Gored skirts divide the skirt into panels (gores) that flare from waist to hem, with each panel width calculated from the waist and hem circumferences divided by the number of gores.