Gauge in stitches per 10 cm and fabric width in cm—stitch count to cast on (rounded).
The page uses stitches = round((width ÷ 10) × gauge) where gauge is stitches per 10 cm and width is a positive length in the same unit system as the pattern (typically cm). Yarn thickness and edge stitches are not modeled.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.