A smaller hook pulls less yarn through each stitch, producing tighter, smaller stitches. With the same yarn, going down one hook size (e.g. from 4.0 mm to 3.5 mm) typically increases your stitch count per 10 cm by 2–4 stitches, making the finished piece smaller.

Yes — especially if you are changing yarn weight or hook size. A 10×10 cm swatch takes 10 minutes and can save hours of re-work. Crochet it in the round (tube) since flat and in-the-round tension differ for most makers.

Yarn covers stitches, which tile the surface of your piece. If you double the finished size (SF = 2), each dimension doubles, so total surface area = 2 × 2 = 4× larger, requiring 4× as much yarn. That is why SF² is the correct multiplier.

Yes. Two makers using identical yarn and hook can produce different gauges. If your tension is looser than average for a given weight, your gauge will be lower and your piece larger. Always measure your swatch and enter the measured gauge as a custom value for greatest accuracy.

Absolutely. Enter the pattern’s original size and a lower gauge (thicker yarn), and the calculator shows you the predicted size and how much more yarn and stuffing you will need. Keep in mind that very large amigurumi may need wire or foam armatures to hold their shape.

If the original pattern uses 30 g of polyfill, and your stuffing multiplier is 2.7×, you will need approximately 81 g. This prevents both under-stuffing (floppy toy) and over-buying stuffing you will not use.