Finished Size Calculator
Most amigurumi patterns give you a stitch count, not a finished size. Enter your yarn weight, hook, actual gauge, and the stitch count at the widest round, and find out roughly how big it'll actually turn out before you're three rounds from the end.
Materials
Your actual gauge
Measure this from a swatch or your work in progress — it's what actually determines the finished size, not the yarn label.
From your pattern
The inverse of building a pattern from scratch
Our Amigurumi Round Counter starts from a size you want and works out the stitch counts to get there. This tool runs the same relationship backwards: it takes the widest-round stitch count already fixed by whatever pattern you're following, divides by your actual stitch gauge to get the circumference at that round, and converts that to a diameter — the same circumference-to-diameter math, just solving for the size instead of the stitch count. If you add straight rounds at the widest point (common for an egg-shaped body rather than a perfect sphere), each one adds roughly one row-height of extra length beyond the sphere's diameter.
Why the same pattern turns out different sizes for different people
Two crocheters following the identical pattern, with the identical yarn weight and hook size listed, routinely finish at different sizes, because gauge is personal tension as much as it is materials — that's exactly why this calculator asks for your actual measured gauge rather than only the yarn weight and hook. The yarn weight and hook fields above are there to flag when your gauge is running unusually tight or loose for that combination, not to replace the measurement; if your hook is well outside the typical range this calculator notes for your chosen weight, expect a noticeably more open or dense fabric than the pattern photo shows, on top of whatever size difference the stitch count math gives you.
Using this alongside a stuffing estimate
Once you have an estimated diameter here, it's the same number our Stuffing Estimator asks for to estimate fiberfill weight — running both in sequence turns "I wonder how big this will be" into an actual materials plan before you've committed a single skein.