I have not used this, just came across it, but it looks helpful. This is for those times when you can’t get the gauge needed for a sweater pattern (maybe yarn was subbed or we just like the fabric in a tighter or more open gauge or want to use a different weight yarn) and then the question of which size to make, or if it’s even possible.
I usually do the calculations myself but this calculator apparently works out which size to make in the pattern.