help with crochet math

[churrokkii]

hello! im trying to make garments that fit the body. is there a formula to use that i could use for shaping. for example the gauge is 3 sts x 3 rows in 1 In. the waist measurement is 36" or 108 stitches and the hip measurement is 48" or 144 stitches. what math or formula can i use to get from 108 sts to 144 stitches?

[Granny Square]

I think there is a missing piece of information that is important - what is the distance from waist to hip?  That is not a 'constant' for every size or body type.  I'm short, I just measured 'me' and that distance is about 4".  A tall willowy fashion model's measurement is undoubtedly a lot greater.  If you are making a custom fit outfit that is skin fitting, you need more measurements than the usual horizontal bust, waist, hip.  People are long waisted, short waisted (I'm short waisted and that makes a big difference in fit for some clothing styles), and there's probably a lot of variation between waist to hip or crotch  distance as well.

1 stitch at your gauge is 0.33" in height and width.  For 'me' as an example. 3 stitches per inch, times 4 inches, is 12 rows.  So to get from 108 sts at the waist to 144 stitches at the hip, the difference between the numbers is 36 stitches, in 12 rows (I swear I wasn't making up my measurement to make the math easier, honest).  So mathematically,108 divided by 3 is 36, so for the first round I'd increase in every 36th stitch in the first round, and increase in each increase until you reach the 12th row.

BUT that is just math, and my measurement which may not be helpful.  There is a human body shape to consider in general, (and the individual you are trying to fit specifically) which probably flares out at a greater or different angle than those numbers above would allow, meaning you might start with the number above but increase the number of increases the closer you get to the hip.

Designing something that is a 'small, medium or large' and not tight fitting is hard enough.  Custom fitting something skin tight is exponentially more difficult.  Something else to consider, I assume you are adding buttons, zippers, snaps or ties as crochet fabric is unlikely to stretch enough to put on otherwise (take it from someone who learned to sew in the 1960's, all of these undoubtedly zipped up the back; these are fabric and not crochet, but still.  You'd be more likely to not need a closure for a skin-tight hand knit item in small gauge yarn, crochet fabric isn't as stretchy.).