Mitten goof

Hi everyone,
My fav thing to knit is mitts and i can do it with my eyes closed so I thought id do a pair of mittens for my first fair isle (stranding) project. I did a pair with two stitches dark brown and two stitches light brown all the way to the top and it turned out really really cute. However, they are WAY to tight for any human to wear, I obviously forgot to account for the double thick material. My question is this, is there some sort of formula for converting a normal pattern to a stranded pattern? Like how many stitches should i have added to account for the extra layer?

I suppose the answer would be to do a swatch and figure out what you need mathematically. Bummer huh? Maybe someone knows a short cut, but I don’t. :slight_smile: