A ? about sizing

[Orangey]

How do I figure out how many stitches I need based on someones chest size?
I just finished a tank top for my daughter and a friend asked me to make one for her daughter, but I used the biggest size and Im not sure how many stitches I need to increase the patter by to fit my friends daughter.
I hope that made sense...

[Ingrid]

You measure how many stitches per inch you got with your yarn and needles and then multiply that by the number of inches you need to knit.