How many stitches to cast on when changing needle sizes?

I have an infant hat pattern which calls for size 15 needles and a cast on of 36 stitches. I want to use size 13 needles but do not know how many stitches to cast on. I am using chunky yarn and want to make sure the diameter of the hat is approx. 13-14 inches. I know I have to cast on more than 36 stitches but I am very confused on how to calculate this??

I don’t think you can calculate. You just have to accept that you may be ripping out, guess at a number and go until you’ve got enough substance to get a good measurement.

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The only way to calculate something like that is to make a swatch and figure out the number of stitches per inch. You measure them over 4" and divide for best results.

Thank you–your answer was very helpful and something I will use now and in the future.

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What’s your pattern?