New knitter help please

So i finished my first hat today by following along to a youtube video and it was just for a basic beanie with a peal/ knit band at the bottom. Anyways i casted 45 stitches and it fits me perfectly (I’m a 17 year old). Since i loved it so much i wanted to make one for my sister who is 7 and i looked up how to figure out how to calculate the number of cast ons and it says I should do 80 for her head. but i know that will be way to big for her tiny head what am i missing.

I’m using 9mm circular needles,Wool Ease Thick and Quick super bulky yarn, and I knit about 5 stitches an inch.

For the circumference of her head I’m going with the average number because the hat is a surprise so like 19-21inches

What am I messing up?

You were probably looking at instructions intended for thinner yarn. With super bulky yarn and needles a size smaller I get about 3 st/in so I think that you probably aren’t getting 5 sts/in in stockinette with this yarn, but that’s OK, you don’t have to have super accurate gauge numbers. You could probably use about 3 or 4 stitches fewer and you’ll get a hat she can use. When you’ve knit the bottom band you’ll have a good idea of the size of your hat and can tell if it’s right or not. This is a lovely thing for you to do for your little sister, it will be a great surprise for her. We love pictures so if you’d like to share a photo of your hat, please do. We’d love to see the little sister hat too. Be sure to let us know if what I wrote isn’t helpful so someone else can make sure you get helped. And all your questions are welcome!