One of my favourite knitting tools is a calculator. My students are often surprised that I keep a calculator in my knitting bag... and they're even more surprised, I think, how often I use it.
It seems obvious to me, but I've been asked about it often enough I figure it's worth explaining why.
I use it for calculating yarn requirements when making a substitution.
I use it to calculate where the increases go when I'm told to "increase 6 stitches evenly across row".
I use it for making any adjustments to a pattern.
And I use it to simply check my arithmetic. (Ok, so I started with 54 stitches and I increased 2 stitches 12 times which means I should have how many?... )
And in my pattern drafting class, I also explain why I love graph paper so much.
And my Dad thinks I don't use my Mathematics degree...