Building 40+ free calculators with no signup — what would you add?

Launching BreezyCalc on Product Hunt in a few days. It's a collection of 41 free calculators for money, health, and everyday math (mortgage, take-home pay, BMI, calorie needs, GPA, and more), no account required, no paywall.

The idea started from a simple annoyance: most calculator sites either bury the tool under ads and popups, or just spit out a number with zero explanation of how they got there. Every calculator on BreezyCalc links to a short guide explaining the actual formula behind the result, whether that's the 28/36 rule for mortgages or the Widmark formula for blood alcohol.

Everything runs instantly in the browser. Nothing gets stored, no account needed.

Curious what this community thinks: what calculator do you find yourself needing that most sites either don't have or get wrong? Happy to build it.