Building a free calculator site, which calculator do you wish existed?

Hey everyone,

I'm launching BSM Sites tomorrow, a growing collection of free calculators for everyday life (finance, health, math, dates, and a few fun ones like dog/cat age and a World Cup league table). The whole idea came from being frustrated with calculator sites that bury a simple tool under pop-ups and sign-up walls. So everything here is free, instant, and needs no account.

Before I go live, I'd genuinely love input from this community:

• Which calculators do you actually reach for most?

• Is there one you've always wished existed but could never find a good version of?

I'm adding new ones regularly, so I'll happily build the best suggestions. Thanks in advance and I'll be launching here tomorrow if you'd like to follow along!!!