What's the free finance calculator you wish existed but can't find anywhere?

I've spent the last few weeks building Tally — free personal finance calculators (budget split, debt payoff, emergency fund, and a few more) with the actual math shown instead of hidden behind an email wall or a paywall.

While building it I kept running into the same pattern: sites that market themselves as "free calculators" usually aren't — you get a teaser number, then a signup form, or the formula is a black box.

Curious what's missing for you. If you could get one calculator built for free, with the formula shown, what would it be? Genuinely looking for ideas for what to build next — launching on Product Hunt July 22 and would love this community's take before then.