CentsMath is a growing set of free US finance calculators for the money decisions people actually face - debt payoff, mortgage extra payments, Roth vs Traditional, HYSA vs CD vs Treasury, Coast FIRE, and more. Every tool shows its formula in plain English, cites the primary source (IRS, CFPB, TreasuryDirect), and has its math covered by automated tests. No signup, no paywall, and nothing you type ever leaves your browser.