I built KnownSum after noticing how many people were miscalculating the new 2026 federal overtime tax deduction. The actual rule is that only the premium half of overtime pay qualifies — so at $30/hr regular rate paid at time-and-a-half ($45/hr), only the extra $15/hr is deductible, not the whole $45. Nearly every explanation I found online got this wrong or glossed over it, and I couldn't find a calculator that modeled it correctly. That first calculator led to two more: a mortgage recast calculator (recasting lowers your payment without changing your rate or term — most people confuse it with refinancing or extra principal payments) and a Texas vehicle tax/title/license calculator, since that's another area where the actual math is more nuanced than people expect (Texas taxes private-party sales on whichever is higher: the price you paid or 80% of the vehicle's presumptive value). Everything runs client-side — no signup, no data leaves your browser. Built solo with Astro and Cloudflare Pages, and I'm adding calculators weekly. I'd love feedback on what to build next, and I'm happy to answer questions about the tax mechanics or the build itself.

KnownSum

Free US tax and money calculators

Free US tax and money calculators