Hi PH. Solo dev, this is my first launch. Built Mathstub because the same question keeps coming up in tech-worker circles: my RSU/ESPP/ISO vested, now what do I owe in April? The math is public domain — IRS Pub 505, §3402(g), Form 6251 — but the existing tools either lock answers behind TurboTax's funnel or assume you already speak CPA. Five calculators on the site: - RSU tax shortfall — the gap between 22% supplemental withholding and your real marginal rate (32–37% for most tech workers). - ESPP qualifying disposition — splits §423(c) sale into ordinary income and LTCG, with NIIT. - ISO exercise AMT — Form 6251 walk for exercise-and-hold vs same-year-sell, with AMT credit carryforward. - Quarterly estimated tax safe-harbor — §6654 90%/100%/110% rules with per-quarter cumulative schedule. - State stock-comp lookup — top marginal + supplemental withholding + state-AMT status for all 50 states. Tech: Next.js 15 + TypeScript. Tax math is pure functions (no React/DOM imports), 99.76% test coverage on the calc layer because the math has to be right. Things I'm NOT trying to be: tax software (no return-prep), tax advice (every page has a disclaimer + IRS Pub citations), or a SaaS (no signup, calculators run client-side, no data leaves the browser). Monetization: ads + affiliate links. Three Notion templates on Gumroad ($19/$29/$39) for the deeper "track-your-equity-comp-all-year" use case. The calculators stay free forever. Happy to answer questions about tax math, monetizing free tools, or the build pipeline. First launch — feedback wanted.

Mathstub

Free tax calculators for tech workers with equity comp

Free tax calculators for tech workers with equity comp