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Mathstub
Free tax calculators for tech workers with equity comp
RSU, ESPP, ISO/AMT, quarterly estimated tax, and a 50-state stock-comp lookup. Free, no signup, math straight from IRS publications. The calculators run client-side; no data leaves your browser.
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.
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About Mathstub on Product Hunt
“Free tax calculators for tech workers with equity comp”
Mathstub was submitted on Product Hunt and earned 0 upvotes and 1 comments, placing #43 on the daily leaderboard. RSU, ESPP, ISO/AMT, quarterly estimated tax, and a 50-state stock-comp lookup. Free, no signup, math straight from IRS publications. The calculators run client-side; no data leaves your browser.
Mathstub was featured in Productivity (652.3k followers), Finance (5.9k followers) and Personal Finance (2.8k followers) on Product Hunt. Together, these topics include over 142.5k products, making this a competitive space to launch in.
Who hunted Mathstub?
Mathstub was hunted by Authentae. A “hunter” on Product Hunt is the community member who submits a product to the platform — uploading the images, the link, and tagging the makers behind it. Hunters typically write the first comment explaining why a product is worth attention, and their followers are notified the moment they post. Around 79% of featured launches on Product Hunt are self-hunted by their makers, but a well-known hunter still acts as a signal of quality to the rest of the community. See the full all-time top hunters leaderboard to discover who is shaping the Product Hunt ecosystem.
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