About Mathstub
Free tax calculators for US tech workers with equity compensation — RSUs, ISO/AMT, ESPP, NSO, bonuses, quarterly estimates, state lookups. Independent. No signup. Inputs never leave your browser.
Who builds it
Singharash Rashasing
Solo indie maker. Built Mathstub after losing $6,000 to an April-surprise RSU underwithholding penalty — the kind of mistake off-the-shelf tax software does not flag because the math happens months before filing. Every calculator on this site exists because a real-world situation was painful enough that a focused tool was worth shipping.
What I actually do here: read IRS publications + the controlling IRC sections, transcribe them into pure TypeScript math modules, cross-check against worked examples in IRS Pubs and major CPA blogs, and ship the result as a free browser-side calculator. The pure math is unit-tested (571 tests across 26 files as of June 2026) so the same engine can power the Chrome extension and the Anthropic-skill version we’re building next.
What I am not: a CPA, an enrolled agent, or a tax attorney. Mathstub does not give individual tax advice — it gives the math and the IRC citation behind that math. For high-stakes situations ($10,000+ exposure, ISO exercise timing, multi-state moves), every calculator routes you to a CPA matching service. The whole methodology is documented at /methodology.
Find me on Indie Hackers · email me at [email protected].
Case studies — what the math actually catches
Real (composite) tax-prep saves, with the IRC citations.
Three deep narrative case studies showing the kind of finding the calculators surface for typical $200k–$700k tech-worker scenarios:
- Priya’s $11,300 cash + $77,000 tax-shelter Annual Review — $310k MFJ NY senior engineer, 90-minute year-end audit. Withholding shortfall, Mega-Backdoor Roth capacity, ISO state-AMT, RSU cost-basis flag, concentration risk.
- Maya’s $2,574 RSU cost-basis fix — $280k California engineer, 100-share RSU sale. The 60-second Form 8949 column (g) adjustment that prevents broker $0-basis double-taxation.
- Daniel’s $34,000 CA→TX surprise — senior engineer moved CA→TX mid-vest, didn’t realise FTB Pub 1004 still claims a share of every post-move vest. Caught it in October, defeated the §19136 underpayment penalty.
Names are composites; IRC citations, dollar figures, and form mechanics are concrete and verifiable in IRS primary sources.
What Mathstub covers
- RSU withholding — the 22% federal supplemental gap that creates an April surprise.
- ISO & AMT — exercise-year exposure and multi-year credit recovery on Form 8801.
- NSO exercise — bargain element, FICA, and supplemental withholding shortfall.
- ESPP — qualifying vs disqualifying disposition math under §423.
- Bonus tax — same supplemental-wage engine, applied to cash bonuses.
- Quarterly estimates — IRC §6654 safe-harbor calculator to avoid the underpayment penalty.
- State stock-comp — top marginal, supplemental, AMT, LTCG by state.
How the math is sourced
Every calculation cites the IRC section or IRS publication that controls it. Brackets, supplemental rates, FICA wage bases, and AMT exemptions are version-controlled in code with one constant per tax year. Pure math lives in a single tested module so the same engine can later power the Chrome extension and the Anthropic skill we plan to ship next.
We currently mark every blog post and calculator as "Reviewed against IRS primary sources" until traffic justifies the spend on a real CPA reviewer. Until then, the math is unit-tested against worked examples from IRS publications and major CPA-blog articles — see the editorial policy for details.
How Mathstub makes money
Every calculator is free to use. We earn from two sources:
- Display ads — Google AdSense, served only inside content blocks (never above-the-fold or in the calculator).
- Affiliate commissions — disclosed inline at each card. We only recommend products that solve the problem the calculator just identified: TurboTax / TaxAct for filing, Harness Wealth for high-complexity CPA matching, Carta / Empower for tracking. Affiliate relationships never influence the math we display.
Privacy
Every calculator runs entirely in your browser. Inputs are never sent to a server. We use Vercel Web Analytics for traffic aggregate stats (no personally-identifying information). Full details in our privacy policy.
Get in touch
Email [email protected] for: math corrections, calculator requests, partnership questions, or just to say what is broken. Every report gets read.
