About Mathstub

Mathstub ships small, focused calculators for the financial situations that catch people off guard — starting with the RSU withholding gap that leaves tech workers with a surprise tax bill every April.

What we cover

• Equity comp tax math — RSU withholding shortfall, ESPP qualifying disposition, ISO/AMT exposure (planned).
• Job-change money decisions — vesting cliffs, FSA carryover, COBRA timing (planned).
• Anything else that takes a tax pro 15 minutes to calculate but should take a calculator 15 seconds.

Who builds it

This site is run by a small team that has personally been blindsided by a four-figure RSU tax bill more than once. Tools are reviewed for accuracy by a CPA before launch and updated annually as IRS thresholds change. See our editorial policy for details.

How we make money

Every tool is free to use. We make money two ways: display ads (Google AdSense) and affiliate commissions when readers sign up for products we’ve vetted (TurboTax, TaxAct, Carta, Empower, Harness Wealth). We only recommend products that solve the problem the calculator identifies. Affiliate relationships never influence the math.

Get in touch

Spotted a calculation mistake or want to suggest a tool? Open an issue on our public repo (linked from the footer once we move it).