Small tools for big money moments.

Mathstub ships 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.

Tools

📈 RSU Tax Withholding Shortfall Calculator
Estimate the gap between what your employer withholds at RSU vest (22% or 37%) and what you actually owe at your marginal rate.
🧾 ESPP Qualifying Disposition Tax Calculator
Split a §423 ESPP sale into ordinary income vs. long-term capital gain, apply federal LTCG + NIIT + state tax, and compare against disqualifying treatment.
⚡ ISO Exercise AMT Calculator
Estimate the Alternative Minimum Tax on an ISO exercise, the cash required, and the AMT credit you’ll carry forward — plus a same-year-sale comparison.
🗓️ Quarterly Estimated Tax Safe-Harbor Calculator
Compute your IRS §6654 safe-harbor target, see the per-quarter cumulative payment schedule, and get the exact dollar amount to send before the next due date.
🗺️ State Stock-Comp Tax Lookup
Top marginal rate, supplemental withholding, AMT status, and LTCG treatment for RSU/ESPP/ISO income across all 50 states + DC.