Mathstub

What it does

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.

Small, fast utility calculators for things that actually matter. RSU tax shortfalls, equity comp planning, and more — free and ad-supported.

Your employer withheld 22% . Your real rate is probably closer to 37% . Find the gap before April finds you. Every claim references an IRC § or IRS Pub#. Read the math, not the marketing. All math executes client-side. Your inputs never leave your machine. Every calc is cross-checked against worked examples from IRS publications. 2024, 2025, 2026 brackets / limits / phaseouts all baked in. Updated every November. No VC, no enterprise sales motion. Just Earth, mathstub.com, on his own taxes. The 22% supplemental flat rate is a floor , not your bracket. Most engineers find a five-figure surprise in their first vest. Selling company stock at a loss? An RSU vest within 30 days — before or after…from mathstub.com

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