VestingGap is a free RSU tax calculator built out of a simple frustration: the people who need this information most are the last to get it — and by the time they find out, they already owe.
My name is Jonathan. I'm not a CPA, and I'm not in finance. But over the past few years, I've watched several friends land their first serious tech jobs — good companies, real equity, the kind of compensation packages that look incredible on paper.
And then, one by one, I watched the same thing happen to all of them. They'd vest their first batch of RSUs. Their broker would send a statement showing 22% withheld. They'd assume that was the end of it — the same way you assume your employer handles payroll correctly. Then April would come around, and they'd get a bill. Sometimes $8,000. Sometimes $18,000. Once, north of $30,000 for a senior engineer in California who'd had a strong vesting year and hadn't set aside a dollar.
"Nobody at my company ever told me I'd owe extra taxes on top of what the broker withheld. I assumed the sell-to-cover was it. It wasn't."
I heard some version of that sentence from every single person it happened to. Not one of them felt stupid — they felt blindsided. Because the system is genuinely confusing: the 22% supplemental withholding rate is a legal IRS mechanism, not a reflection of what you actually owe. Your broker is following the rules. The rules just aren't designed to protect you from a shortfall.
I looked at the landscape and found a handful of tools — most behind a paywall, most requiring an account, most giving vague advice rather than real numbers for your specific situation. So I built VestingGap. One page. No login. No ads. You put in your shares, your salary, your state — it tells you exactly what your broker withheld, what you actually owe, and what you need to do about it before April.
That's it. That's the whole thing.
VestingGap uses 2026 IRS federal tax brackets (from IRS Rev. Proc. 2025-32), the 2026 Social Security wage base of $184,500, and state marginal rate brackets for all 50 states plus Washington D.C. The federal supplemental withholding rate (22% / 37% above $1M) is sourced from IRS Publication 15.
That said, this is an estimation tool — not a substitute for professional tax advice. There are scenarios the calculator intentionally simplifies:
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