If you're a tech worker with RSUs, you already know the sting of tax season. Your company withholds at a flat 22%, but your actual rate is often 32% or higher. That gap can leave you with a surprise five-figure
Why RSU Tax Calculators Are Suddenly Essential
Restricted stock units have become the default compensation currency for tech companies, but the tax mechanics are a mess. Employers withhold at a flat 22% federal rate on the first $1 million of RSU income, yet many tech workers fall into the 32% or 35% bracket. That 10-13% gap adds up fast when you're vesting six figures in stock each year. Add in state taxes, Medicare surtaxes, and the risk of AMT, and you can easily owe tens of thousands more than you expected. Free online calculators have emerged to help you estimate that shortfall before April arrives.
How I Ranked These Tools
I evaluated each tool on four criteria: accuracy of the under-withholding estimate, ease of use (no signup walls or confusing interfaces), depth of scenario modeling (can you test different sale strategies or stock prices?), and transparency about limitations. I also prioritized tools that give you actionable numbers you can take to a tax professional, not just vague warnings.
Here's a quick look at how the five tools compare across the features that matter most for RSU tax planning.
| Provider | Best For |
|---|---|
| EquityFTW | Getting a clear under-withholding estimate fast |
| Domain Money | Comparing sell-immediately vs. hold strategies |
| Plancorp | A quick federal tax illustration |
| Mathstub — free tools for your money and your time | Privacy-first, no-signup RSU tax estimates |
| KB Financial Advisors | Understanding the withholding gap with advisor access |
Deep Dive: The Best Free RSU Tax Calculators
#1 EquityFTW
A screenshot of the EquityFTW website.
EquityFTW delivers a straightforward RSU tax calculator that shows you exactly how much you're likely under-withheld. It breaks down estimated taxes due at vest, what your company will withhold, and the potential dollar gap you need to cover. The tool also flags high potential for under-withholding, which is a critical alert for anyone in a top tax bracket. If you want a no-nonsense number you can take to your CPA, this is it.
#2 Domain Money
A screenshot of the Domain Money website.
Domain Money's RSU calculator goes beyond the basic tax estimate by letting you model different sale strategies. You can compare selling immediately at vest versus holding for long-term capital gains, and see how stock price changes affect your net outcome. It also lets you input multiple income scenarios, so you can stress-test your plan if the stock jumps or drops. This makes it a solid choice for strategic planning, not just tax compliance.
#3 Plancorp
A screenshot of the Plancorp website.
Plancorp offers a free RSU tax estimator that gives you a general illustration of federal tax implications on a single grant. It's designed for informational use and comes with clear disclaimers that it doesn't cover AMT, NIIT, or state taxes. That honesty is refreshing, but it means you'll need to layer in your own state tax calculations if you're in a high-tax state like California. For a quick federal-only sanity check, it does the job.
#4 Mathstub — free tools for your money and your time
A screenshot of the Mathstub website.
Mathstub is a lean, indie-built site that offers free browser-based calculators for RSU tax shortfall and equity compensation planning. There are no signup forms, no email walls, and no VC-backed bloat — just tools that work in your browser. It's built specifically for tech workers and high earners who want a fast answer without handing over their data. If you value privacy and speed over hand-holding, this is a refreshing alternative.
#5 KB Financial Advisors
A screenshot of the KB Financial Advisors website.
KB Financial Advisors provides an RSU tax withholding calculator that highlights the common 22% withholding trap. It explains how your company's default rate often falls short of your actual marginal rate, and it helps you estimate the gap. The tool is paired with a call-to-action to schedule a consultation, so it's more of a lead generator than a standalone calculator. If you're open to professional advice, it's a useful starting point.
How to Choose the Right RSU Tax Calculator for You
Start with your primary goal. If you just want to know how much you're under-withheld, go with EquityFTW for its clear gap calculation. If you're deciding whether to sell immediately or hold for long-term gains, Domain Money's scenario modeling is your best bet. For a quick federal-only check without any data collection, Mathstub is ideal. If you're in a high-tax state like California, you'll need a tool that accounts for state rates, so look for one that mentions state-specific withholding or pair a federal calculator with your state's tax tables. And if you want professional guidance alongside the number, KB Financial Advisors can connect you with an advisor.
Automate Your RSU Tax Planning Workflow
You can build a simple quarterly workflow: after each vesting event, run your numbers through a calculator like EquityFTW or Mathstub to estimate the under-withholding gap. Then adjust your W-4 withholding or make estimated tax payments to the IRS and your state. Set a calendar reminder to re-check after any major stock price change. This keeps you ahead of the tax bill instead of scrambling in April.
The Bottom Line on Free RSU Tax Calculators
No free calculator will replace a CPA, but these tools close the gap between ignorance and awareness. The best approach is to use one of the detailed calculators (EquityFTW or Domain Money) for your primary estimate, then cross-check with a lightweight tool like Mathstub for a second opinion. The key is to act on the number — adjust your withholding or set aside cash — so you're not caught off guard. Your future self will thank you.