Find your safe harbor.
The IRS underpayment penalty is entirely avoidable if you know your number. This free calculator finds your cheapest safe harbor and exactly what's left to pay in 2026.
Form 1040, line 24. The whole year's tax, not the refund or balance due.
Your best estimate of this year's total federal tax. Last year's number is a fine starting point if income is similar; add the tax on anything new (RSU vests, a business, a sale).
Full-year W-2 withholding across all jobs (check a recent pay stub's year-to-date and project it forward), plus any other federal withholding.
Any Form 1040-ES payments you've already sent for tax year 2026.
Your cheapest penalty shield is the prior-year safe harbor, $41,800 for the year, and you're $5,800 short. The quarterly deadlines for 2026 have passed, but extra W-4 withholding before December 31 still counts as paid evenly across the year and can erase the penalty.
Federal estimate under IRC §6654 for tax year 2026: no underpayment penalty if withholding and timely estimates reach 90% of this year's tax or 100% of last year's (110% when prior-year AGI exceeded $150,000, or $75,000 married filing separately), and none when the balance due is under $1,000. Withholding counts as paid evenly through the year; estimated payments count only for their quarter, so catching up late with estimates can leave penalty on earlier quarters (Form 2210 computes it, at 7% annually as of mid-2026). Ignores the annualized income method and state rules. A planning estimate, not tax advice. Runs entirely in your browser; nothing is stored or sent.
A target, not a guess.
Estimated taxes feel like guesswork because people aim at their unknowable final bill. The safe harbor turns it into a fixed target you can hit on purpose. RSU vests, side income, and a second W-2 job all funnel into the same three steps.
The two targets
The law gives you two ways out of the penalty: 90% of this year's tax, or 100% of last year's (110% for AGI over $150,000). The calculator computes both and picks the cheaper one.
What's already covered
Withholding across all your jobs counts first, and it counts as paid evenly through the year. Estimated payments you've already sent count for their quarters.
What's left, and when
The gap between the target and your coverage, split across the remaining 2026 deadlines, or covered with extra W-4 withholding, which can even cure quarters you already missed.
Asked every quarter.
What exactly is the estimated tax safe harbor?
The IRS charges an underpayment penalty when you pay too little tax during the year, even if you pay in full by April. IRC §6654 provides escape hatches: no penalty if your withholding and timely estimates reach 90% of the current year's tax, or 100% of last year's total tax, which rises to 110% if your prior-year AGI was over $150,000. Meeting the cheaper of those targets is called reaching the safe harbor.
Why aim for the safe harbor instead of my exact tax bill?
Because the safe harbor is knowable today and your exact 2026 tax is not. The prior-year harbor is a fixed number from a return you already filed. Hit it and the penalty disappears no matter how much you end up owing in April. Owing a large balance inside the safe harbor is not a penalty, just a payment you kept in your own account all year.
Is extra paycheck withholding better than quarterly estimates?
Usually, yes. Withholding is treated as paid evenly across the year even if it all happens in December, so raising your W-4 line 4c amount late in the year can retroactively cure an underpayment from an earlier quarter. An estimated payment only counts for the quarter you make it. If you missed a deadline, withholding is the only lever that reaches backward.
What happens if I miss the safe harbor?
The penalty is interest on each quarter's shortfall at the federal underpayment rate, 7% annually as of mid-2026, computed on Form 2210. On a small gap for a few months it is modest; on a $20,000 gap for a full year it is real money. There is also no penalty at all if your balance due is under $1,000.
Does this calculator store my information?
No. It runs entirely in your browser. Nothing you type is sent to a server, stored, or shared.
Pay the IRS on schedule, not with interest.
Multiple jobs, equity comp, or a business make the safe harbor a moving target. Thirty minutes sets the number and the mechanism, W-4 or 1040-ES, for the rest of the year.