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Safe harbor is the IRS rule that switches off the underpayment penalty once you've prepaid enough during the year, even when a large balance is still due in April. Cover 90% of this year's tax, or 100% of last year's (110% if prior-year AGI topped $150k), and the penalty cannot apply.
Safe harbor caps the penalty. It does not size your final bill, and it does not mean you prepaid the exact amount. You can finish the year with a large balance due and still owe zero penalty, as long as your payments cleared the harbor along the way. The rule lives in IRS Publication 505 and Form 2210.
For most filers the prior-year figure is the one to chase, because it is locked in and easy to hit. When income is rising, the prior-year route wins outright, because it locks onto a year that is already closed and cannot grow on you.
Most taxpayers avoid the penalty by clearing the smaller of these two amounts. When income is rising, prior-year wins:
Useful when your income is flat or falling, but it leans on a forecast of a number you do not yet know. Miss the estimate and the harbor moves under you mid-year.
The target is already on last year's return at Form 1040, line 24. Divide it by four and pay without guessing. Dependable — a fixed target that cannot move on you mid-year.
Higher earners pay more to qualify. If prior-year AGI cleared $150,000 (or $75,000 filing separately), the prior-year bar moves from 100% to 110%. Pay exactly 100 when you actually owed 110 and the IRS still hits you with a penalty on the gap.
You qualify for safe harbor by clearing whichever of the two figures is smaller — so we run both, and put you on the cheaper route. A safe-harbor calculator returns one number and never checks the other.
We take line 24 from your prior return and apply 100% or 110% based on your prior-year AGI. That fixes the target — no forecast, no moving number.
We split the target across the four due dates (April 15, June 15, September 15, and January 15 of the following year) and hand you the amount to send each time.
If your income is dropping, we test whether the 90% current-year route lets you pay less. If it does, we move you to it. If it doesn't, we don't — the prior-year route is safer. See late estimated tax payment if you have already fallen behind, or quarterly estimated taxes for the full framework.
A safe-harbor calculator returns one number from figures you type in. It does not read your actual prior return, and it will not tell you the 90% route is cheaper this year. We check both routes against the real numbers and put you on the lower one.
Price tracks what your year involves: a one-time safe-harbor calculation off last year's return at one end, quarterly support across the year at the other. Number of income sources and whether last year's return needs a review sit in between.
Send your prior 1040 and we quote the exact figure. You do not need this year's income figured out to lock in a safe harbor — last year's return already holds the number that protects you.
The 110% rule is the single most common way the safe harbor is missed by accident. Someone reads "100 percent of last year's tax" and stops there, without checking whether AGI cleared $150,000. Pay exactly 100 when you actually owed 110, assume you're safe, and the IRS still hits you with an underpayment penalty on the gap.
Two other traps: safe harbor caps the penalty, not the bill (a large April balance is still due), and the harbor is only cleared if each quarterly installment is paid on time — a lump-sum payment in Q4 does not retroactively cover Q1-Q3.
Sources: IRS Publication 505; Form 2210 instructions; IRC §6654
A good fit if:
New to quarterlies? Start with quarterly estimated taxes. Already fell behind? See late estimated tax payment. Self-employed? Self-employed quarterly taxes covers the SE tax side.
Send us last year's Form 1040 and we will calculate your safe harbor target and the four payment amounts. Reviewed by a CPA. You do not have to figure out this year's income first — the number that protects you is already on last year's return.
"Dimov Tax recommended changes to my tax strategy which had a significant positive impact on my return this year."
— Julie N., Google review
One document (last year's 1040). One number back. Peace of mind for the whole year.
