Estimated tax safe harbor rule explained
Learn the 90%, 100%, and 110% thresholds used to reduce underpayment penalty risk.
Current-year 90% rule
One route to avoiding an underpayment penalty is paying at least 90% of the current year's total tax through withholding and estimated payments. This can work well when income is predictable.
If income changes sharply during the year, revisit the estimate before each deadline rather than relying on the first quarterly number.
Prior-year 100% or 110% rule
The prior-year safe harbor compares this year's prepayments to last year's total tax. Most taxpayers use 100% of prior-year tax; higher-income taxpayers generally use 110%.
The calculator asks for prior-year tax and whether prior-year AGI exceeded $150,000 so it can show the lower of the current-year 90% target and the prior-year target.
When safe harbor still leaves a balance due
Safe harbor is about penalty risk, not the final amount owed. You can satisfy safe harbor and still owe additional tax when filing if the current year turns out much stronger than last year.
The result wording deliberately says what to pay to avoid penalties and keeps the full annual estimate visible next to it.
The guide explains the rule. The calculator shows how it changes your next quarterly payment.
Open the calculator