Safe harbor on huge windfall

by law if you meet the 110% safe harbor rules there are no underpayment of estimated tax penalties even if you owe a substantial amount, come 4/15/2027 - the latest date for paying the balance due. To be safe I would not wait until then to pay because if there is an issue, then the payment could be late, and you would be subject to late payment penalties. if you have problems filing either your return or extension by 4/15, then you are subject to late filing and payment penalties. 

Thank you for your question.

The 110% safe harbor rule protects you from underpayment penalties.  Your AGI above the IRS level of High income of $150,000 puts your safe harbor target of estimated taxes at 110% of your prior year tax liability (line 24 of Form 1040 for 2025).  You still need to make sure your withholding and estimated payments will cover the amount.  Also if you are making installment you still need to make sure your payments are made evenly to meet the annual requirement.

If you are in a payroll situation you may check your withholding and adjust your W-4.

If you are paying the 110% via quarterly estimated tax vouchers, you are expected to have paid them in four equal, timely installments (April, June, September, and January).
For your remaining liability you will need to be sure you will be able to meet the 4/15 deadline for any additional tax payment.

https://www.irs.gov/payments/underpayment-of-estimated-tax-by-individuals-penalty

Not all states honor the 110% rule, so you will need to check the specific rules of your state(s) to make sure you do not get an underpayment penalty there.

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