September 15 is close. What Q3 estimates actually are.

Federal estimated tax is due four times a year: April 15, June 15, September 15, and January 15 of the following year. Florida has no individual income tax, so the voucher is federal (and, if you have it, another state’s). Missing September does not generate a separate notice. It generates an underpayment penalty that shows up on next year’s return, quietly.

Safe harbor, in one sentence

Pay in 100% of last year’s total tax (110% if last year’s AGI was over $150,000) and the IRS cannot penalize you for underpayment, even if this year’s income is much higher. That is the safe harbor. It is not always the cheapest path, but it is the one that lets you sleep.

When safe harbor is the wrong number

If income fell — a slow season, a sale that closed last year and not this one — paying last year’s tax is a loan to the Treasury. We project the current year and pay 90% of this year’s tax instead. The projection is the work. The voucher is just the output.

S-corps and the wage piece

Owner wages already had federal withholding. Estimated payments cover the rest: the K-1 ordinary income, the 3.8% NIIT if it applies, and any under-withheld W-2. If the wage is set correctly, the voucher is smaller. If the wage is a leftover from 2019, the voucher is doing work the payroll should have done.

We send Q3 vouchers to planning clients in the first week of September. If you are not on that list and want to be, the Tax Center will give you a first-pass number in about four minutes.