@baldietax
Based on the taxpayer's statement that the locum job started in July, and they had regular withholding from their regular job all year (per normal procedure), they will only owe estimated taxes on the locum income for the last two quarters.
They should probably file form 2210 (penalty calculation) with schedule AI (annualized income method) with their tax return, even if the software does not automatically suggest it. The annualized income method shows the IRS that even though tax payments were not evenly spread over the whole year, neither was the income (due to the sudden increase in July), and the tax payments did keep pace with the income, which is what the IRS wants to see.
Even if they paid 75% now and the other 25% in January, they could still be assessed a late payment penalty, representing interest due backdated to April 15, June 15 and Sept 15, since that's when the payments were technically due. Schedule AI should negate that. If the IRS does assess a penalty anyway, the taxpayer can ask for a first-time waiver, or a waiver for cause (not understanding the requirements for the locum job).
Going forward into 2026, the taxpayer can either increase their W-2 withholding to cover the extra tax on the locum job, or make quarterly estimated payments.
