I partially agree with the two previous answers. You owe 2.9% Medicare tax plus income tax at your highest tax bracket for your Schedule C locum work. You should also get a 20% QBI (Qualified Business Income) deduction unless you're in certain professional fields, such as medicine. I wouldn't worry about that for now.
@VolvoGirl is right in that rather than making estimated tax payments, you should first try to increase your withholding for the anticipated additional taxes. The reason is that, as a general rule, IRS considers withholding to be paid evenly throughout the year.
We pay estimated tax in order to avoid IRS underpayment penalties. The best way to do so, IMO, is to increase your tax withholding so that your total 2025 withhold will equal or exceed 110% of your 2024 total taxes (Line 24 of Form 1040). If that's not possible, then pay the difference in the form of estimated tax, preferably using Direct Pay at www.irs.gov. Most states follow the federal rules on underpayment penalties. But check to confirm that.
Underpayment of estimated tax by individuals penalty | Internal Revenue Service
Avoid a penalty
You may avoid the Underpayment of Estimated Tax by Individuals Penalty if:
- Your filed tax return shows you owe less than $1,000 or
- You paid at least 90% of the tax shown on the return for the taxable year or 100% of the tax shown on the return for the prior year, whichever amount is less. If your adjusted gross income (AGI) for 2023 was more than $150,000 ($75,000 if your filing status for 2024 is married filing separately), substitute 110% for 100%.
