If you are self-employed or work as an “independent contractor” (who receives a Form 1099 instead of a Form W-2), neither estimated federal income tax nor the FICA taxes are withheld from your income and you could have a tax liability when you file your return. You may be required to make periodic estimated tax payments in order to avoid penalties for underpaying or failing to make estimated payments throughout the year. More information about whether you are required to make estimated payments and how to calculate them can be found at https://www.irs.gov/pub/irs-pdf/f1040es.pdf.