Underpayment Penalty.

In plain English

The underpayment penalty applies when you do not pay enough of your tax as you earn income during the year, through paycheck withholding or quarterly estimated payments. The IRS charges it like interest on the amount you came up short, using the quarterly underpayment interest rate, for the period the money was due but unpaid. You generally avoid it if you paid at least 90% of this year's tax or 100% of last year's tax (110% if your prior-year income was above $150,000), or if you owe less than $1,000. Form 2210 figures out whether you owe it and how much.

01Why it matters

People with freelance income, big bonuses, or investment gains often owe this penalty without realizing it, because no employer is withholding tax for them, and it stacks on top of the tax they already owe.

02The math, step by step

You freelance and owe $8,000 in tax for the year, but you made no estimated payments and had no withholding. Because you paid nothing during the year and owe far more than $1,000, the IRS charges an underpayment penalty figured as interest on the unpaid amount across the four quarters it should have been paid.

03What this is NOT

04Receipts

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