The United States income tax system is a pay-as-you-go tax system, which means that you must pay income tax as you earn or receive your income during the year. You can do this either through withholding or by making estimated tax payments. If you didn't pay enough tax throughout the year, either through withholding or by making estimated tax payments, you may have to pay a penalty for underpayment of estimated tax. Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and refundable credits, or if they paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller. There are special rules for farmers and fishermen, certain household employers and certain higher income taxpayers. For more information, refer to Form 1040-ES. Generally, taxpayers should make estimated tax payments in four equal amounts to avoid a penalty. However, if you receive income unevenly during the year, you may be able to vary the amounts of the payments to avoid or lower the penalty by using the annualized installment method. Use Form 2210, Underpayment of Estimated Tax by Individuals, Estates, and Trusts to see if you owe a penalty for underpaying your estimated tax. The law allows the IRS to waive the penalty if: You didn't make a required payment because of a casualty event, disaster, or other unusual circumstance and it would be inequitable to impose the penalty, or You retired (after reaching age 62) or became disabled during the tax year or in the preceding tax year for which you should have made estimated payments, and the underpayment was due to reasonable cause and not willful neglect. For more information, refer to the Instructions for Form 2210.