IRS 網站這麽說的：

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. 

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.