If you file your tax returns using form 1040, you are expected to make quarterly estimated payments to ensure that your tax liability at the end of the year should not be excessive. These quarterly payments include estimates of both income tax and self-employment tax. It is important to estimate reasonably accurately. If you underestimate, your payment at the end of the year will be high, and you might also pay an underpayment penalty. If you overestimate, you will receive a large refund at the end of the year.