When you file your annual return, you'll have to pay an additional tax charge (ATC). The ATC is equivalent to interest on the estimated tax that you didn't pay by the due date. Calculate this charge as follows:
ATC = tax x interest x # days late / 365
Example: You owed $1,000 in estimated tax on June 15 but did not pay until June 30. If the interest rate is 3 percent, the ATC on your return is $1.23, as calculated below:
ATC = $1,000 x 0.03 x 15 / 365 = $1.23