TurboTax will not calculate the late payment penalty/interest nor should you. Pay the amount your tax return says that you owe. The IRS will calculate any additional amount based on the amount you owe and when they receive your payment, and then they will bill you separately for that amount.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
