There is no such thing as interest for underpayment of estimated tax. I think you might be conflating two different things. The penalty for underpayment of estimated tax is only that - a penalty for underpaying. If you paid 100% of your prior year tax, then you are all set with regards to that penalty. It is avoided entirely. HOWEVER, if you still have a balance and that balance is not paid by the due date, then you will owe penalties and interest for making a late payment. This is not related to estimated tax, but simply paying the tax owed by the due date.