You will be charged interest at 7% per year, compounded monthly, on income tax not paid by the due date (April 15, 2015 for calendar-year filers). An extension allows only additional time to file; it does not allow additional time for payment of tax due or prevent accrual of interest.
In addition to interest, a penalty is assessed for late filing. A separate penalty is assessed for the late payment of tax. The penalty for late filing is $25 or 10% of the tax due, whichever is greater. If a tax return is not filed upon demand, the penalty for late filing is the greater of $25 or 25% of the tax due. The penalty for late payment of the tax is 1% per month up to a maximum of 25%. Both penalties are assessed when the return is filed late and the tax is paid late. The law also provides for penalties for underpaying estimated tax, preparing or filing a fraudulent income tax return, and for understating income.