The caveat about getting an extension is that it is an extension TO FILE and NOT an extension TO PAY. When you file your extension, which generally will allow you to file an individual tax return as late as October 15th, you must still pay, by April 15th, the total of your estimated tax liabilities, whether by with-holdings, estimated tax payments, or as a payment with your extension request. It is better to pay a little too much than not enough as you will be charged interest on any amount not paid by April 15th and, depending on your tax situation, may be charged penalties as well.

This is one of the biggest misconceptions in the tax field for a lot of people. All it does is gives you 6 extra months in the event you have not received all of your tax info from say your job or mortgage comp. It is one of the biggest fallacies...IRS can deny the extension, if you don't have a valid reason for it. And yes the amount even if a estimated amount needs to be paid by April 15th...the penalties and interest will start on that date.