Form 4868 is the one to use to gain an extra six months of time. That's better than making a mistake that costs you money or makes you file an amended return. But the form doesn't give you an extension to pay taxes due. The IRS will charge interest on an unpaid balance, plus a late-payment penalty if you don’t pay at least 90 percent of the amount you owe on time. The extension form can be filed using IRS Free File. And if you file for an extension, don't forget your state taxes. If your state levies an income tax, you need to check its policy on extensions and file the appropriate form, with payment.