If you made a mathematical error on your return, often times the IRS will automatically fix the error. After correcting the mistake, the IRS will send you a notice that shows the changes made to your return (usually a CP11 or CP12). Before correcting your return for a mathematical error, you may want to call the IRS at 1-800-829-1040 to see if they have fixed it for you.
If you forgot to include a form or schedule to your return, it is very likely that the IRS will send you a notice requesting just the missing form(s) and/or schedule(s) which means you will not need to do anything other than send in the requested items.
If the IRS has not corrected your return, you should file a Form 1040X, Amended U.S. Individual Income Tax Return. Remember that if you file an amended Federal tax return, you may also need to file an amended State tax return.
If your amended tax return generates a refund, generally you need to file the amended return within three years of the date the return was due including extensions or within two years of the date the tax was paid, whichever is later. So if you filed an extension for your 2009 personal income tax return, then filed that return on 10/15/2010, and paid it on 11/01/2010, you would have until either 10/15/2013 or 12/01/2012 to receive a refund from your amended return. Obviously the 10/15/2013 date is later, so that would be your deadline for filing an amended return for a refund. After that date, your refund is lost.
If your amended return results in tax due, the sooner you file the return the better. Interest and penalties are calculated on the new tax due beginning on 4/15 of the tax year you are amending. For example, if you find an error on your 2009 tax return which results in additional tax due and file an amended return on 12/01/2010, the penalty and interest for late payment of the tax will be calculated from 4/15/2010, not 12/01/2010.
In order to prepare and file an amended return, you need to have first filed an original return. You cannot file more than one original tax return per tax year unless it is before the first tax filing deadline of the tax year. For instance, if you file your 2010 personal income tax return on 2/15/2011 and realize that you made a mistake, you can file another original return on or before 4/15/2011. The last original return filed on or before 4/15 will be your official tax return.
Once you have filed an amended tax return, be patient. It often takes the IRS between 8 to 12 weeks to process the return. It may even take longer if you file around the busiest times for the IRS, mainly 3/15, 4/15, 9/15, or 10/15 of the tax year since these are the deadlines for individual, corporate, partnership and other tax returns.
For more information go to irs.gov.