It happens to the best of us. You gather all your tax documents, crunch the numbers, and double check everything on the tax return. No sooner have you dropped the return in the mail or pressed the efile button, but a new W2 or 1099 comes in the mail. Ah, now you remember. And now you need to revise your tax return.

Revising your tax return is done by filing an amendment. You need to use Form 1040X, along with a new Form 1040, plus any other necessary schedules and forms. Amended tax returns have to be mailed to the IRS on paper for manual processing. The IRS typically processes an amended return about 8 to 12 weeks, but this process can take longer during the IRS's busiest times.

Why File an Amendment to Your Tax Return?

You should file an amended tax return if you need to correct any piece of information that will alter your tax calculations. You can use an amended return to make corrections to your filing status, dependents, income, deductions, or tax credits. For example, if you need to report additional income from a W-2 that arrived after you filed your original return, you'll need to file an amendment. Similarly, if you need to remove dependents because you were not eligible to claim them, you should file an amendment.

You should not file an amended return if you are only correcting math errors as the IRS computers will check your math and correct any errors in calculation. Math errors are errors in adding or subtracting items on your tax return.

Be aware that you have three years to make any corrections that result in additional tax refunds. That's because there's a three-year statute of limitations on issuing tax refund checks. This three-year period is measured from the date you filed your original tax return. If you filed your return before April 15th, the three-year period begins from April 15th. If you requested an extension, the three-year period runs from October 15th (Internal Revenue Code section 6511(b)(2)(A)).

If you are beyond the three-year period, you can only receive refunds for overpaid taxes that were actually paid during the previous two years.

People who need to report additional income and to correct overstated deductions can file an amended return at any time. Be aware that the IRS has three-years to audit your tax return, and may have a longer period of time if there's substantial under-reporting.

Reasons for filing an amendment include:

Claiming additional dependents,

Removing dependents you previously claimed,

Reporting your proper filing status,

Reporting additional income – from a W2, 1099, or other income statement,

Making changes in your above-the-line deductions, standard deduction or itemized deductions,

Changing your personal exemptions,

Claiming additional tax credits, removing tax credits mistakenly taken, or recalculating the amount of the credits;

and Reporting additional withholding – from a W2 or 1099.