Other users indicate Amended Return Calc has same error for their 1040-R's: When I report my missing 1040-I amount (~$1,000) in the 1040-X return, the amount of my income is increased by double my entry (~$2,000)! Of course, the calculations then double the amount of the tax that I owe for the addition! I've tested this numerous times without saving. Your amended return program clearly has a bug! Call or email me if you can't replicate it, and I will send you a pdf of the printed copy of the errored calculation. My amended return is simple. Users with large amendments could have large tax mistakes/costs that could be costly to them, and you.
The simplest way to amend is by hand.
You must have a copy of the original tax return you want to update (or a transcript).
You must be able to do arithmetic.
Prepare your correct tax return (using TurboTax for example, or other means*) and print it.
Then lay the paper printouts of both returns (correct and original) side-by-side.
You now have all the information needed to prepare Form 1040-X by hand.
It's pretty straight-forward.
The blank form in fillable PDF format and its instructions are available at www.irs.gov.
Refer to the Instructions for what is to be attached to Form 1040-X.
That is - only forms that are new or changed due to your amendment/
You must find these by visual inspection, TurboTax won't break them out for you even when it is working.
Do not include your old 1040 nor your revised 1040 because the Form 1040-X reflects any changes there and becomes your new tax return.
Mail to the address shown in the Instructions.
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* There are other tax prep sites on the internet that will do your return again (federal) for no charge.