Ask a question (or tell us what's wrong)Can I calculate if it's worth it to amend my tax return for a large charitable donation that I forgot to include? Thanks!

Charitable donations are reported on Schedule A. If the total of itemized deductions on Schedule A is greater than the standard deduction, then TurboTax will automatically use the itemized total.

So your first question is what is the total on Schedule A? Since you are using the Online product instead of the desktop version of the software, you can't go look at Schedule A. All you can do is look at the "Preview my 1040" which can be accessed by going to Tax Tools->Tools->View Tax Summary (on the left) and then clicking on Preview my 1040 on the left.

Look at Line 9 on the 1040. If you are itemizing deductions, then this number will be something other than your standard deduction (see amounts in the left margin of the 1040).

If the number you see if greater than the standard deduction, then every dollar you add to Schedule A will reduce your tax by that amount times your marginal tax rate (see https://turbotax.intuit.com/tax-tips/irs-tax-return/current-federal-tax-rate-schedules/L7Bjs1EAD). So a $1,000 contribution at the 25% tax bracket will yield you $250.

If, however, the amount in line 9 is the same as your standard deduction amount, you will have to grab a paper copy of Schedule A and add up the deductions for yourself to see how close you are to exceeding the standard deduction. If the amount of the new charitable contribution doesn't push you over the standard deduction amount (or not by much), then the amended return may not pay for itself.

[Edited 3/31/2020 1:27 pm CDT - updated for 2019]