I am filing "single". Last year I was separated and my ex filed us as "joint". He makes way more. How do I know what amount to put as AGI as it's a combined amount?

[xmasbaby0]

Were you divorced by the end of 2017?  If so you can file Single for 2017.  As for the 2016 joint return: Your 2016 tax return shows your 2016 Adjusted Gross Income  (AGI)  If you filed a joint return then the AGI is the same for each of you.
Form 1040EZ    line 4
Form 1040A       line 21
Form 1040       line 37

[AJ]

I believe that you might need to add additional information for this question because I don't think this one is has enough information for a definitive answer.  I will try to give some information.  First, with regard to AGI: it is difficult to understand what AGI you are attempting to get.  If it is for this year, and if you are filing without your ex, then your AGI will simply be the result of what you enter for your own income items, which are purely related to your own individual income documents.  The other person's income will not matter on your return.  (Deductions can be a significant issue if you are itemizing and if you are still married.)  But the amount that your ex makes does not appear to be relevant here if you are divorced.  I believe I might need to state some information relative to your filing status.  You filed as Married Filing Jointly (MFJ) for 2016.  MFJ is only correct if you were still married on 12/31/2016.  And if you are filing as "single" for 2017, then this is only correct if you obtained a divorce prior to 12/31/2017.  If you are still married, and if I assume that you have no dependents, then your only choices would be Married Filing Jointly (MFJ) or Married Filing Separately (MFS).  You should file Single if still married. And if not married, then only your own income counts toward your AGI.

[AJ]

Typo.  You should NOT file Single if still married!

[AJ]

I am back. I now understand the issue.  Too bad I cannot delete the first answer.  I assume you need the AGI to efile.  When this occurs, it comes to a few choices.  If you are on good terms with your EX then get a copy of last year's return.  You can get a transcript from the IRS, but that will likely take more time than you want.  So the best choice without being able to efile is to simply print your return and mail it via regular US Mail.