What is my Modified Adjusted Gross Income for 2018?

There are about a dozen different definitions of Modified Adjusted Gross Income (MAGI), depending on what it's being used for. Why are you looking for your Modified Adjusted Gross Income?

Or did you mean Adjusted Gross Income (AGI). Your 2018 AGI is on Form 1040 line 7 of your 2018 tax return.

evidently the MAGI, which doesn't actually show on the 1040, is used by Medicare to determine and calculate IRMAA. If you can tell me what it was for 2018 it would help me understand the calculation. I just want visibility.

The Social Security Administration (SSA) says that "Your MAGI is your total adjusted gross income and tax-exempt interest income." 

Please read Medicare Premiums on page 2 for more background. I assume that the SSA has only one MAGI definition (and not many, like the IRS).