Depends what you need it for. There are different ways to calculate the MAGI.
MAGI is calculated differently for different purposes. There are about a dozen different definitions of MAGI, depending on what it's being used for. What do you need your MAGI for? Are you looking for MAGI for 2021 or for an earlier year?
I’m trying to calculate how much I can put in a Roth account (guesstimate for the coming year.) I’ll have maxed out what I can contribute in 2022 and I’m over 55.
I assume that the "Roth account" you are talking about is a Roth IRA, because the limit on contributions to a Roth 401(k) does not depend on your income.
Use Worksheet 2-1 on page 39 of IRS Publication 590-A to calculate your MAGI for Roth IRA contributions. See "How Much Can Be Contributed?" starting on page 38 of Publication 590-A.
I need to calculate MAGI because Social Security wants to increase my Medicare Pt. B premium. For reference I retired at the end of last year and turned 65 in APR2022. Thank you in advance.
MAGI for determining your Medicare premiums is your AGI plus tax-exempt interest. The calculation of your Medicare premiums uses your MAGI from two years earlier. Your 2023 premiums will be based on your 2021 MAGI. On your 2021 tax return, the MAGI for calculating Medicare premiums is Form 1040 line 11 plus line 2a.
Your age and when you retired have nothing to do with your MAGI.