There is no single uniform value for MAGI. The "Modification" rules for the MAGI depend on the specific credit or other purpose. Since you are receiving Social Security, I will make the assumption that your question might refer to the MAGI for the Income-Related Monthly Adjustment Amount (IRMAA), which affects the surcharge for Medicare B premiums. That MAGI is calculated by adding the AGI and the tax-free interest income. (See Line 8b of the old Form 1040, or Line 2a of the new draft 1040.) If you have a different purpose for a MAGI, then please state the purpose where the MAGI needs to be calculated in order to determine the correct MAGI rule.
Here is a link for some of the common MAGI rules other than for the IRMAA: https://turbotax.intuit.com/tax-tips/irs-tax-return/what-is-the-difference-between-agi-and-magi-on-y...
Some Medicaid programs and other health-related programs can have additional rules, and some of those programs might add untaxed Social Security. It all depends on the program. MAGI is a confusing topic.
