Is non-taxble Social Security included in MAGI?

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-your-taxes/L7kHckNS3

     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.

Yes- This is confusing.  It looks like the two agencies have different definitions for MAGI for different purposes.

Even TurboTax  lists "non-taxable Social Security" to be included in MAGI 

https://turbotax.intuit.com/tax-tips/irs-tax-return/what-is-the-difference-between-agi-and-magi-on-your-taxes/L7kHckNS3

For Social Security IRMAA purposes, I believe this can be misleading- (just my opinion).

I believe for SS IRMAA  purposes, my 15% non-taxed amount is NOT to be added back in (See SSA doc reference below). 

But I am not going to risk it - I will add that 15% back in as a safety margin when calculating how much Roth conversions I can do.

Of course standard disclaimer applies- rely on a tax professional.

FYI- Copied  from the SS Handbook at their site: 

https://www.ssa.gov/OP_Home/handbook/handbook.25/handbook-2501.html

What is Modified Adjusted Gross Income (MAGI)?

I guess as more of us age into this situation this question continues to be relevant. 

The most useful "official" reference I could find -- one that clearly shows the different MAGI calculations for different uses -- is the Congressional Research Service's Dec. 2018 publication The Use of Modified Adjusted Gross Income (MAGI) in Federal Health Programs.  It has a very handy Table 1 that lists 5 different federal uses of MAGI and what goes into each calculation.

With respect to the original question: when determining which bracket you fall into when determining IRMAA adjustments to premiums for Medicare Part B and Part D, untaxed social security income is NOT included in MAGI .