How does TurboTax calculate any Additional Medicare Tax owed based on total income and withholdings?

[najeed]

What data does it use from the W2 and what does the calculation look like?

[Mike9241]

Turbotax uses form 8959 to compute the tax as per IRS instructions. you should see it in your forms list but probably can't see the actual form until you pay your fees. you are asking from live for which you paid extra

https://ttlc.intuit.com/turbotax-support/en-us/help-article/product-setup/connect-tax-expert-turbota...

5 AM to 5 PM, PT, Monday–Friday

https://www.irs.gov/forms-pubs/about-form-8959