IRMAA Part B and IRMAA Part D premiums are based on the data you give to the IRS during your yearly income tax return. This information is then given to the Social Security Administration (SSA) to calculate what your premium adjustment should be.
Your IRMAA is based on your income tax from two years ago. That means that your 2018 IRMAA will be calculated using your 2016 income tax by the SSA.
If you brought in less than $85,000 in 2018, you will not have to pay IRMAA adjustments along with your monthly premium. This is also the case if you and your spouse, filing jointly, brought in less than $170,000 in 2018. According to the Social Security Administration, less than 5% of Medicare members are required to pay IRMAA Part B and Part D premium adjustments.