IRMAA stands for Income-Related Monthly Adjustment Amount, or more commonly called Means Testing. In short, it is an additional tax on wealth. In Medicare, the Modified Adjusted Gross Income on your taxes from 2 years previous is used to determine the amount of your Part B premium and the amount additional you may have to pay for your Part D.
This table shows the threshold levels for 2018 for means testing. People whose MAGI falls under $85,000 from 2016, will pay the base Part B premium for 2018. Those between $85K and $107K will pay the same percentage of increase as in previous years.
New tiers are of $107,000 – $133,500 and $133,500-$160,000. The upper tier begins at $160,000 (down from $214K in 2017). The actual amounts of the increased charges for both Parts B & D, will not be available until October or November 2017.