The Reddit r/Bogleheads and r/financialindependence forums are full of posts from 58-year-old married couples sitting on roughly $2.2 million in a traditional 401(k), still working, and wondering whether to start Roth conversions now or wait until retirement. The answer hinges on a deadline most people miss: the window to convert without triggering a Medicare IRMAA surcharge closes at the end of the year they turn 62.
Why the Conversion Window Closes at Age 62
Medicare premiums use a two-year lookback on modified adjusted gross income. The 2033 premium for someone turning 65 that year is set off the 2031 tax return. Any Roth conversion done in 2031 or later lands on a Medicare premium bill. Conversions done in tax years 2026 through 2030, while the reader is age 58 through 62, never touch a Medicare premium because the lookback years fall before enrollment. That is the five-year window, and it is the cleanest tax planning gift the system gives a high-balance saver in their late 50s.
The cost of missing it is concrete and measurable. A single IRMAA tier bump on a married couple adds roughly $70 to $100 per person per month. The top tier runs more than $400 per person per month. Over a 20-year retirement, even a middle tier surcharge compounds into a five-figure tax on the conversion that nobody puts on the spreadsheet.
The Bracket Math That Drives the $315,000
For a married couple filing jointly in 2026, the 24% bracket runs up to $211,400, and the 32% bracket starts at $403,550. The standard deduction is $32,200. A couple with $180,000 in wages has roughly $255,000 of headroom inside the 24% bracket before the next jump.
Fill that headroom with conversions for five straight years and roughly $1.25 million moves from traditional to Roth at a 24% marginal cost. Skip the window and the same dollars come out as required minimum distributions starting at age 73, stacked on top of Social Security and a portfolio that has compounded for 15 more years. A $2.2 million balance growing at 7% becomes north of $6 million, and the first RMD lands around $230,000. Combined income at that point sits in the 32% to 35% bracket with the full IRMAA stack on top. The 8 to 10 percentage point spread on roughly $1.25 million of conversions is where the $315,000 lifetime tax differential comes from.
The Catch-Up Rule That Changed in January
Anyone earning more than $150,000 in 2025 must direct 401(k) catch-up contributions into the Roth side of the plan starting in 2026. The standard cap is $24,500 with an additional $8,000 catch-up for the 50-and-over crowd, for a total of $32,500. For a 58-year-old running a conversion ladder, this is a feature: the $8,000 quietly builds a parallel Roth bucket every year without any conversion tax. At age 60 the catch-up jumps to $11,250, lifting the total to $35,750 through age 63.
Where Rates Fit In
The Fed funds upper bound sits at 3.75% after 75 basis points of cuts over the past year, and the 10-year Treasury yields almost 4.5%. Holding the conversion tax bill in a Treasury or short-duration bond fund earns enough to offset roughly a full year of inflation drag while the cash waits for the April filing deadline. The conversion tax must come from outside the 401(k); paying it from the converted balance defeats most of the strategy.
What to Do in the Next 90 Days
- Pull last year’s tax return and calculate exactly how many dollars of room sit between your taxable income and the top of the 24% bracket at $211,400. That number is your 2026 conversion target. Do the conversion before December 31, not in April.
- Open a Roth IRA today with a $1 contribution if you do not already have one that is at least five tax years old. The five-year clock starts the year the account is funded, and a converted balance rolled into a brand new Roth restarts that clock under IRS rules.
- If your wages plus a full-bracket conversion would push combined income above the first IRMAA threshold in any year from 2031 onward, hire a fee-only advisor. The premium math alone justifies the engagement.
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