Quick Read – High earners can funnel an extra $36,250 annually into a Roth by filling the gap between the $72,000 IRS cap and their deferrals and employer match. – Only 24% of 401(k) plans allow after-tax contributions, and even fewer permit in-service Roth conversions.
Both features are required for the strategy to work. – Roth withdrawals don’t count toward IRMAA MAGI, helping retirees avoid Medicare surcharges of up to $6,936 per person annually triggered above $218,000 joint income. – A 56-year-old software engineer on the r/fatFIRE subreddit recently laid out a familiar problem
She earns $310,000, already maxes her 401(k) deferrals and catch-up, funds a backdoor Roth IRA, and still has cash left over that lands in a taxable brokerage. Her HR portal mentions “after-tax contributions” but she has never used them. She wanted to know if that box was worth checking.
For high earners with a compatible plan, checking that box is the single most valuable retirement move available in 2026. It can push an additional $36,250 into a Roth account every year, on top of what already goes in through payroll. The Math Behind the $36,250 The IRS caps total annual additions to a 401(k) under Section 415(c) at $72,000 in 2026.