What Happens When You Inherit a $500,000 401(k) and the 10-Year Tax Rule Most Beneficiaries Don’t Know About

Inheriting a $500,000 traditional 401(k) triggers mandatory distributions under the SECURE Act‘s 10-year rule, and the tax consequences can accumulate across the full distribution window.

The SECURE Act eliminated the stretch inherited 401(k) for most non-spouse beneficiaries, replacing it with a hard deadline: the entire balance must be distributed within 10 years of the original owner’s death. What many beneficiaries still don’t know is that the rules split depending on one key detail. If the original owner had not yet begun RMDs, beneficiaries can distribute on any schedule they choose within the 10-year window. If the original owner had already begun RMDs, annual distributions are required throughout all 10 years, not just a lump sum at the end. That distinction meaningfully changes how much tax planning flexibility you actually have.

The $50,000-a-Year Tax Problem

A beneficiary who withdraws $500,000 evenly over the 10‑year window takes $50,000 per year in ordinary income. On its own, that amount fits comfortably within the 22% federal bracket for single filers in 2026 (covering income from $50,401 to $105,700). Layer in wages, Social Security, investment income, or a pension, and the distribution can push the beneficiary into the 24% bracket with little effort.

At a 22% rate, that $50,000 generates roughly $11,000 in federal tax each year, or about $110,000 over the decade.

Bracket creep is only part of the issue. Adding $50,000 of taxable income annually can push a beneficiary into higher marginal rates year after year. For retirees already receiving Social Security, that extra income can cause up to 85% of benefits to become taxable once combined income exceeds $34,000 for single filers or $44,000 for married couples filing jointly.

The Medicare Surcharge Nobody Budgets For

IRMAA (the Income‑Related Monthly Adjustment Amount, which is the Medicare premium surcharge applied when income exceeds certain thresholds) uses a two‑year lookback. Distributions taken in 2026 affect premiums in 2028. For 2026, the first IRMAA tier triggers when MAGI exceeds $109,000 for single filers or $218,000 for married couples. A beneficiary who crosses that line pays roughly $1,044 per person per year in combined Part B and Part D surcharges.

Crossing the second tier, which starts at $137,000 single or $274,000 joint, raises that to about $2,496 per person annually. A married couple in Tier 2 faces nearly $5,000 in annual IRMAA surcharges, on top of any additional income taxes.

Front-Loading vs. Back-Loading: How Distribution Timing Affects Total Tax

The tax‑optimal strategy is to front‑load distributions in lower‑income years and back‑load in higher‑income years, or to reverse the order if the beneficiary expects rising income. A beneficiary who retires early in the distribution window, before Social Security begins, may have two or three years with lower taxable income. Taking $80,000 to $100,000 in those years while staying below IRMAA thresholds, then reducing distributions in years when Social Security and other income are fully active, can meaningfully reduce the total tax bill across the decade.

The inverse applies to someone still working. Taking smaller distributions now and larger ones after retirement, when income drops, keeps each withdrawal in a lower bracket. The account continues to grow tax‑deferred in the meantime, though that growth also becomes taxable upon distribution.

Who the 10-Year Rule Does Not Apply To

Several categories of beneficiaries qualify for exceptions. Surviving spouses are exempt from the 10‑year rule entirely and can roll the inherited 401(k) into their own IRA or 401(k), treating it as their own and deferring RMDs until their own required beginning date. Beyond spouses, eligible designated beneficiaries include the deceased’s minor children, disabled or chronically ill individuals, and individuals no more than 10 years younger than the deceased. Minor children qualify for the stretch period until they reach the age of majority, at which point the 10‑year clock begins. Disabled and chronically ill beneficiaries can stretch distributions over their own life expectancy.

If none of those exceptions apply, the 10‑year rule is mandatory.

Distribution Planning Steps to Consider

1. Mapping income over the 10 years before distributions can help identify the years when taxable income will be lowest, while factoring in Social Security start dates, pension income, and any part-time work. Those are typically the years where larger withdrawals keep the beneficiary below IRMAA thresholds.
2. Review recent MAGI figures. Because IRMAA uses a two-year lookback, 2026 Medicare premiums are based on the 2024 tax return. If your 2026 or 2027 distributions will push MAGI above $109,000 single or $218,000 joint, budget for the surcharge or adjust the distribution schedule accordingly.
3. If your combined income from distributions, Social Security, and other sources exceeds the first IRMAA threshold, the surcharge math across a 10-year window can easily run to five figures per person, which is a range where professional tax planning is commonly engaged.