He Retired With Room for a $100,000 Roth Conversion. Social Security Could Make the Same Move Much More Expensive.

A longtime public employee retires in his mid-60s and, for the first time in decades, his tax return gets quiet. The paycheck is gone. His pension has started, but he has not claimed Social Security yet. Required minimum distributions (RMDs) are still years away. Then his former employer adds an in-plan Roth conversion option, giving him the chance to move $100,000 of pretax retirement money into a Roth.

His instinct is to wait. There is no deadline stamped on the offer. That is exactly what makes the opportunity easy to miss. The $100,000 conversion will still be available later, but the empty space around it may not be.

Retirement Can Create a Brief Tax Valley

A Roth conversion moves money from a pretax retirement account into a Roth account and makes the converted amount taxable in the year of the move. The conversion itself does not become cheaper because someone retires. What can change dramatically is everything surrounding it.

Suppose his pension produces $45,000 of annual income and little else is landing on the return. A $100,000 conversion sits on top of that relatively modest base. Now move the exact same conversion a few years forward. Social Security has started. Investment income may be higher. Eventually required distributions begin pulling money from pretax accounts whether he wants it or not. The $100,000 did not change. The tax return underneath it did.

Social Security Can Crowd the Same Space

Once Social Security starts, a Roth conversion can affect more than the tax rate on the converted dollars. Federal rules use provisional income to determine how much of a Social Security benefit becomes taxable. A large conversion increases that calculation and can cause as much as 85% of the benefit to enter taxable income.

That can produce an unpleasant stacking effect: the retiree pays tax on the conversion while the conversion itself causes more of his Social Security to become taxable. It does not mean claiming Social Security makes Roth conversions a bad idea. It means the same $100,000 move can carry a different tax price before and after the benefit begins. That is why the first quiet retirement years can be unusually valuable.

Medicare Can Arrive Two Years Later

There is another clock running in the background. Medicare generally uses modified adjusted gross income (MAGI) from two years earlier to determine the income-related monthly adjustment amount (IRMAA) added to Part B and Part D premiums. A large conversion can therefore create a second bill long after the income tax is paid.

In 2026, IRMAA begins above $109,000 of MAGI for a single filer and $218,000 for married couples filing jointly. Someone converting $100,000 can cross one or several tiers depending on what else is already on the return. The important point is timing. Even a conversion completed before Medicare enrollment can matter later if that tax year becomes part of the two-year lookback. The silent year is not invisible to Medicare just because the Medicare card has not arrived yet.

Waiting Has a Cost of Its Own

None of this means he should automatically convert the entire $100,000 today. A large pension may already leave little room in lower tax brackets. Future tax rates matter. So do heirs, required distributions and the amount of pretax money still sitting in the account. But “I’ll deal with it eventually” is not a neutral decision. Every new income stream that starts can make the same conversion harder to fit cleanly into the return.

Timing Is Everything

The useful exercise is not simply deciding whether to convert. It is deciding when the conversion has the least competition:

1. Mark the year Social Security is expected to begin and estimate how much of the benefit could become taxable alongside a conversion.
2. Identify which conversion years could later feed Medicare’s IRMAA calculation, even if Medicare coverage has not started yet.
3. Compare several smaller conversions with one $100,000 move instead of assuming the entire amount has to cross the tax return at once.

Retirement finally removed his paycheck from the tax return. The opportunity is recognizing what that empty space is worth before Social Security, Medicare and eventually required withdrawals begin filling it back in.

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