The same $100,000 Roth conversion: $7,640 or $16,812 in federal tax.The difference is when you do it.
What a conversion costs depends on what is already on the return underneath it. Before Social Security, the low brackets are empty. Once benefits flow, each converted dollar can drag more of them into tax. Once RMDs start, the forced withdrawal has used the room before you convert a dime, and a $25,000 conversion that was free at 63 costs $4,331 at 76. The printable shows every amount across all three stages, with the honest caveats (IRMAA lookback, the five-year clocks, why bigger is not better). From Jim Swiech, CPA.
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Quick answers
- What is the Roth conversion window?
- The years between when your paycheck stops and when Social Security and required minimum distributions begin. For most retirees that stretch is the lowest-bracket period of their adult life, so a conversion done then costs less tax than the same conversion done later. In the example on this page a $100,000 conversion costs about $7,640 in federal tax before Social Security and $16,812 once RMDs stack on top.
- Why does the same Roth conversion cost more later in retirement?
- Because it lands on top of more income. Once Social Security is flowing, each converted dollar also pulls more of the benefit into taxable income, so the effective rate runs above the printed bracket. Once RMDs start at 73, the conversion stacks on income you are forced to take anyway. Same $100,000, higher rate, bigger bill.
- How much should you convert in a year?
- Usually enough to fill your current bracket without spilling into the next one, while watching two other lines: the Medicare IRMAA thresholds, which use this year's income to set premiums two years out, and the five-year clock on converted dollars. The right amount is a calculation, not a rule of thumb, and it changes every year.
Updated . Reviewed by Jim Swiech, CPA.
This printable is educational and general in nature, it is not tax, legal, or investment advice, and it is not a recommendation to convert. Figures are 2026 federal amounts under stated assumptions; outcomes depend on your specific facts. Go Beyond Tax is a brand of Swiech Consulting LLC.