How this is calculated
Each year the plan converts exactly the space left in your chosen bracket: the bracket's top minus your taxable income. Tax on each conversion is computed with the real 2026 brackets - including any lower brackets a large gap spans - while the unconverted balance keeps growing at your rate.
Why spread conversions at all
Converting everything at once shoves most of it into the top brackets - the lump-sum figure above shows that penalty on your numbers. Spreading keeps every converted dollar at your chosen rate or below. The trade: the remaining balance keeps growing, so there is more to convert later - which is why the schedule sometimes never finishes at low ceilings.
Windows that make conversions golden
Early retirement before Social Security and RMDs; any low-income year; a market crash (converting depressed shares moves the whole recovery into the Roth). Watch two side effects: conversions can raise Medicare IRMAA premiums two years later, and each conversion carries its own 5-year clock for penalty-free access under 59 1/2.
Notes
Assumes constant brackets and other income (2026 rules throughout); state tax on conversions not included - check your rate in the tax tool.