Why does the same conversion cost more once Social Security starts?

Because converted dollars do two things at once. They are taxed themselves, and they raise the income figure that decides how much of your Social Security is taxable. In the example, the conversion pulls $17,800 of additional benefits into taxable income. That is why the real rate on a $25,000 conversion for the couple at 68 is 19.8%, even though they started in the 10% bracket.

What is the real tax rate on a Roth conversion?

The federal tax the conversion causes divided by the amount converted. It is often higher than the bracket you think you are in. At 63 in the example, $100,000 converted costs $12,360, a 12.4% real rate. At 68 it costs $17,607, a 17.6% real rate, because part of the conversion is taxed at 22% and part of it makes Social Security taxable.

Roth conversions · cost

How much tax do you pay on a $100,000 Roth conversion?

By Jim Swiech, CPA · Updated September 5, 2026

Quick answer

For a married couple in 2026, about $12,360 in federal tax if they are 63, retired, drawing $40,000 from an IRA and not yet collecting Social Security. About $17,607 if they are 68 and collecting $48,000 of Social Security on top of the same draw. About $18,017 at 76, once a $60,000 required minimum distribution has replaced the draw.

Same conversion, same couple. The bill depends on what else is on the return that year.

The example, stage by stage

Everything below is 2026 federal law: a $32,200 standard deduction for the couple, $1,650 more per spouse once they are 65, the $6,000 per-person senior deduction that runs through 2028, and the Social Security worksheet that decides how much of a benefit is taxable. The figures come from the same engine as the calculator on the Roth conversion page, so you can change any input and watch them move.

Federal income tax caused by the conversion, and the real rate on the converted dollars. Married filing jointly, 2026.
Convert	Age 63, no Social Security	Age 68, $48,000 Social Security	Age 76, $60,000 RMD + Social Security
$25,000	$2,660 (10.6%)	$4,950 (19.8%)	$3,096 (12.4%)
$50,000	$5,660 (11.3%)	$7,950 (15.9%)	$6,357 (12.7%)
$100,000	$12,360 (12.4%)	$17,607 (17.6%)	$18,017 (18.0%)
$150,000	$23,360 (15.6%)	$29,267 (19.5%)	$29,677 (19.8%)

Stage 1, age 63: the open window

The couple draws $40,000 from the IRA and has no other income. Before the conversion their federal tax is $780. Add $100,000 of conversion and it becomes $13,140. The conversion cost $12,360, a real rate of 12.4%. Most of the converted dollars landed in the 10% and 12% brackets, and the last slice reached 22%. Nothing else on the return was disturbed, because there was nothing else on the return.

Stage 2, age 68: Social Security is flowing

Same $40,000 draw, plus $30,000 and $18,000 of Social Security. Before converting, $23,000 of their $48,000 in benefits is taxable and their federal tax is $1,550. The $100,000 conversion raises the income figure the Social Security worksheet uses, so an additional $17,800 of benefits becomes taxable. The conversion now costs $17,607, a real rate of 17.6%.

Look at the $25,000 row for this couple. It costs $4,950, a real rate of 19.8%, for a couple who started the year in the 10% bracket. That is the Social Security tax torpedo: each converted dollar is taxed once as itself and again by pulling up to 85 cents of benefits into taxable income. The printed bracket badly understates what a small conversion costs once benefits have started.

Stage 3, age 76: RMDs have used the room

By 76 the couple's required minimum distribution is $60,000, so the draw is no longer a choice. With that RMD and their Social Security, $40,000 of their $48,000 in benefits is already taxable before any conversion. The $100,000 conversion stacks entirely on top, mostly in the 22% bracket, and costs $18,017, a real rate of 18.0%. The window closed not because a rule changed but because the low brackets were already full when the couple arrived.

What the example leaves out

Medicare. A $100,000 conversion at stage 2 or 3 puts this couple's modified adjusted gross income near or above the first 2026 IRMAA line of $218,000, which would raise their Part B premiums two years later. The calculator shows that separately. New York adds about $4,285 of state tax on the stage 1 conversion for a couple already using their $20,000-per-person exclusion on the IRA draw. State income tax elsewhere, capital gains, itemized deductions and a pension all change the answer, which is why the real work is done on your return, not on an example.

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Quick answers

What is the federal tax on a $100,000 Roth conversion for a married couple in 2026?: About $12,360 if the couple is 63, retired, drawing $40,000 from an IRA and not yet collecting Social Security. About $17,607 if they are 68 and collecting $48,000 of Social Security on top of the same draw. About $18,017 at 76 when a $60,000 required minimum distribution has replaced the draw. Same conversion, same couple, three different bills.
Why does the same conversion cost more once Social Security starts?: Because converted dollars do two things at once. They are taxed themselves, and they raise the income figure that decides how much of your Social Security is taxable. In the example, the conversion pulls $17,800 of additional benefits into taxable income. That is why the real rate on a $25,000 conversion for the couple at 68 is 19.8%, even though they started in the 10% bracket.
What is the real tax rate on a Roth conversion?: The federal tax the conversion causes divided by the amount converted. It is often higher than the bracket you think you are in. At 63 in the example, $100,000 converted costs $12,360, a 12.4% real rate. At 68 it costs $17,607, a 17.6% real rate, because part of the conversion is taxed at 22% and part of it makes Social Security taxable.

Updated September 5, 2026. Reviewed by Jim Swiech, CPA.

Educational and general in nature. Not tax, legal or investment advice, and not a recommendation to convert. Figures are 2026 federal amounts under the assumptions stated on this page; your return has more lines than any example. Go Beyond Tax is a brand of Swiech Consulting LLC.