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Field guide

What a Roth conversion actually is — and when it makes sense.

A Roth IRA conversion means moving money from a tax-deferred Traditional IRA into a Roth IRA. You pay ordinary income tax on the converted amount in the year of the conversion, and after that the money grows and is withdrawn tax-free in retirement — forever.

That upfront tax bill is the entire cost. Whether the conversion is a good idea depends on one core question: will you pay more or less tax on this money today than you would when you take it out of the Traditional IRA in retirement?

The math: now vs. later

Assuming you pay the conversion tax from outside the IRA (the financially correct approach — never use IRA funds to pay the tax), the net lifetime advantage of a Roth conversion is:

• C: amount converted
• r: annual return (decimal)
• Y: years to retirement
• t_ret / t_now: retirement / current tax bracket (decimal)

The result is positive (Roth wins) when your retirement bracket exceeds your current bracket, and negative (Traditional wins) when you expect a lower rate in retirement. Critically, the number of years and the return rate do not change which strategy wins — they only amplify the bracket difference. Time is not your enemy in this math; the bracket gap is.

The break-even year (payback period)

A complementary metric is the payback period: how many years before the tax-free compounding advantage accumulates enough to equal the taxes you paid upfront. The formula is:

If the break-even year is before your retirement, the conversion has paid for itself by the time you start drawing down. This is a useful sanity check even when the net-advantage formula already tells you which direction to go.

When a Roth conversion makes strong sense

• Lower-income year: Job change, sabbatical, early retirement, or a year without a bonus can drop you into a temporarily low bracket — a perfect window to convert.
• Expecting higher taxes later: Young earners, people with large pension income starting later, or anyone who expects tax rates to rise legislatively.
• Large Traditional IRA and concern about RMDs: Required Minimum Distributions begin at 73, forcing taxable withdrawals that can push you into higher brackets. Converting reduces the Traditional IRA balance — and future RMDs.
• Estate planning: Roth IRAs have no RMDs during the owner's lifetime and pass tax-free to beneficiaries. For heirs in high brackets, this is a significant advantage.
• Tax diversification: Even if the math is roughly neutral, holding both Roth and Traditional assets gives you flexibility to manage your taxable income in retirement.

When to keep the Traditional IRA

• Higher bracket now than at retirement: If you expect significant income reduction in retirement (no pension, modest Social Security), your retirement bracket may be lower — making Traditional withdrawals relatively cheap.
• You’d have to use IRA funds to pay the tax: This eliminates compounding on the tax dollars and almost always makes the conversion unfavorable. Only convert if you can pay taxes from a taxable account.
• Near retirement (under 5 years): You may not have enough time for the Roth’s tax-free growth to offset the upfront tax cost, especially if you start drawing the account shortly after converting.
• The conversion would spike your income: A large conversion can trigger higher Medicare premiums (IRMAA), affect financial aid eligibility, or phase out deductions and credits.

Partial conversions: the sweet spot

You don’t have to convert everything at once. Many financial advisors recommend partial Roth conversions over several years, filling up a specific bracket without crossing into the next one. For example, a taxpayer in the 22% bracket might convert just enough each year to bring taxable income to the top of that bracket — without tipping into 24%.

This strategy is especially powerful in the years between early retirement and Social Security / RMD onset — a window when income and therefore tax brackets are often at a career low.

Important considerations not in this calculator

• State income tax: Most states tax Roth conversions as ordinary income. Add your state rate to the effective cost of conversion.
• 5-year rule: Converted funds must remain in the Roth for at least 5 years before penalty-free withdrawal of principal. Each conversion starts a new 5-year clock.
• IRMAA (Medicare surcharges): High-income years can increase your Medicare Part B and D premiums two years later. A large conversion could trigger IRMAA.
• Backdoor Roth: If your income exceeds the Roth contribution limit, a Backdoor Roth (non-deductible Traditional contribution followed by immediate conversion) is a related but separate strategy.