Roth Conversion Optimizer
Find the right amount to convert each year. Not too little (leaves money on the table), not too much (triggers IRMAA or kills ACA subsidies). The optimizer calculates the exact conversion amount for every year of your retirement using your actual numbers.
Why Roth conversion timing matters
A Roth conversion moves money from a traditional IRA or 401(k) into a Roth account. You pay income tax on the converted amount today, but all future growth and withdrawals are tax-free. The question isn't whether to convert — it's how much to convert each year.
Convert too little and you leave tax savings on the table. Convert too much and you push yourself into a higher bracket, trigger Medicare IRMAA surcharges two years later, or lose ACA healthcare subsidies if you're retiring before 65. The optimal amount changes every single year based on your income, account balances, age, and life events.
The three constraints on Roth conversions
Tax bracket ceiling
You want to convert up to the top of your target bracket (often 22% or 24%). Go above it and every additional dollar costs more. The optimizer calculates your bracket headroom after accounting for all other income — salary, pension, Social Security, RMDs.
IRMAA threshold
Medicare uses your MAGI from two years ago to set Part B and Part D premiums. A $50K conversion at age 63 can add $1,000+ per year in surcharges starting at age 65. The optimizer uses two-year lookback logic to avoid crossing IRMAA tier boundaries.
ACA subsidy cliff
If you're retiring before 65 and using ACA marketplace insurance, conversions increase your MAGI and can reduce or eliminate premium subsidies worth $10-30K per year. The optimizer caps conversions to preserve ACA subsidies when this constraint is enabled.
How ThunderHarbor optimizes Roth conversions
ThunderHarbor runs a year-by-year projection from now to age 95. For each year, the optimizer:
- Calculates all income sources — salary, pension, Social Security, RMDs, capital gains
- Finds the remaining headroom in your target federal tax bracket
- Checks the IRMAA two-year lookback to avoid crossing into higher Medicare surcharge tiers
- Checks ACA subsidy limits if you're pre-Medicare and using marketplace insurance
- Sets the conversion amount to the maximum that satisfies all three constraints
- Shows you the result — tax bill, account balances, and the cumulative impact on your retirement
You can also run manual conversions — set specific amounts for specific years — and compare strategies side by side in the What-If Workshop.
Frequently asked questions
When should I do Roth conversions?
The best time is during years when your income is in a lower tax bracket than you expect during RMDs. Common windows include early retirement before Social Security starts, years with low or no wage income, and any year where you have bracket headroom before hitting the next tier.
How much should I convert to Roth each year?
The optimal amount depends on your specific situation. Generally, you want to convert enough to fill your current bracket up to the top of your target rate, but not so much that you trigger IRMAA surcharges or lose ACA healthcare subsidies.
Do Roth conversions affect Medicare premiums?
Yes. Roth conversions count as income for IRMAA purposes, and Medicare uses your MAGI from two years prior to set your premiums. A large conversion at age 63 could increase your Medicare Part B and Part D premiums at age 65.
Do Roth conversions affect ACA healthcare subsidies?
Yes. Roth conversions increase your MAGI, which can reduce or eliminate ACA premium subsidies. If you retire before 65 and use ACA, conversions can cost you more in lost subsidies than they save in future taxes.
What is the Roth conversion window?
The Roth conversion window is the period between retirement and when RMDs begin at age 73. During this window, your income may be lower, allowing you to convert at lower tax rates.
See your optimal Roth conversion strategy
Enter your numbers and see your year-by-year Roth conversion plan. Free to start.
Build your free plan →Free plan included · Full Roth optimizer with Premium ($47/year)