If you’re asking what the best time to convert a 401(k) to a Roth IRA is, here’s the short answer: it’s not a date on the calendar, it’s a low-income year. A conversion is taxed at your marginal rate the year you do it, so the entire game is converting when that rate is as low as it will ever be. Get the timing right and you move money to the tax-free side cheaply; get it wrong and you hand the IRS more than you needed to.
Why timing is everything
When you convert pre-tax 401(k) money to a Roth, the converted amount is added to your ordinary income for the year. Convert $40,000 and you’re taxed on an extra $40,000. So the question isn’t really “when should I convert?” — it’s “in which year will that extra income be taxed the least?” Every good conversion window is just a version of that same idea: catch a year when your other income is low and your bottom tax brackets are sitting empty.
The five best windows to convert
1. The early-retirement gap years. This is the prime window. Between the day you stop working and the year Social Security and required minimum distributions begin, your taxable income can fall through the floor. Those are the cheapest conversion years you will ever get. Filling your low brackets with conversions during this gap is the single highest-value timing move in retirement planning, and it’s the heart of how to decide how much to convert each year.
2. Any unusually low-income year. You don’t have to be retired. A gap between jobs, a sabbatical, a year of self-employment losses, or a year you took unpaid leave all open the same door. If your income dropped this year, your conversion got cheaper.
3. A market downturn. When your balance is temporarily down, converting is a bargain: you pay tax on the lower value, move more shares into the Roth, and every dollar of the recovery is then tax-free. Converting into a down market is one of the few ways to turn a bad year into a tax win.
4. Before required minimum distributions start. RMDs begin at 73 or 75 depending on your birth year, and they force taxable money out of your traditional accounts whether you need it or not. The larger your pre-tax balance at that point, the larger those forced distributions — and they can push you into higher brackets in your 70s and 80s. Converting in your 60s shrinks the balance that drives those RMDs, smoothing the spike before it arrives.
5. Before a known income jump. If you know Social Security, a pension, or an annuity is about to switch on, the years before it are your last cheap window. Once that steady income starts filling your low brackets, there’s far less room left to convert without spilling into a higher rate.
Don’t convert just because you’re afraid rates will rise
For a few years the big talking point was “convert before the 2017 tax cuts expire at the end of 2025.” That deadline is gone: the 2025 tax law made those lower brackets permanent. So the decision is no longer driven by a looming sunset — it’s driven by your income, this year versus your expected future. Could a future Congress raise rates? Sure. But that’s a guess, and it’s a much weaker reason to convert than a concrete low-income year sitting right in front of you.
How much to convert once you’ve picked the year
Timing tells you when; bracket math tells you how much. The usual approach is to convert up to the top of a target bracket — often the 12% or 22% bracket — and then stop, so none of the conversion gets taxed at the next rate up. Converting your whole 401(k) in one year almost always backfires, because the back half of that big number gets taxed at much higher rates. Spread it across several low years instead.
The mechanics
There are two ways to get 401(k) money into a Roth. If you’ve left the employer, you roll the 401(k) into a traditional IRA and convert from there. If you’re still working, some plans let you convert inside the plan — see in-plan Roth conversions for how that works. Either way, pay the resulting tax from a separate savings or brokerage account, not by having it withheld from the conversion. Paying with outside money keeps the full balance growing tax-free and avoids a penalty on withheld dollars if you’re under 59½.
A quick example
Picture someone who retires at 60 with $600,000 in a traditional 401(k) and no income until Social Security at 67. That seven-year gap is the best time to convert a 401k to a Roth IRA they will ever see. Each year they roll a slice into a Roth IRA — enough to fill the 12% bracket, roughly $50,000 of taxable income for a single filer in 2026 — and pay the tax from savings. Over seven years that quietly moves a few hundred thousand dollars to the tax-free side at a 10–12% average rate, instead of letting it compound into six-figure RMDs taxed at 22% or more a decade later. Same money, far less lifetime tax, purely because of when they converted.
Watch-outs before you pull the trigger
- It’s permanent. You can’t reverse a conversion — that option ended in 2018.
- The pro-rata rule. If you have other pre-tax IRA money, a conversion may be partly taxable in a way you didn’t expect; the IRS adds up all your traditional IRAs to figure the taxable share.
- IRMAA. At 63+, a big conversion can raise your Medicare premiums two years later.
- ACA subsidies. Conversion income can shrink a marketplace health-insurance subsidy before Medicare.
- The 5-year clock. Each conversion starts its own clock for penalty-free access to the converted principal before 59½.
The best time to convert a 401(k) to a Roth IRA is whenever your tax rate dips — and the smart move is to be ready for those years before they arrive, so you can act while the window is open. The IRS covers the conversion and rollover rules on its Roth IRAs page.
This is educational, not tax advice. Conversions are irreversible and the right timing and amount depend on your own numbers — talk to a tax professional before converting.
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