The Roth Conversion Window: Filling the Bracket Before RMDs Start

A traditional 401(k) or IRA isn’t really your money — not all of it. Part of every balance is a tax bill you deferred years ago and haven’t paid yet. You chose when to skip the tax going in. The problem is the IRS gets to choose when you pay it coming out, and that choice arrives as Required Minimum Distributions (RMDs) the year you turn 73.

A multi-year Roth conversion is how you take that timing back. Done right, it drains the pre-tax account on your schedule, in the years and brackets you pick, instead of the government’s. Here’s the shape of the strategy — the moves that make it work and the tripwires that quietly wreck it.

The problem: a deferred bill with a deadline

Leave a large pre-tax balance alone and two things happen. First, RMDs kick in at 73 and force taxable income out whether you need the cash or not — often pushing a retiree into a higher bracket than they ever planned for. Second, whatever is left when you die passes to your heirs, and under current rules most non-spouse beneficiaries have to empty an inherited account within ten years. If those heirs are in their peak earning years, that money gets taxed at their top rate, stacked on top of their salary.

So “do nothing” isn’t neutral. It’s a decision to let RMDs and the next generation’s tax brackets decide what the account is worth.

The core move: fill the bracket, don’t blow past it

The federal system is marginal — income gets taxed in layers, each at its own rate, and there’s usually a wide band between where one bracket ends and the next, much higher one begins. The whole strategy lives inside that band.

In the years before RMDs start, taxable income is often unusually low. That creates room at the top of a moderate bracket. The play is to convert just enough from the pre-tax account to fill that bracket to its ceiling — and then stop. Do it again the next year, and the year after, for as many low-income years as you have. You’re deliberately realizing income now, at a rate you control, to avoid having it forced out later at a rate you don’t.

Why spread it over several years instead of one big conversion? Because a single large conversion spikes your income straight through the moderate bracket and into the high one — you’d pay top rates on the overflow. A series of right-sized conversions keeps every dollar taxed at the moderate rate you targeted. Patience is the entire edge.

Pay the tax from outside the account

This is the one rule that separates a conversion that builds wealth from one that just shuffles it. Pay the conversion tax with money from outside the retirement account — taxable brokerage funds or cash — not by withholding from the conversion itself.

Here’s why it matters. If you convert and skim the tax out of the IRA, fewer dollars actually land in the Roth, and you’ve shrunk the tax-free base that’s supposed to grow untouched for the rest of your life. Pay the tax from a side account and the full converted amount keeps compounding tax-free. The outside cash you spend on tax was already taxable anyway — you’re simply moving it into the most efficient bucket you have.

No outside money to cover the tax? That’s usually the signal that aggressive conversions aren’t your move this year. The strategy assumes you can fund the tax bill without raiding the account you’re trying to fill.

The ripple effects you have to model

A conversion doesn’t happen in a vacuum. Raising your income on purpose touches a half-dozen other things, and a good plan prices all of them in before you commit:

• IRMAA (Medicare surcharges). Higher income raises the premiums you pay for Medicare Part B and D — and it works on a two-year lookback, so a conversion today can raise premiums two years from now. The surcharge jumps in tiers, so the goal is to fill your target bracket without tipping over an IRMAA threshold by a few dollars.
• The cost of raising the tax cash. If you sell from a brokerage account to fund the tax, those sales can trigger capital gains — a real cost that belongs in the comparison, not an afterthought.
• A state move. State tax treatment of conversions varies widely. If a relocation is on the horizon, when you convert relative to that move can change the state tax bill meaningfully — sometimes enough to wait, sometimes enough to hurry.
• High-deduction years. A year with large itemized deductions — say, around a home sale or a big charitable gift — can be the cheapest year to convert, because those deductions absorb some of the conversion income.
• Everything else that phases with income. Social Security taxability, the 3.8% net investment income tax, and various income-based deductions all move when you push income up. None are deal-breakers; all deserve a look.

When it pays off — and when it doesn’t

The conversion window works best when several things line up: a large pre-tax balance, a stretch of low-income years between retirement and age 73, cash on the side to pay the tax, heirs who’ll likely be in higher brackets than you, and a belief that tax rates aren’t headed down.

It works against you if you’ll genuinely be in a lower bracket later, if you have no outside money for the tax, if your horizon is short, or if you’re charitably inclined — Qualified Charitable Distributions can move pre-tax money out tax-free after 70½ and may beat conversions for giving. The point isn’t that everyone should convert. It’s that the years before RMDs are a window that closes, and it’s worth knowing whether it’s open for you.

This is a plan, not a transaction

One more thing that’s easy to miss: this isn’t a thing you do once. A conversion plan is built on today’s brackets and rules carried forward, and those change. The right move is to revisit it every year — re-check the bracket target, confirm you haven’t drifted into a new IRMAA tier, and re-measure what’s left to convert. Some years you’ll convert to the top of the band; some years a one-off event will tell you to convert less, or more.

Get money into a Roth and the rules for funding one are a separate playbook — we covered those in the four ways to fund a Roth. Getting money out of a pre-tax account efficiently is this one. Most retirees with a large 401(k) need both.

This article is general information, not tax, legal, or investment advice. Bracket targets, IRMAA thresholds, state treatment, and whether a Roth conversion makes sense at all depend on your income, your account balances, and your goals — let's look at your specific situation before you convert.