The Roth conversion trap: don't let withholding shrink the account

Why would anyone pay a tax bill with the very money they are trying to shield from future taxes? Yet that is exactly what the withholding option invites you to do during a Roth conversion. It sounds like convenience. In our view, it is a small trap with a long tail.

A Roth conversion means moving money from a traditional IRA (or a similar pre-tax account) into a Roth IRA, and the pre-tax amount you move is added to your taxable income for that year. The question is where the cash for that tax comes from. These are U.S. rules, and they differ elsewhere, so read what follows as general information.

The advice is nearly unanimous: pay from outside the account

Wells Fargo says that while it is possible to use retirement assets to pay the conversion tax, it generally doesn't make sense, and it suggests using assets outside retirement accounts instead. A Covenant Wealth Advisors guide dated October 9, 2026 makes the same point: paying with outside cash keeps the full conversion in the Roth, where it can grow tax-free.

When the cautious voices of a big bank and an advisory firm line up like that, we think it is worth listening. Not because they are infallible, but because the logic is hard to argue with.

Withholding quietly undoes the point of converting

The logic of a conversion is simple: pay tax now, get tax-free growth later. Every dollar taken out of the conversion to cover the bill is a dollar that never gets that growth. You are, in effect, converting less than you planned. Meanwhile the tax is still owed on the pre-tax amount you did convert.

The age problem makes it worse. Wells Fargo notes that there is no additional 10% tax on the amount converted, but if you take a distribution or elect tax withholding to pay the taxes and are under age 59½, you may owe that 10% additional tax on the portion not converted. So the money set aside to cover the bill can itself trigger a penalty. That is a bad deal by any standard. And even if you are older and escape the penalty, you still end up with a smaller Roth than you intended.

A Reddit commenter put it bluntly: you should not convert unless you have separate funds to cover the tax. We wouldn't cite a forum as an authority, but we think the instinct is sound. If you lack outside cash, the conversion may simply be too big.

The fix is sizing the conversion, not finding a cleverer payment method

The better lesson is about planning. Schwab points out that you can convert part of an account, and that partial conversions over several years can make the tax impact easier to manage. It also warns that a larger conversion can push income into a higher bracket and affect Medicare premiums, taxes on Social Security benefits, and some credits and deductions.

So we'd flip the order of questions. Don't start with "how much can I move?" Start with "how much tax can I pay from cash that isn't in a retirement account?" Convert an amount whose bill you can cover from savings or a taxable brokerage account. If that means converting less this year and more next year, so be it. Schwab also notes that Roth conversions made in 2018 or later generally can't be recharacterized, so a miscalculation is hard to unwind.

There is a counterpoint. Using retirement money for the bill is possible, and for someone with no outside cash and a modest conversion it may look pragmatic. We just think it should be a conscious, last-resort decision rather than a default box ticked on a form. This is general information, not personal advice, and a tax professional can run your numbers.

Our verdict

If you are converting this year, set aside the tax money first, outside the account, and think twice before choosing withholding from the conversion. Talk to your preparer about when the bill comes due, since it still arrives. If you can't afford the tax without dipping into the converted funds, shrink the conversion. In our view, a smaller Roth that is fully funded beats a bigger one that was quietly drained on the way in.