Should I pay the conversion tax from savings?

A conversion is a taxable event: the converted amount is added to ordinary income for the year, and the tax on it is due for that year. The tax can be withheld from the converted amount itself, or paid from money held outside the IRA. Either way the tax is a real cost in the year of the conversion, and any comparison of "convert" against "do nothing" is only meaningful if it charges that cost consistently in both paths.

Withholding from the IRA

Part of the distribution is withheld for taxes and only the remainder is converted. Example: converting $100,000 with 24% withheld puts $76,000 in the Roth IRA and sends $24,000 to the IRS.

Two consequences of withholding:

• Less money ends up tax-free. The withheld portion permanently leaves the retirement environment.
• Penalty before age 59½. The withheld amount is a regular distribution, not a conversion. If the owner is under 59½, the withheld portion is subject to the 10% early-distribution penalty on top of ordinary income tax (conversions themselves are exempt from the penalty; withholding is not).

Paying from outside cash

The full converted amount lands in the Roth IRA and the tax is paid from a taxable account. This raises the Roth balance relative to withholding, and it is the mode most conversion calculators default to.

It is not free. Those dollars leave a taxable account where they would otherwise have stayed invested and compounded for the rest of the horizon. A comparison that credits the Roth with the full converted amount while charging only the nominal tax has quietly dropped the forgone compounded growth on the outside cash. The size of the omission grows with the return assumption and with the length of the projection: at a 6% return over 10 years, tax money spent today gives up roughly 79% of its own value in growth that the "do nothing" path still gets to keep.

Modeling this path honestly requires tracking a third pool — the taxable account the tax came from — including the drag of tax on its own returns. A calculator that offers "pay from outside cash" without that third pool will systematically overstate the case for converting.

What actually decides whether a conversion helps

The dominant factor is the comparison between the marginal rate paid on the converted dollars now and the marginal rate that would have applied to those dollars when withdrawn later:

• Rate now lower than rate later — converting moves income into a cheaper year, and the conversion tends to come out ahead.
• Rate now equal to rate later — with the tax withheld from the conversion, the two paths produce the same after-tax result. Converting is a wash.
• Rate now higher than rate later — converting pays tax at a premium and tends to come out behind.

Because a conversion stacks on top of existing income, a large single-year conversion can push the converted dollars into brackets well above the owner's usual marginal rate, which is what makes staged conversions ("fill to bracket top") different in outcome from converting everything at once.

How the QuantFP calculator models this

The QuantFP Roth conversion calculator withholds the tax from the conversion and does not offer an outside-cash option. The reason is comparability: the withholding path can be stated exactly with the pools already modeled, while the outside-cash path cannot be stated correctly without also modeling the taxable account the tax was drawn from. Offering it as a toggle would let the projection show a gain that comes from an omitted cost rather than from the rate comparison above.

Estimated taxes

A conversion creates tax due that withholding on wages may not cover. Underpayment penalties can apply if estimated taxes are not paid during the year of the conversion. Withholding (from the conversion or from other income sources late in the year) is treated as paid evenly through the year, which is sometimes used to cure an underpayment.