Your Social Security Has a Gross and a Net

A conversation with an advisor last week, about a client stuck on a Roth conversion. The client's objection was the standard one, and it is not wrong on its face: a conversion moves a tax bill from the future into the present. If the rates are similar, why bother?

Here is the framing that landed.

Your Social Security has a gross and a net — just like a paycheck. The award letter states a gross figure. What reaches the bank account is smaller, because Medicare premiums and income-based surcharges are deducted from the benefit before it is paid — that is federal law, not a plan choice. And what the retiree keeps is smaller still, because up to 85% of the gross benefit is taxable.

A Roth conversion is one of the few moves that changes the net without touching the gross. In the scenario modeled below, both paths — convert and don't convert — pay the client the identical $1,575,798 of gross Social Security over 26 years. The conversion path delivers $193,305 more of it.

Same benefit. Bigger deposit. Three mechanisms do the work, and they compound in that order.

The IRS decides how much of a benefit is taxable with a formula: one-half of benefits, plus all other income — including tax-exempt interest. Traditional IRA withdrawals are ordinary income inside that formula. Qualified Roth withdrawals are excluded from gross income entirely, so they never enter it.

Every dollar sitting in a traditional account is a dollar that will eventually feed that formula, because required minimum distributions force it out. Every converted dollar stops feeding it, forever.

In the modeled scenario the difference shows up exactly where the mechanism predicts: the year required distributions begin. At 75, the client who didn't convert has $48,492 of the benefit taxable — the 85% maximum, immediately. The client who converted has $31,984 taxable — $16,508 less, in one year. The gap persists for nine years before rising income pins both paths at the cap.

Two honest boundaries on this claim. High-income retirees sit at the 85% maximum with or without a conversion — for them this first leak barely moves. And taxes on benefits are normally paid at filing, not withheld from the check, so this leak reduces what the retiree keeps, not the deposit itself. The deposit is the next mechanism's job.

Medicare Part B premiums are collected, by statute, "by deducting the amount thereof" from the monthly benefit. The income-based surcharges — IRMAA — ride the same rail. They never arrive as a bill; the check simply shrinks.

And IRMAA runs on modified adjusted gross income from two years prior. That delay is why almost nobody connects the surcharge back to its cause.

Watch the two-year lookback do its work. Without the conversion, required distributions begin at 75 — and the surcharge lands at 77, starting at $3,450 and climbing the tiers to $33,983 a year by 90 as distributions compound. Lifetime surcharges: $220,162.

The conversion path faces the same rules with a different input. The $500,000 conversion at 61 cut the traditional account that generates those distributions — lifetime RMDs fall from $2.37 million to $980,000 — so the income that sets the surcharge stays under the thresholds until 88. Lifetime surcharges: $26,857.

One precision point worth getting right in client conversations: the conversion does not touch the standard Part B premium — everyone pays that. It only removes the income-based surcharge stacked on top. In this scenario that stack was worth $193,305.

The modeled client is single, so this scenario cannot show the third leak — but for couples it is the sharpest one. When one spouse dies, the survivor keeps roughly the larger Social Security benefit and most of the household income, then files single: about half the bracket room, and IRMAA thresholds at half the married level. Every mechanism above gets meaner at exactly the moment one person is left depending on the check.

We've published the year-by-year math on that separately — the same retirement that costs $82,352 in surcharges as a couple costs $342,386 for the widow. A conversion executed while both spouses are alive — at married-filing-jointly brackets and married IRMAA thresholds — is the one version of this move the survivor cannot make alone later.

None of this says a conversion is right for any particular client, and nothing here recommends converting $500,000 in a single year — that shape is this scenario's, not a prescription. Converted in the wrong year, the first two mechanisms run in reverse: the conversion itself is ordinary income, so converting while collecting benefits maximizes the taxable share of the benefit that year and can trigger surcharges two years later. This scenario works because the conversion happens at 61 — before benefits, and far enough ahead of Medicare that the income spike never touches a premium.

What it does say: the decision is bigger than a tax-rate bet. For an advisor, the checklist looks like this.

• Model Social Security net of Medicare, both paths. The gross benefit is identical by construction; the difference lives entirely below the line.
• Project the MAGI that required distributions will create, and read the surcharge two years forward from the first spike.
• Check the taxable-share window between RMD start and the 85% cap — that is where the tax leak concentrates.
• For couples, run the plan as the survivor, not just as the couple. Single thresholds, one benefit gone.

Modeled in RetirementAdvisorPro: single filer, $4,500 monthly benefit at 65 with a 2% cost-of-living adjustment, $500,000 converted in one year at 61, required distributions from 75, projected to age 90. Every figure past 65 inherits the scenario's premium inflation assumptions — Part B at 7.3% a year (the sixty-year historical average), Part D at 6%. Lower assumptions shrink the late-year figures; they do not change the mechanisms, the thresholds, or the two-year lookback.