Many retirees look at their average tax rate as a quick measure of how much they pay in taxes each year. It feels like a tidy summary. The problem is that this number is useful for reviewing the past but surprisingly poor for guiding your next decision. Your average tax rate is mostly a rearview-mirror statistic. It tells you what already happened, but does not tell you what your next decision will cost.
The Impact of the Next Dollar
What matters is your effective marginal rate, which is the tax cost of the next dollar of income you generate. That next dollar might come from a Roth conversion, an IRA withdrawal, realizing capital gains, or even part-time work. In retirement, that next dollar can behave in ways that surprise people. The tax code doesn’t operate in neat, isolated brackets. It’s more like a web. Pull one string and three others move. This is why retirement tax planning is less about what bracket you are in today and more about how each income decision changes what happens next.
Additional income often triggers side effects that don’t show up in a simple tax bracket chart. It can cause more of your Social Security income to become taxable, push you over a Medicare premium threshold, or trigger certain capital gains rules.
Put these together, and it’s common for retirees to believe they are in the 12 percent bracket while the real cost of the next dollar lands closer to 18 or 20 percent once the tax impact is factored in. That gap is where good planning lives.
The “Tax Torpedo” in Action
Let’s look at a simplified example.
John and Susan are both 67. They are receiving $40,000 per year in Social Security benefits and withdrawing $30,000 from their traditional IRA to supplement it. They have very little other income. On paper, they appear to be comfortably in the 12 percent federal bracket. Their average tax rate looks modest. Because their combined income sits in the range where Social Security benefits are still phasing into taxation, additional withdrawals have a magnified effect.
Now suppose they decide to withdraw an additional $10,000 from their IRA to fund a kitchen renovation. They assume that $10,000 will simply be taxed at 12 percent, or about $1,200 in tax.
But that is not what happens.
With the way Social Security is taxed, that extra $10,000 doesn’t just add $10,000 to taxable income; it also causes more of their Social Security benefits to become taxable.
For many retirees in this income range, each additional dollar of IRA withdrawal can pull up to 85 cents of Social Security into the taxable column. In effect, that $10,000 withdrawal could increase taxable income to roughly $18,500 rather than $10,000.
At a 12 percent bracket, that translates into about $2,200 in federal tax rather than $1,200. Their marginal rate on that decision was roughly 22 percent, not 12 percent.
That phenomenon is often called the “tax torpedo.” Income in this range causes a rapid increase in taxable income because Social Security is phased into taxation at the same time. From the outside, it looks like you’re in a low bracket. Under the surface, the math is doing something very different. This example highlights why the marginal cost of income matters far more than the average rate shown on a tax return.
Why This Matters
Notice what did not change in John and Susan’s situation. Their average tax rate for the year still looks perfectly reasonable. If you only glance at that number, nothing seems alarming. Yet the real cost of the next dollar was far higher than expected, and that is where planning either helps or quietly hurts.
That extra withdrawal did more than increase their tax bill for the year. It nudged their income closer to levels that could raise future Medicare premiums, and if they had also realized capital gains, it might have pushed some of those gains out of the 0 percent range and into a taxable one.
None of these effects feels dramatic in isolation, but together they show how a single income decision can ripple across multiple parts of the system. One year might not seem consequential, yet repeating those kinds of marginal decisions over time can steadily chip away at tax efficiency and impact your longer-term retirement plan. Good retirement tax planning isn’t about minimizing this year’s bill. It’s about managing how income shows up across many years.
Planning Happens One Decision at a Time
Retirement tax planning is not about chasing the lowest possible lifetime average rate. It is about understanding how each income decision interacts with the broader system and recognizing that timing and sequencing matter just as much as the amounts involved. The tax code is full of moving parts, and when income interacts with Social Security, Medicare premiums, and capital gains rules, small changes can have outsized effects.
The average rate may be interesting to look at, but the marginal rate is where real strategy lives. In retirement, that is what helps preserve financial flexibility and protects your ability to make future decisions on your own terms instead of letting the tax code make them for you.
For readers who would like to explore these coordination decisions in more depth, including how Roth conversions, capital gains harvesting, and Social Security timing fit together across multiple years, the Creating Tax Efficiency for Retirement Income workshop provides a structured way to evaluate those tradeoffs in practice.
Want to learn more? Listen to Episode 216 of the Retire With Style Podcast.
