Two numbers, one confusion
Ask someone what their tax bracket is and they'll give you a single number — "I'm in the 24% bracket." Then they look at their actual tax bill and see 17% or 18% of their income went to federal tax. Are they lying? Bad at math?
Neither. Those are two different rates, and mixing them up is how most people misunderstand what pre-tax contributions actually save them.
The bracket system in one picture
The U.S. federal income tax is progressive. Your income gets sliced into layers, and each layer is taxed at its own rate. For a single filer in 2025, those layers are roughly:
- First ~$11,900: 10%
- Next ~$36,400: 12%
- Next ~$54,700: 22%
- Next ~$93,500: 24%
- ... and so on up to 37%
If you make $120,000, you don't pay 24% on all of it. You pay 10% on the first slice, 12% on the next slice, 22% on the next, and 24% only on the income that fell into the 24% bracket.
Your marginal rate is the rate on the last dollar you earned — the rate your "bracket" refers to.
Your effective rate is total tax paid ÷ total income — a blended average of every bracket your income touched.
The effective rate is always lower than the marginal rate (as long as you have any income below your top bracket). For a $120K single filer, the marginal is 24% but the effective is about 17.5%.
Why this matters for every pre-tax dollar
When you contribute to a 401(k), Traditional IRA, or HSA, you're reducing your taxable income. The dollars you contribute come off the *top* of your income — the highest brackets you were in.
This means a pre-tax contribution saves you your marginal rate, not your effective rate. If you're in the 24% bracket and you contribute $10,000 to a 401(k), you save $2,400 in federal tax — not $1,750.
That's the single most important thing to understand about tax-advantaged accounts. People see "effective rate 17%" and think "my 401(k) only saves me 17%." No — it saves you the rate on the dollars you took off the top, which is higher.
Where FICA doesn't play along
There's a footnote that trips people up: FICA (Social Security + Medicare payroll tax) works differently.
- Social Security is 6.2% on your first $176,100 of wages (2025). Flat, not progressive. 401(k) contributions don't reduce it — you still pay 6.2% on your gross salary up to the cap.
- Medicare is 1.45% on all wages, plus an additional 0.9% surtax above $200K for singles / $250K MFJ. Also flat. Also not reduced by 401(k) contributions.
- HSA is the exception — HSA contributions via payroll *do* reduce FICA, which is why they're sometimes called "triple tax-advantaged."
So when you contribute $10K to a 401(k), you save $2,400 in federal income tax (at 24%), but you still pay ~$765 in FICA on those dollars. The total tax break is federal rate only.
The Traditional IRA deduction phaseout
Here's where it gets weirder. If you have access to a 401(k) at work, your Traditional IRA deduction phases out at higher incomes. For single filers in 2025, the phaseout starts at $79,000 and ends at $89,000 of modified adjusted gross income. Above $89K, your Traditional IRA contribution is still allowed but it's no longer deductible — which usually means you should contribute to a Roth instead (or use the backdoor Roth).
This is one of the sneakier corners of the tax code, and it's exactly the kind of thing a simulator should make visible.
Run the math on your own numbers
The Tax Bracket Calculator on STWLTH shows the full picture: your marginal rate, your effective rate, every bracket your income touches, and what happens when you move the 401(k), HSA, and Traditional IRA sliders around. It also flags when you hit the FICA cap or the Medicare surtax threshold, and it applies the Traditional IRA phaseout automatically.
The "with vs without max contributions" comparison card is the punchline: it shows you exactly how much taxable income you could cut, how much federal tax you'd save, and what your new effective rate would be. For a lot of people, that number is bigger than they expect — because they were anchoring on the effective rate, not the marginal one.