By Increase your 401(k) contribution from 0% to 5% and your paycheck drops by about $113 per biweekly period — not $144. That difference is the tax code working in your favor. Here’s the exact math.
The Core Mechanism: What 401(k) Actually Reduces
A traditional 401(k) contribution reduces your taxable income for:
- ✓ Federal income tax
- ✓ Most state income taxes
- ✗ Social Security (6.2%)
- ✗ Medicare (1.45%)
FICA is calculated on your gross pay BEFORE the 401(k) deduction. This is a deliberate design — your future Social Security benefit is based on your full earnings, not your reduced taxable income.
| Tax | 401(k) Reduces It? | Why |
|---|---|---|
| Federal income tax | ✓ | Contribution is “pre-tax” for income tax purposes |
| State income tax | ✓ (most states) | Most states conform to federal treatment |
| Social Security | ✗ | FICA uses gross pay, not adjusted gross income |
| Medicare | ✗ | Same as Social Security |
| Additional Medicare (0.9%) | ✗ | Threshold based on Medicare wages (gross) |
The Real Cost of a 5% Contribution
Let’s work through a $75,000 salary, single filer, biweekly pay, Texas (no state tax):
Without 401(k):
| Line | Per Check | Annual |
|---|---|---|
| Gross | $2,884.62 | $75,000 |
| Federal tax (22% bracket) | −$295.00 | −$7,670 |
| Social Security | −$178.85 | −$4,650 |
| Medicare | −$41.83 | −$1,088 |
| Net | $2,368.94 | $61,592 |
With 5% 401(k) ($3,750/year):
| Line | Per Check | Annual |
|---|---|---|
| Gross | $2,884.62 | $75,000 |
| 401(k) contribution | −$144.23 | −$3,750 |
| Federal tax (on $71,250) | −$263.27 | −$6,845 |
| Social Security (still on $75,000) | −$178.85 | −$4,650 |
| Medicare (still on $75,000) | −$41.83 | −$1,088 |
| Net | $2,256.44 | $58,668 |
The Result:
- Paycheck reduction: $112.50 per check ($2,925/year)
- 401(k) balance increase: $3,750/year
- Federal tax savings: $825/year (22% × $3,750)
You invest $3,750 but only lose $2,925 in take-home pay. The government effectively contributes $825 through reduced taxes — exactly your 22% marginal rate on the deferral.
Tax Savings by Bracket
The value of the 401(k) deduction depends on your marginal tax rate:
| Marginal Rate | Tax Savings on $10,000 Contribution | Real Cost |
|---|---|---|
| 12% | $1,200 | $8,800 |
| 22% | $2,200 | $7,800 |
| 24% | $2,400 | $7,600 |
| 32% | $3,200 | $6,800 |
| 35% | $3,500 | $6,500 |
Higher earners get more benefit per dollar contributed. A 32% bracket worker investing $10,000 only “feels” a $6,800 reduction in spendable income.
The Employer Match: The Only Guaranteed Return
Most employers match some portion of your contributions. Common formulas:
| Formula | On $75,000 Salary | Annual Match |
|---|---|---|
| 100% of first 3% | $2,250 | $2,250 |
| 50% of first 6% | $2,250 | $2,250 |
| 100% of first 4% + 50% of next 2% | $3,000 + $750 | $3,750 |
| Dollar-for-dollar up to 5% | $3,750 | $3,750 |
Rule: Contribute at least enough to get the full match. A 50% match is an immediate 50% return before any market growth. No investment offers that.
Example: If your employer matches 50% up to 6%, and you contribute 3%, you’re leaving $1,125/year on the table ($2,250 potential match − $1,125 actual match).
Roth 401(k): The Reverse Trade
A Roth 401(k) works the opposite way:
- Contributions are post-tax (no deduction now)
- Growth and withdrawals are tax-free in retirement
| Traditional | Roth | |
|---|---|---|
| Tax deduction now | ✓ | ✗ |
| Paycheck impact | Lower | Higher |
| Tax on withdrawals | Taxed as income | Tax-free |
| FICA impact | Same (no difference) | Same (no difference) |
When Roth Wins:
- You’re in a low bracket now (12% or below)
- You expect to be in a higher bracket in retirement
- You’re young with decades of tax-free growth ahead
- You want tax diversification
When Traditional Wins:
- You’re in a high bracket now (24%+)
- You expect lower income in retirement
- You need the current tax deduction for cash flow
- You live in a high-tax state now and plan to retire to a no-tax state
State Tax Treatment
Most states follow federal rules and allow 401(k) contributions to reduce state taxable income. But there are exceptions:
| State | Treatment |
|---|---|
| Most states | ✓ Conforms — 401(k) reduces state taxable income |
| Pennsylvania | ✗ Does NOT allow 401(k) deduction for state tax |
| New Jersey | ✗ Does NOT allow 401(k) deduction for state tax |
If you live in PA or NJ, your 401(k) still saves federal tax but costs you state tax on the contribution. This makes Roth 401(k) relatively more attractive in those states.
The Compounding Effect: Why Starting Now Matters
The real power of 401(k) isn’t the tax deduction — it’s decades of compound growth.
| Starting Age | Annual Contribution | At Age 67 (7% return) |
|---|---|---|
| 25 | $10,000 | $2,210,000 |
| 35 | $10,000 | $1,060,000 |
| 45 | $10,000 | $480,000 |
| 55 | $10,000 | $180,000 |
Starting at 25 instead of 35 — with the same annual contribution — produces $1.15 million more at retirement. Ten years of delay cuts your retirement fund by more than half.
And remember: each $10,000 traditional contribution only costs a 22% bracket worker $7,800 in take-home pay. You’re investing money that would otherwise go to taxes.
2026 Contribution Limits
| Category | Limit |
|---|---|
| Employee contribution (under 50) | $24,500 |
| Employee contribution (50+) | $24,500 + $8,000 catch-up = $32,500 |
| Total (employee + employer) | $72,000 |
| Total (50+ with catch-up) | $80,000 |
Frequently asked questions
Does a 401k reduce my taxable income?
Traditional 401k contributions reduce federal and most state taxable income, but do NOT reduce FICA taxes. Social Security and Medicare are calculated on gross pay before the 401k deduction. Roth 401k contributions reduce nothing now — they grow tax-free instead.
How much less will my paycheck be with a 5% contribution?
On a $75,000 salary, 5% is $3,750 per year. Your biweekly paycheck drops by about $113 — not the full $144 — because you save about $32 per check in federal income tax at the 22% marginal rate. The exact number depends on your bracket and state.
Roth or traditional 401k — which should I choose?
Choose traditional if your current marginal rate exceeds your expected retirement rate — typically higher earners now. Choose Roth if you’re early-career in a low bracket or expect higher rates later. Many workers split between both for tax diversification.
How much is my employer match worth?
A common 50% match on the first 6% of salary is worth 3% of compensation — $2,850 on $95,000. Always contribute at least enough to capture the full match. It is an immediate guaranteed return that no investment can match.
What is the 2026 401k contribution limit?
The employee limit is $24,500 under age 50, plus an $8,000 catch-up for ages 50+, totaling $32,500. Employer matching contributions are counted separately against a $72,000 combined limit.
How do I calculate my exact paycheck with a 401k?
Enter your salary, filing status, state, and 401k percentage into the take-home pay calculator. It applies 2026 tax rules including the 401k income tax reduction (but not FICA reduction) to show your actual net pay per check.
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