Every dollar you put into a traditional 401(k) reduces your taxable income for that year. It’s one of the simplest, highest-leverage tax moves available to W-2 workers. Here’s the exact math.
The Mechanics
A pre-tax 401(k) contribution is deducted from your gross wages before federal and state income tax is calculated. Your employer skips the step of including that money in your taxable pay. Practically:
- Gross pay: $100,000
- 401(k) contribution: $10,000
- Federal taxable wages shown on your W-2: $90,000
You pay tax on $90,000, not $100,000. The $10,000 grows tax-deferred inside the 401(k) until you withdraw it in retirement.
What That Saves You
The savings depend on your marginal tax rates. For a single filer earning $100,000 in 2026:
- Federal marginal rate: 22% (the $100K salary lands in the 22% bracket)
- Hypothetical state rate: 5% (varies wildly by state — 0% in TX/FL/NV, up to 10%+ in CA/NY)
A $10,000 401(k) contribution saves:
- Federal: $10,000 × 22% = $2,200
- State: $10,000 × 5% = $500
- Total tax savings: ~$2,700
That means $10,000 into the 401(k) only “costs” you about $7,300 of take-home pay. The other $2,700 comes out of taxes you would have paid anyway.
What About FICA?
Here’s the nuance: 401(k) contributions do NOT reduce FICA (Social Security + Medicare) taxes. FICA is calculated on your gross wages before most pre-tax deductions.
This is why you’ll sometimes see different numbers for “taxable wages” on your W-2:
- Box 1 (Federal wages): reduced by 401(k), cafeteria plan, HSA contributions
- Box 3 (Social Security wages) and Box 5 (Medicare wages): reduced by HSA and cafeteria plan, but NOT by 401(k)
So a 401(k) contribution saves you federal and state income tax — but not the 7.65% FICA.
The Exception: HSA Contributions Through Payroll
If you contribute to a Health Savings Account (HSA) through your employer’s payroll, that contribution also reduces your FICA wages. On a $7,500 HSA contribution:
- Federal income tax saved: $7,500 × 22% = $1,650
- State income tax saved: $7,500 × 5% = $375
- FICA saved: $7,500 × 7.65% = $574
- Total savings: $2,599
That extra FICA reduction is why financial planners often say to max the HSA before the 401(k) beyond any employer match.
2026 Contribution Limits
- 401(k) / 403(b): $23,500 (with $7,500 catch-up for 50+)
- IRA: $7,000 ($1,000 catch-up)
- HSA: $4,400 self-only / $8,750 family ($1,000 catch-up for 55+)
- FSA (health): $3,300
Traditional vs Roth
The tax savings described here apply only to traditional pre-tax contributions. A Roth 401(k) contribution uses after-tax money, so it does NOT reduce your current-year taxable income. The tax advantage comes later — Roth withdrawals in retirement are tax-free (including gains).
Choose based on your expected retirement vs current tax rate:
- Traditional wins if you expect a lower tax rate in retirement
- Roth wins if you expect a higher tax rate in retirement (or want tax diversification)
Other Pre-Tax Deductions That Reduce Taxable Income
- Traditional 401(k), 403(b), 457(b)
- HSA (through payroll)
- FSA (health and dependent care)
- Health insurance premiums paid through cafeteria plan
- Commuter benefits (transit and parking)
- Group-term life insurance (first $50K of coverage)
See The Impact on Your Paycheck
Use a paycheck calculator and plug in your pre-tax contribution amount to see the exact effect on your take-home pay. The “Pre-tax deductions” input is where 401(k), HSA, and similar contributions go.