How Your 401(k) Affects Your Paycheck

Q: Is Roth 401k better for take-home pay?

No — Roth 401(k) gives you lower take-home pay than traditional 401(k) for the same contribution amount. Roth contributions are after-tax, meaning they do not reduce your federal, state, or FICA taxes. The full contribution comes directly out of your net pay. The benefit of Roth is that withdrawals in retirement are completely tax-free.

Contributing to a 401(k) changes your take-home pay — but not always by the amount you'd expect. The type of 401(k) you have determines how much your paycheck actually drops for every dollar you contribute. Here's exactly how the math works.

Two Types of 401(k), Two Very Different Tax Treatments

Not all 401(k) plans work the same way on your paycheck. The two types — traditional and Roth — have opposite tax treatments, and understanding the difference is the key to knowing what your next paycheck will look like.

Traditional 401(k): Pre-Tax Contributions

A traditional 401(k) contribution is pre-tax. Your employer deducts it from your gross pay before calculating federal and state income taxes. This means your taxable income goes down, and you pay less in income tax each pay period.

But here's the critical detail most people miss: traditional 401(k) contributions do NOT reduce your FICA taxes. The tax treatment is “exempt federal and state” — it reduces your federal and state taxable income, but Social Security tax (6.2%) and Medicare tax (1.45%) are still calculated on your full gross pay, including the 401(k) contribution.

This matters. On a $150 contribution, you're still paying $150 × 7.65% = $11.48 in FICA taxes on that money. The tax savings come only from the federal and state side.

Roth 401(k): After-Tax Contributions

A Roth 401(k) contribution is after-tax. Your employer deducts it from your pay after all taxes — federal, state, and FICA — have been calculated on the full amount. The contribution does not reduce any of your taxable incomes this period.

The benefit comes later: when you withdraw money from a Roth 401(k) in retirement, those withdrawals are completely tax-free. You pay the taxes now so you don't pay them later.

Both Types Calculate from Gross Pay

Whether you choose traditional or Roth, your 401(k) contribution is calculated from your gross pay — not your net pay. The IRS considers both types to be “elective deferrals from compensation.” If you set a 6% contribution rate and your gross biweekly pay is $2,500, you contribute $150 regardless of whether it's traditional or Roth.

Side-by-Side: The Real Impact on Your Paycheck

Let's walk through a concrete example. Same worker, same gross pay, three different scenarios. We'll use $2,500 biweekly gross, single filer, no state income tax (to keep the comparison focused on the federal and FICA impact).

Scenario 1: No 401(k) Contribution

Gross pay: $2,500
Federal taxable income: $2,500 (full amount)
FICA taxable income: $2,500 (full amount)
Federal tax withheld: ~$194
Social Security (6.2%): $155.00
Medicare (1.45%): $36.25
Total deductions: ~$385
Net pay: ~$2,115

Scenario 2: Traditional 401(k) at 6% ($150)

Gross pay: $2,500
401(k) contribution: $150 (pre-tax)
Federal taxable income: $2,350 (reduced by $150)
FICA taxable income: $2,500 (still full amount — 401k does not reduce FICA)
Federal tax withheld: ~$161 (lower because taxable income is lower)
Social Security (6.2%): $155.00 (unchanged)
Medicare (1.45%): $36.25 (unchanged)
Total deductions: ~$502 ($150 contribution + ~$352 taxes)
Net pay: ~$1,998

Your take-home dropped by about $117 — not $150. The $150 contribution only “cost” you $117 in reduced take-home because you saved roughly $33 in federal taxes. That's the power of pre-tax contributions: every dollar you put in costs you less than a dollar off your paycheck.

Scenario 3: Roth 401(k) at 6% ($150)

Gross pay: $2,500
401(k) contribution: $150 (after-tax)
Federal taxable income: $2,500 (full amount — Roth does not reduce any taxes)
FICA taxable income: $2,500 (full amount)
Federal tax withheld: ~$194 (same as no 401k)
Social Security (6.2%): $155.00
Medicare (1.45%): $36.25
Total deductions: ~$535 ($150 contribution + ~$385 taxes)
Net pay: ~$1,965

Your take-home dropped by $150 — the full contribution amount. With Roth, there is no per-paycheck tax savings. The $150 costs you exactly $150 off your check. The payoff is that your retirement withdrawals will be completely tax-free.

The Real Cost of a 401(k) Dollar

This is the takeaway most people miss. With a traditional 401(k), the “cost” of contributing one dollar is less than one dollar off your paycheck. How much less depends on your marginal tax rate:

In the 12% federal bracket: $1.00 contributed costs you about $0.88 in take-home
In the 22% federal bracket: $1.00 contributed costs you about $0.78 in take-home
In the 24% federal bracket: $1.00 contributed costs you about $0.76 in take-home

Add state income tax savings and the effective cost drops even further. In a state with 5% income tax, a worker in the 22% federal bracket saves 27% total — meaning every dollar in traditional 401(k) only costs about $0.73 in reduced take-home pay.

With Roth 401(k), the cost is always $1.00 per dollar contributed, regardless of your tax bracket. No current savings, but no taxes on the other side.

How to Decide: Traditional vs. Roth

The right choice depends on your current tax bracket versus what you expect in retirement:

Traditional is better for take-home pay now. If you're in a higher tax bracket today and expect to be in a lower bracket in retirement, traditional gives you more money in each paycheck and defers taxes to when your rate is lower.
Roth is better if you expect higher taxes later. If you're in a low bracket now — early in your career, earning less — paying taxes now at a lower rate and withdrawing tax-free later can save more overall.
Many workers split the difference. Contributing some to traditional and some to Roth gives you tax diversification in retirement.

HSA Contributions Save Even More

If you have access to a Health Savings Account (HSA), it's worth knowing that HSA contributions use a different tax treatment than 401(k). HSA contributions are classified as “exempt all” — they reduce your federal income tax, state income tax, and FICA taxes. That means every dollar in an HSA saves you an additional 7.65% compared to the same dollar in a traditional 401(k).

On a $100 contribution, an HSA saves you roughly $34.65 in taxes (at 22% federal + 5% state + 7.65% FICA), while a traditional 401(k) saves about $27 (22% federal + 5% state only). If you want to understand how all your deductions affect your paycheck, the tax treatment category is what determines the savings.

What the App Shows You

TakeHome IQ lets you add either a traditional or Roth 401(k) contribution to your paycheck estimate. Set a fixed dollar amount or a percentage of gross, and instantly see how your federal tax, FICA, and net pay change. You can toggle between traditional and Roth to compare the impact side by side.

The app handles the tax treatment correctly behind the scenes — traditional 401(k) reduces your federal and state taxable income but not FICA, while Roth 401(k) does not reduce any taxable income. No guessing required.

If you're wondering why your paycheck seems small, your 401(k) contribution is often part of the answer — but the tax savings mean it's not as expensive as it looks.

See How Changing Your 401(k) Changes Your Take-Home

Whether you're starting a new 401(k), increasing your contribution rate, or deciding between traditional and Roth, the best way to understand the impact is to see the actual numbers. Enter your gross pay, add your contribution, and watch how your net pay changes. You might find that saving for retirement costs less per paycheck than you thought — especially with traditional contributions.

For more ways to keep more of your paycheck, see our guide on how to increase your take-home pay.

Frequently Asked Questions

Does 401(k) reduce Social Security tax?

No. Traditional 401(k) contributions reduce your federal and state taxable income, but they do not reduce FICA taxes. You still pay the full 6.2% Social Security tax and 1.45% Medicare tax on every dollar you contribute to a traditional 401(k). Only deductions classified as “exempt all” — like HSA and FSA contributions — reduce FICA.

Is Roth 401(k) better for take-home pay?

No — Roth 401(k) gives you lower take-home pay than traditional 401(k) for the same contribution amount. Roth contributions are after-tax, meaning they do not reduce your federal, state, or FICA taxes. The full contribution comes directly out of your net pay. The advantage of Roth is that your withdrawals in retirement are completely tax-free.

How much should I contribute to my 401(k)?

A common starting point is enough to get your full employer match — that's free money you shouldn't leave on the table. Beyond that, contribute as much as you can afford without creating financial stress. Use the app to see the actual impact: a 6% traditional contribution might only reduce your net pay by about 4–5% because of the tax savings. That gap between contribution rate and take-home impact is the reason pre-tax retirement saving is so powerful.