Pre-Tax vs Roth 401(k): How Each Changes Your Paycheck
Traditional and Roth 401(k) choices hit your paycheck differently: FICA never moves, income-tax withholding does, and the estimate math shows the gap on real numbers.
Choosing between a traditional pre-tax 401(k) and a Roth 401(k) is usually framed as a far-off question about retirement tax rates, but the decision changes something much nearer: this month's paycheck. The two contribution types treat today's withholding in completely different ways, and the difference is visible the moment you change your election. This guide walks the paycheck math for both, using one worked example so the numbers stay concrete, and it is precise about the boundary most guides blur: retirement contributions of either kind do not reduce FICA, ever. What changes is the income-tax side, and only for the traditional flavor. One honest scope note applies throughout: this is the arithmetic of take-home pay, not investment advice, and whether pre-tax or Roth is better for you depends on your tax situation now and later, which is a question for a planner or tax professional with your full picture.
CHAPTER 01Two Accounts, One Word Apart, Different Paychecks
Both contribution types move money from your paycheck into your retirement account, and both show up in the deductions block of your stub. The difference is entirely about when the income tax attaches. A traditional, or pre-tax, contribution is excluded from your taxable wages for income-tax withholding purposes this period; a Roth contribution is made from pay that has already been taxed. Same dollars out of your check, different tax timing.
That timing difference is why the same percentage produces different net pay. Elect 5 percent traditional and your income-tax withholding base shrinks by the deferral, softening the hit to take-home. Elect 5 percent Roth and your withholding base stays at full gross, so the whole 5 percent comes straight out of what lands in your bank. Neither is wrong; they are buying the same future with different present costs, and seeing the paycheck cost of each side by side makes the choice less abstract.
A caution about the phrase pre-tax, because it is broader than people assume. Pre-tax for income tax does not mean untaxed for everything. Payroll computes FICA on gross before your deferral either way, so Social Security and Medicare lines do not care which flavor you chose. The word pre-tax refers specifically to the income-tax lines on your stub, and every calculation below keeps that boundary intact.
CHAPTER 02Traditional 401(k): The Deferral That Softens the Hit
Work the example numbers: gross pay of $2,187.50, built from $25 an hour, 80 regular hours and 5 overtime hours at time and a half. Elect a 5 percent traditional deferral and the contribution is $109.38. For income-tax withholding, your base becomes gross minus the deferral, about $2,078.13, so a rough 12 percent-tier withholding estimate on that base is about $249.38 instead of the $262.50 it would be on the full gross. The deferral reduced your withholding by roughly $13.12 this period.
Now assemble the check. FICA is computed on the full gross and stays $167.34. Net pay is gross, minus the $109.38 deferral, minus FICA, minus the $249.38 withholding estimate, leaving about $1,661.41. That is the softening effect in action: you contributed $109.38 to retirement but your take-home fell by less than that relative to a no-contribution world, because the income-tax line shrank along with the deferral.
The effect scales with your withholding rate, which is one reason the same election feels different at different incomes. The higher the income-tax rate that applies to the deferred dollars, the bigger the offset, and the cheaper the deferral feels on this month's check. Simple estimators handle this with a rough tier percentage applied to the reduced base, and the honest ones label the tier an estimate, because real withholding follows your W-4 through progressive brackets rather than one flat number.
CHAPTER 03Roth 401(k): Paying Tax Now, Straight From the Check
Switch the same 5 percent election to Roth and the mechanics change immediately. Roth contributions are not excluded from taxable wages, so your income-tax withholding base stays at the full $2,187.50, and the rough-tier withholding stays at its full figure, about $262.50 on the same assumptions. The Roth contribution itself, $109.38, then comes out of the check on top of normal taxes, so net pay lands noticeably lower than the traditional version of the same election.
That lower take-home today is the price of the Roth bargain: the money grows and, at withdrawal under current rules, comes out tax-free, having been taxed at today's rates. On the stub, the Roth contribution usually prints as its own deduction row so payroll can keep the tax treatment straight. Some plans let you split an election between traditional and Roth, in which case the stub shows both rows and only the traditional part reduces the withholding base.
If you want to model a Roth contribution with a tool that computes traditional-style, be explicit with yourself about what you are doing: set the 401(k) field to zero, read the net, then subtract the Roth amount you intend to contribute from that net mentally. That two-step reproduces the Roth paycheck exactly, because the only mechanical difference is that Roth dollars do not shrink the withholding base. Label the comparison honestly and the numbers stay trustworthy.
CHAPTER 04FICA Never Moves: The Line That Ignores Your Election
Here is the sentence worth taping inside your mental payroll department: no 401(k) flavor, percentage, or match changes FICA. Social Security at 6.2 percent and Medicare at 1.45 percent apply to gross wages before any salary deferral, traditional or Roth, under the law that defines FICA wages. On the example paycheck, FICA is $167.34 whether you contribute nothing, 5 percent traditional, or 5 percent Roth.
This surprises people because the deferral visibly shrinks other lines, and it is the single most common error in napkin math about retirement contributions. If you predicted that a $109.38 deferral would save you 7.65 percent of $109.38 in FICA, the stub will disappoint you, because that saving never existed. Contributions lower your income-tax withholding; FICA stands apart, computed on full gross, with its own annual wage base of $184,500 for 2026 on the Social Security side.
The boundary has a practical consequence for paycheck comparisons. When you model a contribution change, the honest comparison holds FICA constant and moves only the deferral and the income-tax line. An estimator that keeps the two bases separate, the way Toolfyra's pay stub generator does, produces that comparison natively; a calculator that applies every percentage to a single shrinking base will overstate your FICA savings and understate your net, drifting from reality every period.
CHAPTER 05Same Take-Home Today Is Not the Same Money Later
It is tempting to stop at the paycheck and pick whichever flavor leaves more in the bank this month, but the two contributions are not buying identical futures. Traditional dollars were never income-taxed, so withdrawals are taxed as income under the rules then in force. Roth dollars were taxed before contribution, so qualified withdrawals come out free of tax. The correct comparison therefore depends on the tax rates that actually apply to you at contribution and at withdrawal, which no simple paycheck calculator can know.
Honest tools say so. What a paycheck-level calculator can show precisely is today's arithmetic: the deferral amount, the FICA line that does not move, the withholding estimate on the reduced or unreduced base, and the resulting net. What it cannot do is tell you whether your retirement tax rate will exceed your current one, which is the entire crux of the traditional-versus-Roth decision. Treat any tool or article, including this one, that skips that uncertainty as overselling certainty it does not have.
A reasonable workflow uses both kinds of tools for what they are good at. A paycheck tool shows what each election costs you per month and lets you budget accordingly; a conversation with a tax professional or plan advisor weighs rates now versus expected later, your bracket trajectory and your other income. The paycheck arithmetic in this guide is the piece you can verify yourself today, on your own numbers, before anyone makes the long-run call.
CHAPTER 06Modeling Both Sides Without Guessing
Run the comparison as two estimate passes with one variable changed. First pass: your gross, hours and deductions with the traditional percentage you are considering; read the deferral, the withholding estimate and the net. Second pass: the same inputs with the 401(k) field at zero, representing Roth, then subtract your intended Roth contribution from the displayed net yourself. The difference between the two nets is the real monthly cash-flow gap between the flavors on your numbers.
Anchor the passes to reality before trusting them. Enter a recent employer-issued stub's inputs and adjust the rough withholding percentage until the estimated net matches the stub's net; from then on, that tuned percentage makes every comparison meaningful, because the model agrees with payroll on your actual situation. Re-tune after a W-4 change, a raise, or a move that changes state taxes, since those are exactly the events that shift the withholding rate.
Finally, keep the estimate where it belongs: your records and your planning. The stub you hand a lender or landlord is the one your employer's payroll issued, and the estimate that helped you decide between traditional and Roth is private working paper that made you a more informed participant in that decision. For deeper bracket-level modeling, the general paycheck calculator on Toolfyra handles full withholding scenarios, and the pay stub generator shows the per-period breakdown this guide used throughout.
🔑 Key takeaways
- The paycheck difference between traditional and Roth is entirely on the income-tax side: traditional deferrals shrink the withholding base, Roth contributions do not.
- FICA ignores both flavors: 7.65 percent of full gross ($167.34 on a $2,187.50 example) regardless of your election.
- A 5 percent traditional deferral of $109.38 cuts the example's withholding base to about $2,078.13, so net falls by less than the contribution.
- To model Roth with a traditional-style tool, set the 401(k) field to zero, read net, then subtract your intended Roth amount yourself.
- Tune any rough withholding percentage until your estimate matches a real employer-issued stub, and re-tune after W-4, raise or state changes.
- The paycheck math tells you what each election costs per month; whether pre-tax or Roth wins overall depends on future tax rates, which is planner territory.
❓ Frequently asked questions
Does a Roth 401(k) lower my FICA?
No. FICA applies to gross wages before any deferral, traditional or Roth. On the example gross of $2,187.50, FICA is $167.34 in both cases; only the income-tax withholding line differs.
Why is my net pay higher with a traditional 401(k)?
The deferral reduces the wages your income-tax withholding applies to, so the withholding line shrinks along with the contribution. Part of the deferral is effectively offset by lower withholding this period.
How does the employer match fit in?
Matches are typically made as pre-tax money and are not deducted from your pay, so they do not change your paycheck at all. Whether your plan taxes match or Roth elections differently is a plan-document question.
Can I split my election between traditional and Roth?
Many plans allow it, and the stub shows separate rows. Only the traditional portion reduces the income-tax withholding base; the Roth portion comes from already-taxed pay.
Does my 401(k) contribution reduce Social Security tax?
No. Salary-deferral contributions remain FICA wages. Social Security's 6.2 percent applies to gross up to the $184,500 wage base for 2026, and Medicare's 1.45 percent applies to all gross.
Which one is better for me, pre-tax or Roth?
That depends on your tax rates now versus at withdrawal, your bracket trajectory and your other income, which a paycheck calculator cannot know. Use the tools for paycheck cost, and a tax professional for the strategy call.
Does the Toolfyra tool model Roth directly?
Its 401(k) field behaves like a traditional pre-tax deferral for income tax. For Roth, set the field to zero, read the net, then subtract your intended Roth amount to see the resulting take-home.
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