Pre-Tax vs Post-Tax Deductions on Your Paycheck
The table that answers the question
| Deduction | Avoids income tax | Avoids FICA |
|---|---|---|
| Traditional 401(k) or 403(b) | Yes | No |
| Health, dental and vision premiums (Section 125) | Yes | Yes |
| Health FSA and dependent care FSA | Yes | Yes |
| HSA contributed through payroll | Yes | Yes |
| HSA contributed on your own, claimed on the return | Yes | No |
| Commuter and parking benefits | Yes | Yes |
| Employee stock purchase plan (ESPP) contributions | No | No |
| Roth 401(k) | No | No |
| Union dues, garnishments, most charitable giving | No | No |
The second and third columns are different questions, and conflating them is the most common misunderstanding about paychecks. "Pre-tax" is not one status — it depends on which tax.
Why the order on the stub matters
Pre-tax deductions are subtracted from gross before income tax withholding is computed, so they shrink the base the rates apply to. Post-tax deductions come out of money that has already been taxed and change nothing about the tax.
The saving is therefore your marginal rate, not your effective one. A $200 monthly traditional 401(k) contribution for someone whose next dollars are taxed at 22% federal and 5% state costs about $146 of take-home pay, not $200. The other $54 was going to tax either way.
The Section 125 distinction
Benefits offered through a cafeteria plan under Section 125 — workplace health premiums, FSAs, payroll HSA contributions, commuter benefits — escape income tax and FICA. That is a genuine 7.65% on top of the income tax saving, and it is why paying a health premium through payroll beats paying the same premium from your bank account.
Retirement deferrals work differently. A traditional 401(k) defers income tax, but Social Security and Medicare are charged on the full gross regardless. The trade is deliberate: those FICA-taxed wages are what your future Social Security benefit is computed from.
Deferred is not the same as free
A traditional 401(k) or a pre-tax HSA moves the tax rather than removing it, and where it lands differs:
- Traditional 401(k): no income tax now, ordinary income tax when withdrawn in retirement.
- Roth 401(k): taxed now, qualified withdrawals not taxed later. Post-tax on the stub, which is why it does not reduce this year's taxable wages.
- HSA: the unusual one — no tax going in, no tax on growth, and no tax coming out for qualified medical expenses.
- FSA: no tax going in, but the money is generally use-it-or-lose-it within the plan year and its grace provisions.
What this means for estimating a paycheck
When you enter pre-tax deductions into the take-home pay calculator, enter the annual total of the things in the first column of the table — the ones that reduce the income-taxed base. The calculator applies your income tax rates to gross minus those deductions, while charging FICA on the full gross. That is a deliberately conservative treatment: if a chunk of your deductions is Section 125 money that also escapes FICA, your real check will be slightly better than the estimate rather than slightly worse.
A planning estimate of what each paycheck deposits, built from rates you supply — your effective federal and state rates from last year’s return, not a baked-in tax table that goes stale every January.
Frequently asked questions
What is a pre-tax deduction?
Money taken out of gross pay before income tax withholding is computed, such as a traditional 401(k) contribution or a workplace health premium. It lowers the wages the tax rates apply to.
Do pre-tax deductions reduce Social Security and Medicare taxes?
Only some. Cafeteria plan benefits such as health premiums, FSAs and payroll HSA contributions avoid FICA as well as income tax. A traditional 401(k) avoids income tax only.
Is a Roth 401(k) a pre-tax or post-tax deduction?
Post-tax. The contribution is taken from pay that has already been taxed, so it does not reduce this year taxable wages, and qualified withdrawals later are not taxed.
How much does a pre-tax deduction actually save me?
Your marginal rate on the amount deducted. At a 22% federal and 5% state marginal rate, $200 of traditional 401(k) money costs about $146 of take-home pay.
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