The short answer
Pre-tax deductions come out of gross pay before payroll taxes are calculated, so they shrink the wages your taxes are figured on. Post-tax deductions come out after taxes are calculated — they reduce take-home pay without touching the tax bill.
Same dollar, same paycheck, different outcome. A $200 health premium run pre-tax instead of post-tax changes what the employee owes, what the employer owes, and three separate boxes on the W-2.
There is no single "taxable wage"
Every paycheck produces at least three wage figures:
- Federal income tax wages — W-2 Box 1
- Social Security wages — Box 3, capped at the annual wage base
- Medicare wages — Box 5, uncapped
A deduction can reduce all three, some of them, or none. That is the whole game.
Section 125 deductions reduce all three
Qualified benefits under a Section 125 cafeteria plan — employee-paid health, dental, and vision premiums, health FSA, dependent care FSA, and HSA contributions run through the plan — are generally exempt from federal income tax withholding, Social Security, Medicare, and FUTA.
Two exceptions worth knowing: group-term life coverage over $50,000 and adoption assistance stay subject to Social Security and Medicare even though they escape income tax withholding.
The "through the plan" part matters for HSAs: contribute by payroll deduction and you skip FICA; contribute to the same HSA from your bank account instead and you get the income tax deduction on your return but pay FICA on the money.
Traditional 401(k) deferrals reduce only income tax
Elective deferrals to a traditional 401(k), 403(b), or governmental 457(b) are excluded from federal income tax wages but remain subject to Social Security, Medicare, and FUTA. You never escape FICA on retirement deferrals. You defer income tax, nothing else.
That distinction is the most commonly botched item in small-business payroll.
Post-tax deductions reduce nothing but net pay
Roth 401(k) contributions, wage garnishments, union dues, most voluntary life and disability premiums, charitable payroll deductions, and repayments to the employer all come out of already-taxed money.
A worked example
Biweekly gross $2,500, with a $200 Section 125 health premium and a $125 traditional 401(k) deferral.
- Federal income tax wages: $2,500 − $200 − $125 = $2,175
- Social Security and Medicare wages: $2,500 − $200 = $2,300 — the 401(k) does not touch these
- Employee FICA: $2,300 × 6.2% = $142.60, plus $2,300 × 1.45% = $33.35 → $175.95
- The employer owes a matching $175.95
Federal income tax withholding then comes off the $2,175 using the employee's Form W-4 and the methods in IRS Publication 15-T. It depends on filing status and W-4 entries, so nobody can quote it generically. Post-tax deductions come out of what remains.
Now run that same $200 premium as post-tax. Social Security and Medicare wages go back up to $2,500 and FICA becomes $191.25 — $15.30 more from the employee and $15.30 more from the employer, every check. Across 26 pay periods that is roughly $398 each. And $200 more per check is exposed to income tax on top of it.
Four places the order actually moves money
1. Section 125 saves the employer too. A 401(k) doesn't. Cafeteria plan deductions come out before FICA, so the employer's 7.65% share shrinks alongside the employee's. Retirement deferrals give the employer nothing. Running health premiums post-tax when a cafeteria plan is available is the most expensive shrug in small-business payroll — and the cost of entry is a written plan document.
2. Disability premiums flip the taxability of the benefit. Pay long-term disability premiums with post-tax dollars and the benefits generally arrive tax-free. Run the same premiums pre-tax and the benefits are generally taxable, right when you need them. That is a trade, not an error — make it on purpose.
3. Garnishments ignore voluntary pre-tax deductions. Under the Consumer Credit Protection Act, garnishments are computed on disposable earnings: pay after deductions *required by law* — taxes, the employee's FICA, state unemployment, and legally required retirement withholding. Health premiums and voluntary retirement contributions are not subtracted. If anything, a pre-tax deduction nudges disposable earnings up slightly, by lowering the withholding that is subtracted. Ordinary garnishments cap at the lesser of 25% of disposable earnings or the amount above 30 times the federal minimum hourly wage (still $7.25 as of 2026, so $217.50 a week); child support runs higher. Where state law is more protective, it controls.
4. Pre-tax health premiums shrink your Social Security earnings record. Lower Social Security wages now mean a slightly lower figure in the benefit formula later. The near-term savings usually win, but the tradeoff is real.
The 2026 numbers — and check them again next year
Calendar-year 2026 figures. Every one changes annually except where noted, so confirm against the current publication before you rely on it.
- 401(k)/403(b)/457(b) elective deferral: $24,500; age-50 catch-up $8,000; ages 60–63 $11,250 ([IRS Notice 2025-67](https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500))
- Health FSA salary reduction: $3,400; maximum carryover $680, which does not count against the limit ([Rev. Proc. 2025-32](https://www.irs.gov/pub/irs-drop/rp-25-32.pdf))
- HSA: $4,400 self-only, $8,750 family, plus $1,000 at age 55+ ([Rev. Proc. 2025-19](https://www.irs.gov/pub/irs-drop/rp-25-19.pdf))
- Dependent care FSA: $7,500 ($3,750 married filing separately), raised from $5,000 under P.L. 119-21
- Social Security wage base: $184,500 at 6.2%; Medicare is 1.45% with no cap ([SSA](https://www.ssa.gov/oact/cola/cbb.html))
- Additional Medicare Tax: 0.9% on wages above $200,000, withheld from the employee only, with no employer match. That threshold is set in statute and is not inflation-adjusted, so it does not move year to year.
State and local rules do not always mirror federal treatment. Check your state revenue department's withholding guide rather than assuming.
Put it on the pay statement correctly
Whatever order you run, the statement has to show it. Put every deduction on its own line, label pre-tax and post-tax separately, and show current and year-to-date columns. An employee who cannot tell which deductions reduced their taxable wages cannot check your math — and neither can you, twelve months later, when the W-2 disagrees with the payroll register.
[UWageCo](https://uwageco.com) prepares pay statements and related business documents from the figures you supply. Each one carries a disclosure that it was generated from customer-supplied data, and any filing with the IRS or a state agency stays with you or your accountant.
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*General information, not legal or tax advice. Deduction treatment depends on your plan documents, your state, and your facts — talk to a CPA or payroll professional, or contact the agency directly, before changing anything.*
Sources: [IRS 2026 retirement plan limits](https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500) · [IRS Publication 15-B (2026)](https://www.irs.gov/publications/p15b) · [IRS Topic 751](https://www.irs.gov/taxtopics/tc751) · [SSA Contribution and Benefit Base](https://www.ssa.gov/oact/cola/cbb.html) · [DOL Fact Sheet #30](https://www.dol.gov/agencies/whd/fact-sheets/30-cppa)