Pretax vs. Post-Tax Deductions: How Each Changes Your Paycheck

Key takeaways

• Pretax does not always mean exempt from every payroll tax.
• Traditional 401(k) deferrals generally reduce federal income-tax wages but not Social Security or Medicare wages.
• Qualified health benefits elected through a section 125 cafeteria plan are often excluded from federal income-tax and FICA wages.
• Roth 401(k), union dues and many voluntary deductions are generally taken after current federal taxes.
• State tax treatment, benefit eligibility and the employer plan document can change the result.

What pretax and post-tax mean

Payroll starts with gross compensation, then determines several tax bases. Federal income-tax wages, Social Security wages, Medicare wages and state taxable wages can differ. A deduction may reduce one base without reducing another. This is why subtracting all deductions from gross pay before estimating taxes often gives a wrong answer.

Under a qualifying section 125 cafeteria plan, an employee may choose certain qualified benefits instead of cash. IRS Publication 15-B explains that qualified benefits can be provided on a pretax basis and, in most cases, excluded from federal income-tax withholding, Social Security and Medicare tax. By contrast, the IRS says a traditional 401(k) employee deferral is excluded from current federal income-tax wages but remains subject to Social Security and Medicare taxes.

These are general federal patterns, not a classification of a specific employer’s plan. For example, an HSA payroll contribution made through a section 125 salary-reduction arrangement can receive different employment-tax treatment from an ordinary payroll deduction paid outside that arrangement.

A simplified $100 deduction example

Assume gross pay is $2,000, the worker has not reached the 2026 Social Security wage base, and a qualified section 125 health premium is $100. The employee FICA rate on covered wages is normally 7.65%, consisting of 6.2% Social Security and 1.45% Medicare. Excluding $100 from both FICA taxes would reduce those two employee taxes by $7.65. Federal income-tax withholding could also fall, but the amount depends on Form W-4 and Publication 15-T.

If the same $100 were a traditional 401(k) contribution, Social Security and Medicare withholding would generally remain unchanged. Federal income-tax withholding might decline. If it were a Roth 401(k) contribution, neither current federal income-tax wages nor FICA wages would normally be reduced. In every case, $100 still goes to the selected benefit or account; the difference is how payroll calculates taxes around it.

Why taxable wages can be lower than gross pay

Pay stubs often display gross pay, federal taxable wages and FICA wages separately. That is useful rather than contradictory. Start with gross earnings and identify each deduction’s tax treatment. A qualified pretax health deduction may reduce all three federal wage figures, while a traditional retirement deferral may reduce only federal income-tax wages.

Imputed income can work in the opposite direction. A taxable fringe benefit may add to one or more taxable wage bases even though no equivalent cash was paid. IRS Publication 15-B notes that the taxable cost of employer-provided group-term life insurance above $50,000 of coverage is generally included in Social Security and Medicare wages, with special federal income-tax-withholding treatment.

Questions to ask before changing a deduction

• Is the election governed by a cafeteria plan, retirement plan or another written benefit plan?
• Which federal, state and local tax bases does payroll reduce?
• Can the election be changed now, or only after a qualifying event or during open enrollment?
• Does the employer contribute or match, and are there eligibility or vesting conditions?
• Will the deduction affect benefit calculations, overtime base pay or another employer program?

Where this comes from

• IRS Publication 15-B (2026): fringe benefits and cafeteria plans (opens in a new tab)
• IRS Publication 15 (2026): employment taxes (opens in a new tab)
• IRS: tax treatment of retirement contributions (opens in a new tab)
• IRS: Roth accounts in retirement plans (opens in a new tab)