Payroll deductions are amounts withheld from gross pay before you receive your net paycheck. They fall into two categories: pre-tax deductions, which reduce your taxable income for federal income tax and in some cases FICA; and post-tax deductions, which come out after taxes are calculated. Understanding payroll deductions explains why your net pay is consistently lower than your stated salary or hourly rate.
- Pre-tax deductions reduce federal income tax withholding. Section 125 cafeteria plan deductions, such as health insurance premiums and FSA contributions, also reduce Social Security and Medicare (FICA) taxes.
- Traditional 401(k) contributions are pre-tax for income tax but still subject to FICA. You pay Social Security and Medicare on the full gross pay before the 401(k) deduction.
- Post-tax deductions, such as Roth 401(k) contributions, do not reduce any current-year taxes.
- For tax year 2025, the 401(k) employee contribution limit is $23,500. The health FSA limit is $3,300. The HSA limit is $4,300 for self-only coverage and $8,550 for family coverage.
- Wage garnishments are post-tax. They reduce take-home pay but have no effect on income tax withholding or FICA.
What Are Payroll Deductions?
Payroll deductions are all amounts subtracted from an employee's gross pay each pay period. Some are mandatory. Others are voluntary.
Mandatory deductions are required by federal or state law. An employee cannot opt out of them.
- Federal income tax withholding (based on W-4 elections and IRS Publication 15-T tables)
- Social Security tax: 6.2% on wages up to $176,100 for tax year 2025
- Medicare tax: 1.45% on all wages; an additional 0.9% applies to wages above $200,000
- State income tax withholding (applicable in most states)
- Court-ordered wage garnishments (child support, creditor judgments)
Voluntary deductions are elected by the employee, usually during open enrollment or a qualifying life event.
- 401(k), 403(b), or 457(b) retirement contributions
- Health, dental, and vision insurance premiums
- Health Savings Account (HSA) contributions
- Flexible Spending Account (FSA) contributions
- Commuter or transit benefit contributions
- Supplemental life insurance premiums
- Charitable payroll deductions
Pre-Tax vs Post-Tax Payroll Deductions: What Is the Difference?
Pre-tax deductions are subtracted from gross pay before taxes are calculated. They reduce the income that is subject to federal income tax withholding. Some also reduce the income subject to FICA taxes.
Post-tax deductions are subtracted after all taxes have been calculated and withheld. They have no effect on any current-year tax obligation.
The table below shows the most common payroll deductions and how each one is treated for income tax and FICA purposes.
| Deduction Type | Reduces Income Tax Withholding? | Reduces FICA (SS + Medicare)? |
|---|---|---|
| Health / Dental / Vision Premiums (Section 125) | Yes | Yes |
| Health FSA Contributions (Section 125) | Yes | Yes |
| HSA via Cafeteria Plan (Section 125) | Yes | Yes |
| Commuter Benefits (Section 132, within limits) | Yes | Yes |
| Traditional 401(k) / 403(b) Contributions | Yes | No |
| Traditional 457(b) Contributions | Yes | No |
| Roth 401(k) Contributions | No | No |
| Wage Garnishments | No | No |
The most important distinction in this table: traditional 401(k) contributions are pre-tax only for income tax. FICA is still owed on the gross pay before the 401(k) amount is removed. This surprises many employees who expect their Social Security and Medicare deductions to be lower after enrolling in a retirement plan.
Common Pre-Tax Payroll Deductions Explained
Health, Dental, and Vision Insurance Premiums
Premiums paid through a Section 125 cafeteria plan are excluded from federal income tax wages and FICA wages. The employer must operate a properly established Section 125 plan for this exclusion to apply. Employees save on Social Security, Medicare, and income tax on the full premium amount each pay period.
For example, a biweekly health premium of $120 reduces both the income tax withholding base and the FICA taxable wage base by $120 per period. Over 26 pay periods, the annual FICA savings on a $120 biweekly premium is approximately $239 (7.65% of $3,120).
Traditional 401(k) and 403(b) Contributions
Employee elective deferrals to a traditional 401(k) or 403(b) reduce federal income tax withholding each pay period. They do not reduce FICA. Social Security and Medicare are calculated on the full gross pay amount before the deferral is subtracted.
For tax year 2025, the employee contribution limit is $23,500. Employees age 50 and older can contribute an additional $7,500, for a total of $31,000.
Health Flexible Spending Account (FSA)
Health FSA contributions through a Section 125 plan are excluded from both income tax and FICA wages. For tax year 2025, the health FSA annual contribution limit is $3,300. FSA funds must generally be used within the plan year, though the plan may allow a limited carryover or a grace period extension.
Health Savings Account (HSA)
HSA contributions made through payroll under a Section 125 cafeteria plan are excluded from income tax and FICA. An HSA requires enrollment in a qualifying high-deductible health plan (HDHP). For tax year 2025, the contribution limit is $4,300 for self-only HDHP coverage and $8,550 for family coverage. Unlike an FSA, HSA funds roll over indefinitely with no use-it-or-lose-it rule.
Commuter and Transit Benefits
Under IRC Section 132(f), employees can exclude qualified transit passes and vanpool expenses from income tax and FICA up to $325 per month for tax year 2025. Qualified parking at or near the workplace is also excludable up to $325 per month. Amounts above the monthly limit are taxable.
Common Post-Tax Payroll Deductions
Roth 401(k) Contributions
Roth 401(k) contributions use after-tax dollars. They reduce neither income tax withholding nor FICA taxes in the current pay period. The benefit of a Roth 401(k) is deferred: contributions grow tax-free, and qualified distributions in retirement are also tax-free. The same $23,500 annual limit (2025) applies to combined traditional and Roth 401(k) contributions.
Group Term Life Insurance Over $50,000
Employer-provided group term life insurance above $50,000 in coverage generates imputed income under IRC Section 79. The IRS determines the cost of the excess coverage using age-based rates. That cost is added to taxable wages each pay period, increasing the amount on your W-2. This is not a paycheck deduction but a taxable addition, which explains why some employees see a small, unexplained increase in federal withholding and FICA.
Wage Garnishments
Court-ordered wage garnishments are subtracted after taxes. They do not affect income tax withholding or FICA. Federal law under the Consumer Credit Protection Act limits how much of disposable earnings can be garnished each pay period. Garnishments are reported to the affected employee through the garnishment notice, not through standard paycheck deduction codes.
Union Dues and Other Post-Tax Deductions
Union dues, charitable payroll contributions, and supplemental insurance premiums not offered through a Section 125 plan are post-tax. They reduce take-home pay but have no effect on tax calculations.
How Payroll Deductions Affect Your Take-Home Pay
The following example shows how pre-tax and post-tax deductions interact with federal income tax and FICA withholding for a single filer.
Employee: Sarah, single filer, $65,000 annual salary, paid biweekly (26 pay periods).
Gross pay per period: $2,500.00
Voluntary deductions per period:
Health insurance premium (Section 125): $120.00
Traditional 401(k) contribution (5% of gross): $125.00
Health FSA (Section 125): $25.00
FICA taxable wages: $2,500.00 minus $120.00 (health) minus $25.00 (FSA) = $2,355.00
The 401(k) contribution does not reduce FICA. Social Security and Medicare are calculated on $2,355.00.
Income tax withholding base: $2,500.00 minus $120.00 minus $125.00 minus $25.00 = $2,230.00
Withholdings per period (approximate):
Federal income tax: ~$186.00 (2020+ Form W-4 assumed, standard withholding, 2025 Pub 15-T annualized method, single, Step 1 only — math: $2,230 × 26 = $57,980 annualized income tax base; minus $15,750 standard deduction (TY 2025 per P.L. 119-21 §70102) = $42,230 taxable; 10% on first $11,925 + 12% on remaining $30,305 = $4,829.10 annual ÷ 26 = ~$185.73 per period)
Social Security (6.2% of $2,355.00): $146.01
Medicare (1.45% of $2,355.00): $34.15
Estimated Federal Take-Home Pay: approximately $1,864
($2,500.00 minus $120.00 minus $125.00 minus $25.00 minus $186.00 minus $146.01 minus $34.15)
This figure reflects federal withholding only. State income tax, local taxes, and any additional voluntary deductions are not included. Actual net pay will be lower in states with income tax.
The FIT amount assumes a 2020+ redesigned Form W-4 with no Step 2, 3, or 4 elections beyond the standard deduction. Pre-2020 W-4s and any step elections will produce a different result. Use the Paycheck Calculator to enter your exact W-4 elections.
This example illustrates the key FICA point: Sarah's Social Security and Medicare are calculated on $2,355, not on $2,230. The 401(k) deduction of $125 is excluded from income tax but not from FICA. Her Section 125 deductions ($145 total) reduce both bases.
At LMN Tax Inc., we frequently see employees confused when their paystub shows Social Security taxes on a higher amount than expected after enrolling in a 401(k). The 401(k) deferral reduces income tax withholding but not FICA. This is the single most common misconception we encounter in payroll deduction reviews. A second common issue: health insurance premiums deducted outside of a Section 125 cafeteria plan. Without the Section 125 wrapper, those premiums are post-tax and provide no FICA savings. Employers operating small group plans should confirm their plan document is formally established under Section 125 to preserve the FICA exclusion for both the employer and the employee.
When This Breaks: Situations Where Standard Deduction Rules Do Not Apply
- Health insurance premiums offered outside of a Section 125 cafeteria plan are post-tax. They do not reduce income tax or FICA. The employer must have a formally established Section 125 plan document.
- FSA contributions above the annual IRS limit ($3,300 for tax year 2025) are not excludable. Contributions exceeding the cap create taxable income and must be corrected through a payroll adjustment.
- 401(k) deferrals above the IRS annual limit ($23,500 for tax year 2025) are treated as taxable income. Excess deferrals must be returned to the employee by April 15 of the following year to avoid double taxation.
- Imputed income from group term life insurance above $50,000 in employer-provided coverage increases taxable wages each pay period. Employees may see slightly higher withholding than expected from gross pay alone.
- Wage garnishments follow court-ordered priority rules. Child support garnishments take priority over creditor garnishments. Multiple simultaneous garnishments require the employer to apply the Consumer Credit Protection Act limits before distributing withheld amounts.
Enter your gross pay, filing status, and benefit elections to see federal income tax, Social Security, and Medicare per period.
Enter your hourly rate and hours worked to see a full per-period withholding breakdown for 2025.
Frequently Asked Questions About Payroll Deductions
Once you understand how deductions affect your taxable income and FICA base, the next step is confirming the numbers match your actual paycheck. The Take-Home Pay Calculator applies the FICA treatment rules from this guide directly: enter your 401(k), health insurance, FSA, HSA, and post-tax deductions to see your exact net pay after every line.
For a base federal withholding estimate without benefit deductions, the Paycheck Calculator and Hourly Paycheck Calculator cover income tax, Social Security, and Medicare. To understand how each line on your paycheck stub corresponds to these deductions, see the How to Read a Pay Stub guide.
- IRS Publication 15 (Circular E, Employer's Tax Guide), 2025 — including Special Rules for Various Types of Services and Payments (Section 125 FICA exclusion)
- IRS Publication 969 (Health Savings Accounts and Other Tax-Favored Health Plans), 2025
- IRS Retirement Topics: 401(k) and Profit-Sharing Plan Contribution Limits (2025 limit $23,500; catch-up $7,500)
- IRS Rev. Proc. 2024-40 — 2025 FSA limit ($3,300) and commuter/transit monthly limit ($325)
- IRS Rev. Proc. 2024-25 — 2025 HSA limits (self-only $4,300; family $8,550)