Payroll Tax Rules for Salary Sacrifice: FICA, FUTA, and Reporting

Under U.S. tax law, the payroll tax rules for salary sacrifice arrangements depend entirely on which Internal Revenue Code section authorizes the benefit: a salary reduction routed through a Section 125 cafeteria plan is excluded from Social Security, Medicare, and federal unemployment taxes, while an elective deferral into a 401(k) or similar retirement plan reduces only federal income tax withholding and remains fully subject to FICA.¹ That single distinction drives thousands of dollars in tax differences each year, and misapplying it triggers deposit penalties that begin accruing within days of the missed payment.

The Two Legal Mechanisms Behind “Salary Sacrifice”

U.S. tax law doesn’t use the phrase “salary sacrifice.” The mechanism lives in two provisions: Section 125 of the Internal Revenue Code, which governs cafeteria plans, and the elective deferral rules for qualified retirement plans such as 401(k)s. Both let employees redirect part of their gross pay before certain taxes are calculated, but they hit different tax lines.

A Section 125 cafeteria plan is the only legal way an employer can offer workers a genuine choice between taxable cash and nontaxable benefits without that choice itself making everything taxable. The statute defines a cafeteria plan as a written plan under which all participants are employees and each may choose among two or more benefits consisting of cash and qualified benefits.² Without that formal structure, offering employees a pick-one-or-the-other option would cause the IRS to treat the entire benefit as constructively received cash.

A 401(k) salary deferral is simpler on the surface. The employee elects to have a portion of each paycheck sent straight into a retirement account instead of paid out as cash. The deferred amount doesn’t count as taxable income for federal income tax purposes. But it still counts as wages for Social Security and Medicare.³

Which Payroll Taxes Each Arrangement Reduces

The federal payroll tax system has three components: Social Security tax (6.2% each for employer and employee, up to the wage base), Medicare tax (1.45% each, with no cap), and FUTA, the federal unemployment tax paid only by the employer. How a salary reduction interacts with each depends on the authorizing code section.

Section 125 Cafeteria Plan Reductions

Salary reduction contributions under a qualifying Section 125 plan are not actually or constructively received by the employee. The IRS treats them as though they were never wages, which means they are excluded from Social Security, Medicare, and FUTA taxable wages.¹ The statutory authority sits in IRC Section 3121(a)(5)(G), which excludes payments under a cafeteria plan that would not be treated as wages if Section 125 didn’t exist, provided it’s reasonable to believe the plan wouldn’t cause wages to be constructively received.⁴

An employee earning $70,000 who redirects $6,000 through a Section 125 plan toward health insurance premiums has a FICA taxable wage of $64,000 rather than $70,000. The employer calculates its matching 6.2% Social Security and 1.45% Medicare on the lower figure. Both sides pay less.

401(k) and Other Retirement Deferrals

Elective deferrals into a 401(k), 403(b), or 457(b) plan reduce federal income tax withholding but do not reduce FICA wages. Social Security and Medicare taxes still apply to the full pre-deferral amount.³ An employee deferring $24,500 into a 401(k) still owes Social Security and Medicare tax on that $24,500, and so does the employer. The income tax savings are real, but the payroll tax bill doesn’t budge.

Employer matching and nonelective contributions are treated differently: they are not subject to Social Security or Medicare taxes.³

Common Benefits and 2026 Contribution Limits

IRS Publication 15-B lays out which fringe benefits are exempt from Social Security, Medicare, and FUTA taxes, and which are not.⁵ The benefits employers most commonly offer through salary sacrifice arrangements:

Pre-tax contributions toward employer-sponsored accident and health coverage are excluded from all payroll taxes when run through a Section 125 plan. This is the most common salary sacrifice benefit in the U.S. and typically produces the largest tax savings for both parties.
Employee HSA contributions through payroll are excluded from income tax, Social Security, and Medicare when made through a Section 125 plan. For 2026, the maximum contribution is $4,400 for self-only coverage and $8,750 for family coverage, with an additional $1,000 catch-up for employees 55 and older.
Salary reductions directed to a health care flexible spending account are excluded from all payroll taxes. The 2026 maximum employee contribution is $3,400.⁶
Dependent care FSAs carry the income tax exclusion, but dependent care assistance is still subject to Social Security, Medicare, and FUTA taxes. The 2026 household maximum is $5,000 ($2,500 if married filing separately).
The 2026 basic 401(k) elective deferral limit is $24,500, or 100% of compensation if lower. These reduce income tax but remain subject to full FICA and FUTA.⁷
Employer-paid group-term life insurance up to $50,000 in coverage is excluded from all payroll taxes. Coverage above that threshold is taxable for Social Security and Medicare purposes and must be reported in Boxes 1, 3, and 5 of Form W-2.

Employers should verify these limits annually. The IRS adjusts HSA, FSA, and 401(k) caps for inflation, and missing a limit change can cause contributions to become taxable.

Employer FICA and FUTA Savings

Every dollar an employee redirects through a Section 125 plan reduces the employer’s FICA obligation by 7.65% (6.2% Social Security plus 1.45% Medicare) on that dollar, at least until the employee’s earnings hit the Social Security wage base, which is $184,500 in 2026.⁸ Above that threshold, only the 1.45% Medicare portion applies, so the per-dollar savings drops to 1.45%.

FUTA savings are smaller but real. The federal unemployment tax applies to the first $7,000 of each employee’s wages at a nominal 6.0%, though most employers pay an effective rate of 0.6% after the standard state tax credit. For employees already above the FUTA wage base early in the year, the Section 125 reduction won’t affect FUTA. For lower-wage employees or mid-year hires, the reduction can push taxable wages below the $7,000 threshold and shave off a few dollars per person.

The Social Security Benefit Trade-Off

Section 125 salary reductions lower the wages the Social Security Administration uses to calculate future retirement benefits. Social Security benefits are based on a worker’s highest 35 years of indexed earnings, and every dollar routed through a cafeteria plan is a dollar that doesn’t appear in that earnings record. For someone consistently sacrificing $6,000 a year over a 30-year career, the cumulative effect on monthly retirement benefits can be noticeable.

This trade-off doesn’t apply to 401(k) deferrals, because those remain in FICA wages and show up in the Social Security earnings history. It also doesn’t apply to any employee who already earns above the Social Security wage base ($184,500 in 2026), since additional reductions below that ceiling don’t change the Social Security math.⁸

Setting Up a Valid Arrangement

A salary sacrifice arrangement doesn’t work just because an employee and employer agree to it. The IRS requires specific structural elements, and skipping any of them can cause the entire benefit to be treated as taxable cash.

Written Plan Document

Section 125 requires a cafeteria plan to be a written plan.² The document must describe each available benefit, spell out who is eligible to participate, set out the plan year, and explain how elections are made and how contributions flow from payroll. A verbal agreement or a casual email trail won’t hold up.¹

Prospective Elections

Employees must make their salary reduction elections before the start of the plan year, not retroactively after work has been performed. For new hires, the election must be made before the first day of coverage. If an employee tries to redirect pay for work already completed, the IRS treats that amount as constructively received cash, and the payroll tax exclusions vanish.

Irrevocability During the Plan Year

Once an election is locked in, it generally cannot be changed until the next open enrollment. Mid-year changes are only permitted when a qualifying life event occurs. The IRS regulations list specific triggers: marriage, divorce, birth or adoption of a child, death of a spouse or dependent, a change in employment status (for the employee, spouse, or dependent), loss or gain of other coverage, and a change in residence that affects available plan options.⁹ The change must also be consistent with the event that triggered it.

Nondiscrimination Testing

Section 125 plans can’t be designed to funnel tax savings primarily to owners and highly paid executives. To keep its tax-favored status, a cafeteria plan must pass three annual tests:

The eligibility test. The plan cannot restrict participation in a way that favors highly compensated individuals. It can’t require more than three years of employment for eligibility, and the covered group must be defined by objective business criteria like job category or work location, not compensation level.
The benefits and contributions test. The benefits available and the contributions offered must be uniform across the workforce. If highly compensated employees end up electing a disproportionate share of nontaxable benefits relative to compensation, the plan fails.
The key employee concentration test. No more than 25% of the plan’s total qualified benefits can go to key employees, generally officers, owners, and top earners.

When a plan fails, the tax consequences fall on the highly compensated participants, not the rank-and-file employees. The tax-favored treatment of the benefits elected by those highly compensated individuals is reversed: their salary reductions are added back to taxable wages, and the employer owes the corresponding payroll taxes.

Minimum Wage and Overtime Considerations

A salary reduction agreement cannot push an employee’s cash pay below the federal minimum wage. The Fair Labor Standards Act requires all covered non-exempt employees to receive at least the applicable minimum wage for every hour worked, regardless of any benefit arrangement.¹⁰ If a salary sacrifice would bring the effective hourly cash rate below $7.25 (or the applicable state minimum if higher), the employer cannot legally implement it.

Overtime creates a separate complication. The FLSA defines the regular rate of pay as all remuneration for employment, with only specific statutory exclusions listed in 29 USC §207(e), and that rate cannot be lowered by private agreement.¹¹ Employer-provided health insurance and retirement contributions are among the listed exclusions, so a properly structured Section 125 or 401(k) arrangement typically won’t inflate overtime calculations. An improperly documented arrangement, or one offering benefits that don’t qualify for an exclusion, could.

For exempt employees, the salary threshold matters too. If a reduction drops an exempt employee’s pay below the applicable minimum salary level, that employee loses the exemption and becomes entitled to overtime going forward.

W-2 and Form 941 Reporting

Employers report salary sacrifice amounts on Form W-2 using specific Box 12 codes. The most common are Code D for 401(k) elective deferrals, Code E for 403(b) contributions, Code W for HSA employer contributions (including employee pre-tax contributions made through payroll), and Code DD for the total cost of employer-sponsored health coverage.¹² For 2026, the W-2 instructions introduce additional codes, including Code TP for cash tips reported to the employer and Code TT for qualified overtime compensation.¹³

The critical detail: Social Security wages (Box 3) and Medicare wages (Box 5) must include 401(k) deferrals but exclude Section 125 reductions. Federal taxable wages (Box 1) exclude both. Getting these boxes wrong is one of the most common W-2 errors, and it cascades into the Form 941 quarterly filings where employers report total FICA and income tax withholding deposits.

Employers should retain the written cafeteria plan document, each employee’s signed salary reduction election form, records of gross wages before any reductions, and documentation of the benefits provided. These records must reconcile with the figures on quarterly and annual tax filings.

Penalties for Getting Deposits Wrong

When an employer miscalculates FICA-taxable wages, whether by incorrectly excluding 401(k) deferrals from Social Security wages, failing to gross up taxable fringe benefits, or underreporting Section 125 participants’ wages, the resulting shortfall in employment tax deposits triggers the failure-to-deposit penalty. The IRS calculates the penalty in tiers based on how late the correct deposit is made:¹⁴

1 to 5 days late: 2% of the unpaid deposit.
6 to 15 days late: 5% of the unpaid deposit.
More than 15 days late: 10% of the unpaid deposit.
More than 10 days after the first IRS notice or upon demand for immediate payment: 15% of the unpaid deposit.

The penalties apply to the amount that should have been deposited but wasn’t, and they compound quickly when the employer doesn’t catch the error until an IRS notice arrives. A separate 10% penalty applies when required deposits are not made by electronic funds transfer.¹⁵ Underreported wages can also ripple into incorrect W-2s, which carry their own penalty structure for filing incorrect information returns.

1
Internal Revenue Service. FAQs for Government Entities Regarding Cafeteria Plans
2
Office of the Law Revision Counsel. 26 USC 125 Cafeteria Plans
3
Internal Revenue Service. Retirement Plan FAQs Regarding Contributions
4
Office of the Law Revision Counsel. 26 USC 3121 Definitions
5
Internal Revenue Service. Publication 15-B (2026) Employers Tax Guide to Fringe Benefits
6
FSAFEDS. New 2026 Maximum Limit Updates
7
Internal Revenue Service. Retirement Topics – Contributions
8
Social Security Administration. Contribution and Benefit Base
9
eCFR. 26 CFR 1.125-4 Permitted Election Changes
10
U.S. Department of Labor. Fact Sheet 70 Frequently Asked Questions Regarding Furloughs and Other Reductions in Pay and Hours Worked Issues
11
U.S. Department of Labor. Fact Sheet 56A Overview of the Regular Rate of Pay Under the Fair Labor Standards Act
12
Internal Revenue Service. Form W-2 Reporting of Employer-Sponsored Health Coverage
13
Internal Revenue Service. General Instructions for Forms W-2 and W-3
14
Internal Revenue Service. Failure to Deposit Penalty
15
Internal Revenue Service. 20.1.4 Failure to Deposit Penalty