FICA Wage Base and How It Drives Plan Limits
The Social Security wage base — the FICA OASDI cap — is one of the most-cited numbers in payroll. It also quietly drives the indexing of nearly every other retirement-plan limit in the Internal Revenue Code. For 2025 the base is $176,100, up from $168,600 in 2024.
What the wage base actually does
Under IRC §3121(a)(1) and §230 of the Social Security Act, OASDI (the 6.2% employee + 6.2% employer Social Security tax) applies only to wages up to the wage base. Earnings above the base are exempt from OASDI but remain subject to the 1.45% Medicare tax with no cap, plus the 0.9% Additional Medicare Tax above $200,000 single / $250,000 MFJ under IRC §3101(b)(2).
The base is reset annually by the Social Security Administration based on the national average wage index. The SSA published the 2025 base in its October 2024 fact sheet.
Why it drives retirement-plan limits
Many IRC retirement-plan dollar caps are not independently indexed — they are linked to the wage base through formulas in §415(d). When the wage base rises, the linked limits rise the following year. Examples:
- §401(a)(17) compensation limit ($350,000 in 2025): pegged to a multiple of the wage base.
- §414(q) highly compensated employee threshold ($160,000 in 2025): used in plan nondiscrimination testing.
- §415(b) defined-benefit dollar limit ($280,000 in 2025): wage-base-linked.
- Key-employee compensation threshold ($230,000 in 2025) under §416(i).
The SECURE 2.0 catch-up threshold
The $145,000 FICA-wage threshold that triggers mandatory Roth catch-ups starting in 2026 (SECURE 2.0 §603) is denominated in the FICA wage definition specifically because Congress wanted a number already on the W-2 (Box 3) for administrative simplicity. It is indexed separately for inflation, not tied to the OASDI wage base.
Worked example
A married executive earning $400,000 of W-2 wages in 2025 pays:
- OASDI (6.2% × $176,100): $10,918
- Medicare (1.45% × $400,000): $5,800
- Additional Medicare Tax (0.9% × ($400,000 − $250,000 MFJ threshold)): $1,350
- Total employee FICA: $18,068
The employer pays the matching 6.2% OASDI and 1.45% Medicare. The Additional Medicare Tax is employee-only.
Common mistakes
- Confusing the wage base with the compensation limit. The OASDI base ($176,100) caps the Social Security tax. The §401(a)(17) compensation limit ($350,000) caps how much pay an employer plan can recognize. They are different ceilings.
- Two employers, two bases. If you had two W-2 employers in 2024 and both withheld OASDI on the full wage base, you overpaid. Reclaim the excess as a credit on Schedule 3, Line 11 of Form 1040.
- Forgetting the SE-tax mirror. Self-employed individuals pay the entire 12.4% OASDI on net SE earnings up to the wage base, plus 2.9% Medicare with no cap. Half is deductible above the line.
Sources
- Social Security Administration, "Contribution and Benefit Base" (annual): ssa.gov/oact/cola/cbb.html
- Internal Revenue Code §3121, FICA wages (Cornell LII): law.cornell.edu/uscode/text/26/3121
- Internal Revenue Code §415(d), cost-of-living adjustments for plan limits: law.cornell.edu/uscode/text/26/415
- IRS Notice 2024-80, 2025 plan-limit COLA table: irs.gov/pub/irs-drop/n-24-80.pdf
- IRS Publication 15 (Circular E), employer's tax guide: irs.gov/forms-pubs/about-publication-15
RetirementCheck101 uses the same 2025 limit table the IRS publishes — no stale numbers. Explore the free educational tool.