What this line means
The amount of Social Security tax (FICA) your employer withheld from your paychecks during the year. The rate is 6.2% of your Social Security wages (Box 3), up to the wage base cap of $176,100 for 2025. Your employer paid an additional 6.2% on your behalf — that amount does not appear on your W-2.
Does this apply to you?
- You earned wages and your employer withheld Social Security tax from every paycheck
- You want to verify that Box 4 equals 6.2% of Box 3
- You had multiple employers and need to check whether total Social Security tax withheld exceeded the maximum
Easy to overlook
Your employer matches your contribution dollar for dollar Box 4 shows only the employee portion. Your employer paid an identical 6.2% on your wages — you just never see it on your W-2 or your paycheck. The combined 12.4% funds Social Security. Self-employed workers pay the full 12.4% themselves through self-employment tax, which is why switching from W-2 employment to self-employment causes a noticeable tax increase. 1 SSA — 2025 FICA rate 6.2% employee / 6.2% employer
Multiple W-2s can mean overpaid Social Security tax Each employer withholds independently up to the $176,100 cap. If Employer A paid you $120,000 and Employer B paid you $80,000, you had Social Security tax withheld on $200,000 — but only $176,100 is subject to the tax. The excess withholding ($23,900 x 6.2% = $1,481.80) is refundable when you file your 1040. 2 IRS Schedule 3 — excess Social Security tax credit
Watch out for this
Confusing Box 4 (Social Security tax) with Box 2 (federal income tax). Box 2 is a prepayment toward your income tax that gets reconciled on your 1040. Box 4 is a flat-rate payroll tax with no reconciliation — you do not get it back unless you exceeded the wage base across multiple employers. Filers sometimes add Box 4 to their withholding on Line 25a, which overstates their payments.
Footnotes
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Social Security Administration, FICA Tax Rates. https://www.ssa.gov/oact/cola/cbb.html ↩
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IRS Schedule 3 (Form 1040) Instructions, Excess Social Security Tax. https://www.irs.gov/instructions/i1040s3 ↩