TL;DR:Payroll deductions split into mandatory (taxes, FICA, garnishments) and voluntary (benefits, retirement, FSAs); some voluntary deductions are pre-tax and lower taxable income.
Typical payroll deductions fall into two groups: mandatory (by law) and voluntary (employee-authorized). Mandatory deductions normally include federal income tax withholding, Social Security and Medicare (FICA), state and local income taxes where applicable, and court-ordered wage garnishments (child support, tax levies). Employers also withhold employer/employee contributions required by some states for unemployment or disability programs. Voluntary deductions, taken with employee consent, commonly include employer-sponsored health, dental or vision premiums, retirement contributions (401(k), 403(b), etc.), flexible spending/HSA contributions, life or disability insurance premiums, and union dues. Some voluntary items are pre-tax (lowering taxable income) — e.g., traditional 401(k), health premiums, FSAs — while mandatory items are withheld post- or pre-tax as law requires. Employers must calculate, withhold and remit these amounts correctly and provide pay stubs showing each deduction. For US-specific rules see IRS Publication 15 and for operational guidance see major payroll providers such as ADP or Paychex.
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