Your federal paycheck, line by line
A federal employee's biweekly pay stub is more complex than most generic calculators handle. The deductions interact in specific ways that determine both your take-home pay and your future retirement income. Getting the tax treatment right for each deduction type matters.
FERS contributions and taxes
Your FERS retirement contribution — 0.8%, 3.1%, or 4.4% of your gross pay depending on when you were hired — is not pre-tax. It comes out of your paycheck after income taxes are calculated, meaning it does not reduce your taxable income. This is the most common error in generic federal pay calculators.
The reason this matters at retirement: OPM will calculate a tax-free portion of your annuity (the "cost basis") because you already paid tax on those contributions. But during your working years, you get no deduction.
TSP: pre-tax vs. pre-FICA
Traditional TSP contributions reduce your federal (and state) income tax base — they are pre-tax for income tax purposes. However, they do not reduce your FICA base. You pay Social Security (6.2%) and Medicare (1.45%) on your full gross pay minus FEHB, regardless of how much you contribute to Traditional TSP.
Roth TSP contributions are made with after-tax dollars — they reduce neither your income tax base nor your FICA base. Your FICA tax is identical whether you contribute to Roth or Traditional TSP.
FICA and the wage base
Social Security tax (6.2%) applies only up to the $184,500 wage base in 2026. Earnings above that limit are not subject to Social Security tax — though Medicare (1.45%) applies to all wages with no cap. High earners above $200,000 single / $250,000 married also pay an additional 0.9% Medicare surtax on wages above those thresholds.
FEHB premiums are excluded from both income tax and FICA under IRS premium conversion rules. This makes FEHB one of the most tax-efficient benefits in the federal package.