Quick Answer
Pre-tax deductions (traditional 401(k), HSA, FSA, most health premiums) come out of your gross pay before income tax, lowering your taxable income and your tax bill. Post-tax deductions (Roth 401(k), disability insurance, union dues, garnishments) come out after taxes, so they don't cut your taxes. A $2,000 pre-tax 401(k) contribution in the 22% bracket saves about $440 in tax — reducing take-home by only ~$1,560.
Not all paycheck deductions are equal. Where a deduction sits — before or after taxes are calculated — decides whether it also lowers your tax bill. Understanding the difference is one of the easiest ways to keep more of your money.
Pre-tax vs. post-tax, side by side
Pre-tax
Comes out before income tax — lowers your taxable income.
- Traditional 401(k) / 403(b)
- HSA & FSA contributions
- Most health/dental/vision premiums
Post-tax
Comes out after income tax — no tax reduction.
- Roth 401(k) / Roth IRA
- Disability insurance premiums
- Union dues, garnishments, charity
Why the order matters
A pre-tax deduction shrinks the income the IRS taxes, so you save your marginal tax rate on every dollar. Here's a $2,000 traditional 401(k) contribution for someone in the 22% federal bracket:
401(k) contribution$2,000
Federal tax saved (22%)−$440
Actual drop in take-home≈ $1,560
You moved $2,000 into retirement savings but your paycheck only fell by about $1,560 — the IRS effectively covered the rest. State income tax savings, where they apply, make the gap even bigger.
One nuance: not all pre-tax deductions skip FICA
- HSA, FSA, and cafeteria-plan health premiums usually reduce both income tax and FICA.
- Traditional 401(k) contributions reduce income tax but not FICA — you still pay 7.65% on them. Learn more in what is FICA.
To see how a 401(k) contribution changes your paycheck, use the 401(k) calculator, or run your full breakdown with the paycheck calculator. New to the gross-to-net basics? Start with gross pay vs. net pay.