A pre-taxdeduction comes out of your pay before income tax is calculated, so it lowers the income you're taxed on. That's why putting a dollar into a 401(k) or HSA "costs" you less than a dollar of take-home pay — the government was going to tax part of that dollar anyway.
The key distinction most calculators get wrong
Not all pre-tax deductions reduce the same taxes. There are two payroll-tax bases:
- Income tax wages — what federal (and usually state) income tax is based on.
- FICA wages — what Social Security (6.2%) and Medicare (1.45%) are based on.
A 401(k) or 403(b) contribution reduces your income-tax wages but not your FICA wages — you still pay Social Security and Medicare on it. By contrast, HSA, FSA, and health-insurance premiums taken through a Section 125 plan reduce both income-tax wages and FICA wages — so they save you slightly more per dollar.
A quick example
On a $100,000 salary, contributing $10,000 to a 401(k) lowers the income that federal and state tax see, but Social Security and Medicare are still charged on the full $100,000. Contributing $4,000 to an HSA lowers income-tax wages and the $4,000 escapes the 7.65% FICA tax entirely. The paycheck calculator models exactly this — add 401(k) and HSA amounts and watch how Social Security and Medicare respond differently.
After-tax deductions
Roth 401(k) contributions, wage garnishments, and most voluntary benefits come out aftertax — they don't lower your taxable income today. A Roth trades a tax break now for tax-free withdrawals later.
The takeaway
Pre-tax retirement and health accounts are among the few ways to legally lower your tax bill while paying yourself. Just remember a 401(k) still owes FICA, while HSA/FSA dodge it — a detail that changes your real take-home. See it on your own numbers with the free calculator.