Understanding Payroll Deductions: What Comes Out of Your Paycheck and Why

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Three Buckets, Not One

"Payroll deductions" gets used as a catch-all term, but the deductions on your stub don't all work the same way. There are three distinct buckets, and knowing which bucket a deduction is in tells you exactly how it affects your tax bill.

Mandatory deductions are required by law: federal income tax withholding, state and local income tax where applicable, and FICA (Social Security and Medicare). You don't opt into these and you don't opt out.

Voluntary pre-tax deductions are benefits you elect that reduce your taxable wages before certain taxes are calculated — traditional 401(k) or 403(b) contributions, Section 125 health insurance premiums, and contributions to a Flexible Spending Account (FSA) or Health Savings Account (HSA).

Voluntary post-tax deductions are things you elect or that are imposed on you that come out after taxes are already calculated — Roth 401(k) contributions, union dues, supplemental life insurance, and wage garnishments.

For the layout of where these show up on an actual stub, see How to Read a Pay Stub.

Mandatory Deductions in Detail

Federal income tax withholding is driven by your W-4, not a flat rate. The 2020 redesign of the W-4 removed the old allowance system, so your withholding today is based on your filing status, any dependents, and other income you report — not a number of "allowances" you claim. See Federal Withholding Explained for the mechanics.

Social Security withholds 6.2% of FICA wages, matched by your employer, up to an annual wage base that adjusts each year. Medicare withholds 1.45% of FICA wages with no cap, also employer-matched. Earn above $200,000 in a year and an Additional Medicare Tax of 0.9% applies to the excess — employee-only, no employer match. Full breakdown at FICA Tax Explained.

Curious what your own numbers look like? Run your pay through the calculator to see each line.

Pre-Tax Deductions Don't All Do the Same Thing

This is the part that trips people up: not every pre-tax deduction reduces the same taxes.

A traditional 401(k) or 403(b) contribution reduces the wages used for federal (and typically state) income tax withholding. It does not reduce the wages used for Social Security or Medicare — those are still calculated on your pay before the 401(k) comes out.

A Section 125 health insurance premium (and most FSA/HSA contributions) goes further. It reduces wages for federal income tax and for Social Security and Medicare.

This difference is exactly why your W-2 has separate boxes. Box 1 (federal taxable wages) reflects both types of pre-tax reductions. Box 3 and Box 5 (Social Security and Medicare wages) reflect only the Section 125-style reductions, not the 401(k). Box 3 is also capped at the Social Security wage base; Box 5 is not capped at all.

Post-Tax Deductions

Post-tax deductions come out after your tax withholding is already calculated, so they reduce your net pay dollar-for-dollar but don't touch your taxable wages. A Roth 401(k) contribution is the most common voluntary example — you pay tax on that money now, but qualified withdrawals in retirement are tax-free. Wage garnishments for child support or unpaid debt are mandatory post-tax deductions your employer is legally required to withhold once ordered by a court or agency.

A Worked Example

A salaried employee earns $5,000 gross per month. She contributes 6% to a traditional 401(k) ($300) and pays a $150 Section 125 health premium.

• Federal taxable wages: $5,000 − $300 (401k) − $150 (health) = $4,550
• FICA taxable wages: $5,000 − $150 (health only) = $4,850
• Social Security: 6.2% × $4,850 = $300.70
• Medicare: 1.45% × $4,850 = $70.33
• Federal withholding (from her W-4): $450
• State withholding: $150
• Roth IRA contribution (post-tax, outside payroll but shown for comparison): $200

Total deductions: $300 + $150 + $450 + $150 + $300.70 + $70.33 + $200 = $1,621.03

Net pay: $5,000 − $1,621.03 = $3,378.97

Annualized, her W-2 would show Box 1 (federal taxable) around $54,600 and Box 3/Box 5 (Social Security/Medicare wages) around $58,200 — two different numbers from the same $60,000 in gross pay, purely because the 401(k) only reduces one of them.

Want to see how a new 401(k) election or insurance plan would change your take-home pay? Model it with a pay stub before you enroll.

Why This Matters Beyond Your Paycheck

Understanding which bucket a deduction falls into matters any time someone else is reading your income documents — a lender calculating qualifying income, or an underwriter reconciling your stub against your W-2. If your gross pay, Box 1, and Box 3 don't match in a way you can explain, it slows down every application built on that paperwork. It also matters for your own planning: increasing a traditional 401(k) contribution lowers your federal tax bill immediately, while a Section 125 premium lowers both your federal and FICA tax bill, which is a meaningfully bigger effective discount on that spending.

The Bottom Line

Payroll deductions aren't one undifferentiated pile of money leaving your check. They're three separate systems — mandatory taxes, pre-tax benefits, and post-tax elections — each with its own rules about which wages they apply to. Once you can sort a deduction into the right bucket, you can predict exactly how it will change your net pay and your W-2 before it happens. For the schedule side of payroll, see Weekly vs. Biweekly Payroll, or browse the full payroll resource hub.

Frequently Asked Questions

Related Guides

• How to Read a Pay Stub
• Federal Withholding Explained
• FICA Tax Explained: Social Security & Medicare Breakdown
• Weekly vs. Biweekly Payroll

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Authoritative source: IRS — Topic 751: Social Security & Medicare Withholding Rates

This guide is informational and not legal or tax advice.