Pay and benefits
Pre-tax vs. post-tax deductions
Two deductions of the same size can cost you very different amounts, depending on which taxes they escape.
The short answer
Pre-tax deductions come out of gross pay before withholding is calculated, so they reduce the wages you are taxed on - a $200 pre-tax deduction for someone in the 22% bracket costs about $156 of take-home pay. Post-tax deductions come out after tax and cost the full $200. The important wrinkle: retirement contributions escape income tax but not Social Security and Medicare, while Section 125 health benefits escape both.
Which taxes each deduction escapes
| Deduction | Federal income tax | Social Security & Medicare | Most state income taxes |
|---|---|---|---|
| Traditional 401(k), 403(b), 457, TSP | Exempt | Taxed | Exempt in most states |
| Roth 401(k) | Taxed | Taxed | Taxed |
| Health, dental, vision premiums (Section 125) | Exempt | Exempt | Exempt in most states |
| Health savings account via payroll | Exempt | Exempt | Exempt except CA and NJ |
| Flexible spending account (health or dependent care) | Exempt | Exempt | Exempt in most states |
| Commuter transit and parking benefits | Exempt | Exempt | Exempt in most states |
| Group term life above $50,000 of cover | Added as imputed income | Taxed | Taxed |
| Short and long term disability premiums | Usually taxed by choice | Taxed | Taxed |
| Union dues, charitable payroll giving | Taxed | Taxed | Taxed |
| Garnishments, child support, tax levies | Taxed | Taxed | Taxed |
Pennsylvania taxes 401(k) contributions at the state level, and California and New Jersey tax HSA contributions. Everything marked exempt is exempt from that specific tax only.
What a pre-tax dollar is actually worth
The saving equals the deduction multiplied by the tax rates it avoids. For someone in the 22% federal bracket with no state income tax:
| Deduction of $200 per check | Taxes avoided | Real cost to take-home pay |
|---|---|---|
| Traditional 401(k) | 22% federal only | $156.00 |
| Health premium or HSA | 22% federal + 7.65% FICA | $140.70 |
| Roth 401(k) or union dues | None | $200.00 |
2026 rates. Add your state rate to the first two rows in a state that taxes wages - it widens the gap further.
The 401(k) and health insurance calculator runs this against your own salary and state, and the main paycheck calculator shows the pre-tax and post-tax lines separately in the breakdown.
The order operations happen in
- Gross pay - everything you earned this period.
- Pre-tax deductions come out, producing taxable wages. There are two versions: one for income tax and a higher one for FICA, because 401(k) money is exempt from the first and not the second.
- Taxes are calculated on those wages - federal income tax via your W-4, Social Security, Medicare, and any state and local tax.
- Post-tax deductions come out of what is left, including Roth contributions, disability premiums and garnishments.
- Net pay is deposited.
This ordering is why a stub can show four different wage totals that are all correct, and why the year-to-date columns never quite agree with each other.
When post-tax is the better answer
- Disability cover. Premiums paid with taxed money buy benefits that arrive tax-free. Paying them pre-tax makes any future benefit taxable - a bad trade on a benefit you would only claim when your income has collapsed.
- Roth retirement contributions. You pay tax now at a known rate rather than later at an unknown one. Worth it if you expect to be in a higher bracket in retirement, or want tax diversification.
- Lower incomes and Social Security. Pre-tax retirement deferrals do not reduce Social Security wages, so they do not dent your future benefit - but a large Section 125 election does, because it reduces the wages your benefit is calculated from.
What this means for your W-4
Pre-tax deductions are already handled by payroll before your W-4 is applied, so they should not be entered on line 4(b) of the form - doing so counts them twice and under-withholds. Line 4(b) is for deductions payroll does not know about, such as itemized deductions above the standard deduction. The W-4 calculator keeps the two separate.
FAQ
Frequently asked questions
What is the difference between a pre-tax and a post-tax deduction?+
A pre-tax deduction comes out of your pay before tax is calculated, so it lowers the wages you are taxed on. A post-tax deduction comes out of money that has already been taxed and changes nothing about your tax bill. The same benefit can be either - Roth and traditional 401(k) contributions are the clearest example.
Do 401(k) contributions reduce Social Security and Medicare tax?+
No. Traditional retirement contributions escape federal and usually state income tax, but Social Security and Medicare are still charged on them. Section 125 benefits - health, dental and vision premiums, HSA and FSA contributions - escape both, which makes them worth about 7.65% more per dollar than a 401(k) contribution today.
Is health insurance pre-tax or post-tax?+
Employer-sponsored health premiums are almost always pre-tax, through a Section 125 cafeteria plan, and they escape income tax and FICA. Insurance you buy yourself outside work is post-tax. Disability cover is deliberately often post-tax: paying the premium with taxed money makes any benefit you later receive tax-free.
Is it always better to have a deduction be pre-tax?+
Not always. Pre-tax retirement money is taxed when you withdraw it, so Roth contributions can win if you expect a higher rate in retirement. Post-tax disability premiums buy tax-free benefits. And pre-tax reductions lower the wages Social Security uses to work out your future benefit, which matters at lower incomes.
What order do deductions come out in?+
Pre-tax benefit deductions first, then taxes are calculated on what is left, then post-tax deductions and garnishments. Garnishments are calculated on disposable earnings, which is pay after legally required withholding, so a voluntary pre-tax election does not shrink a garnishment.
Sources
- IRS, cafeteria plans (Section 125)
- IRS Publication 15-B, employer's tax guide to fringe benefits
- IRS, 401(k) contribution limits
- IRS Publication 15 (Circular E), employer's tax guide
- IRS, topic no. 751, Social Security and Medicare withholding rates
General information for planning, not tax or legal advice. Federal figures are checked against the cited IRS, SSA and Department of Labor documents.
Read next
Related guides and tools
- Paycheck calculator with 401(k) and health insurance
How pre-tax deductions change a paycheck, and what $100 into a 401(k) really costs.
- How to read a pay stub
Every section of a pay stub explained, plus a decoder for the abbreviations payroll systems use.
- What is FICA tax?
The 7.65% pair on every US paycheck: what Social Security and Medicare take, and where each one stops.
- Salary vs. hourly pay
Exempt and non-exempt explained, the $684-a-week federal salary floor, and which arrangement pays more.