Pre-Tax Deduction Calculator: 401(k), FSA, and Net Pay

A pre-tax deduction calculator estimates your net pay by subtracting eligible contributions like 401(k) deferrals, health premiums, HSA, and FSA amounts from your gross wages before federal income tax and, in some cases, FICA are applied. The math is straightforward once you know which deductions dodge which taxes. Not every pre-tax deduction saves you the same amount, and that difference is where most quick estimators go wrong.

What You Need Before You Run the Numbers

The accuracy of any calculator depends entirely on what you feed it. Pull your most recent pay stub and your benefits enrollment summary, then gather:

Gross pay per period, exactly as printed on your earnings statement.
Pay frequency: weekly, biweekly, semimonthly, or monthly. Get this wrong and every annual figure the calculator produces will be off.
Filing status: single, married filing jointly, or head of household. Each has different bracket thresholds.
The dollar amount or percentage for each pre-tax deduction category, listed individually on your stub.
Your state of residence. State income tax rates vary, and a few states don’t tax wages at all.

If you’ve adjusted your W-4 recently to claim extra withholding, enter those figures too. A theoretical estimate isn’t much use if your actual withholding is set higher.

Which Deductions Qualify and Their 2026 Limits

Pre-tax treatment is limited to specific categories authorized by the Internal Revenue Code. The main ones, with their 2026 ceilings:

401(k) and 403(b) retirement plans: up to $24,500 in salary deferrals. A 403(b) works the same way but is offered by public schools and certain nonprofits.¹
Health, dental, and vision premiums paid through your employer’s Section 125 cafeteria plan.²
Health Savings Account contributions, if you’re on a high-deductible health plan: up to $4,400 for self-only or $8,750 for family coverage. Participants age 55 or older can add another $1,000.³
Health Care Flexible Spending Account: up to $3,400 for out-of-pocket medical expenses. Qualified expenses cover diagnosis, treatment, and prevention; general wellness items like vitamins typically don’t count.⁴
Dependent Care FSA: $5,000 annual limit for married couples filing jointly or single filers, covering childcare and elder care that lets you work.⁵
Commuter benefits: transit passes and qualified parking each carry a separate monthly exclusion of $340.⁶

Every dollar routed to these accounts bypasses the tax calculation phase of your payroll. Your employer subtracts these amounts from gross pay before reporting wages to the IRS and the Social Security Administration, so the income never appears as taxable earnings in the first place.

Income Tax and FICA Are Two Different Layers

This is where quick calculators oversimplify, and where real money hides. All pre-tax deductions reduce your federal taxable income. But only some of them also reduce your FICA wages.

For 2026, federal tax rates run from 10% on the first $12,400 of taxable income for single filers up to 37% on income above $640,600.⁷ If you sit in the 22% bracket, a $500 monthly 401(k) contribution saves roughly $110 per month in federal income tax alone. Most states with an income tax also exclude these contributions from state taxable wages, though a handful treat certain deductions differently.

FICA is the separate story. Benefits paid through a Section 125 cafeteria plan, including health premiums, FSA contributions, and HSA contributions made through payroll, are exempt from both federal income tax and FICA.² FICA is 6.2% Social Security tax on earnings up to $184,500 in 2026, plus 1.45% Medicare tax on all earnings.⁸ That’s another 7.65% saved on every cafeteria-plan dollar.

Retirement contributions work differently. Traditional 401(k) and 403(b) deferrals dodge federal income tax, but they remain subject to Social Security and Medicare withholding.⁹ A good calculator treats these two layers separately rather than lumping every deduction together.

Working the Math With Real Numbers

Suppose you earn $6,000 per month gross, file as single, contribute $500 to a traditional 401(k), and pay $200 in health insurance premiums through a cafeteria plan. The sequence:

Start with $6,000 in gross pay.
Subtract all pre-tax deductions: $6,000 − $500 − $200 = $5,300 in federal taxable wages.
Apply the 2026 federal brackets. For a single filer in the 22% bracket, withholding on $5,300 comes to roughly $590.⁷
Calculate FICA. Because the 401(k) contribution is still subject to Social Security and Medicare, FICA applies to $5,800, not $5,300. That’s $5,800 × 7.65% = about $444.⁹
Apply your state income tax rate to the state’s taxable wage figure. Rules vary.
Subtract any post-tax items like Roth contributions, life insurance, or wage garnishments.
What’s left is your net pay.

Without those pre-tax deductions, the full $6,000 would face income tax withholding, and the health premium alone would cost an extra $15.30 per month in FICA. Over a full year, the combined tax savings from $500 monthly 401(k) deferrals and $200 in cafeteria-plan health premiums can easily exceed $2,000 for someone in the 22% bracket.

Pre-Tax or Roth 401(k)

Many employers offer a Roth 401(k) alongside the traditional pre-tax version. Both share the same $24,500 annual contribution limit for 2026, but the tax treatment is reversed.¹ Pre-tax contributions reduce taxable income now, but you pay income tax on every dollar withdrawn in retirement. Roth contributions come out of after-tax pay, so your paycheck shrinks more today, but qualified withdrawals in retirement are tax-free.

The choice hinges on whether you expect your tax rate to be higher or lower in retirement. Early in your career and in a lower bracket, Roth locks in today’s rate. In peak earning years and sitting in the 32% or 35% bracket, the immediate deduction from pre-tax deferrals is usually worth more. Both are subject to FICA either way.⁹ Toggling between the two in a calculator shows the take-home difference side by side.

Catch-Up Contributions if You’re 50 or Older

Workers age 50 and older can go beyond the standard limits, and those extra amounts still receive pre-tax treatment in a traditional 401(k) or 403(b). For 2026, the general catch-up is $8,000, taking the total possible deferral to $32,500. Workers who are 60, 61, 62, or 63 during 2026 get a higher catch-up of $11,250, for a potential total of $35,750.¹

HSA participants age 55 or older can add $1,000 on top of the standard $4,400 individual or $8,750 family limit.³ The HSA catch-up is fixed at $1,000 by statute rather than adjusted for inflation. If you’re eligible for both, maxing them during your highest-earning years pulls a lot of income out of the tax base.

A Warning About FSA Contributions

Flexible spending accounts have a trap: unspent money can be forfeited. Unlike an HSA, which rolls over indefinitely, an FSA runs on use-it-or-lose-it within the plan year. Your employer may soften this with one of two options, but never both:

A grace period of 2.5 months after the plan year ends to incur expenses using leftover funds. For a plan year ending December 31, 2025, that pushes the deadline to March 15, 2026.
A carryover of up to $680 of unused health FSA funds into the next plan year for 2026.

Ask HR which option, if either, your plan uses. If neither applies, any balance at year-end disappears. When you plug an FSA number into a calculator, be honest about what you’ll actually spend. Contributing the $3,400 maximum only makes sense if you’re confident you’ll use it; otherwise, a figure based on your recent medical spending keeps more money in your paycheck.

Checking the Calculator Against Your W-2

Once the year ends, your W-2 confirms whether your estimates matched reality. Box 1 (Wages, Tips, Other Compensation) already reflects pre-tax deductions, which is why it’s typically lower than your total annual salary. Box 3 (Social Security wages) and Box 5 (Medicare wages) will be higher than Box 1 if you made 401(k) contributions, because those deferrals dodge income tax but not FICA.⁹

Box 12 breaks out specific pre-tax amounts by letter code. Code D is traditional 401(k) deferrals, Code W is HSA contributions made through your employer (including your own salary-reduction amounts through a cafeteria plan), and Code DD is the total cost of employer-sponsored health coverage.¹⁰ Comparing the Code D figure to the $24,500 limit is the quickest way to confirm you didn’t over-contribute during the year.¹

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Internal Revenue Service. FAQs for Government Entities Regarding Cafeteria Plans
3
HealthCare.gov. Understanding Health Savings Account-Eligible Plans
4
Internal Revenue Service. Medical and Dental Expenses
5
Internal Revenue Service. Child and Dependent Care Credit and Flexible Benefit Plans
6
Internal Revenue Service. Qualified Parking Fringe Benefit
7
Internal Revenue Service. Revenue Procedure 2025-32
8
Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
9
Internal Revenue Service. Are Retirement Plan Contributions Subject to Withholding for FICA, Medicare, or Federal Income Tax
10
Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3