Your FIT taxable wages are almost always lower than your gross pay because your employer subtracts pre-tax deductions, and sometimes adds imputed income, before calculating federal income tax withholding. The comparison of FIT taxable wages versus gross pay comes down to one formula: gross pay, minus pre-tax deductions, plus imputed income, equals the wages the IRS uses to figure your federal income tax. Everything else is detail on which deductions qualify and how they show up on your paycheck and W-2.
What Gross Pay Includes
Gross pay is the total compensation your employer owes you before anything is subtracted. It starts with your base hourly or salary rate and adds overtime, bonuses, commissions, shift differential pay, holiday pay, and the cash value of any paid time off you cash out.1Internal Revenue Service. Publication 525 (2025), Taxable and Nontaxable Income If your employer pays you for services in any form, it feeds into gross pay.
Gross pay is not take-home pay. Take-home is what lands in your bank account after federal and state income taxes, Social Security and Medicare, and every voluntary deduction come out. Gross pay is just where the calculation starts.
The Pre-Tax Deductions That Create the Gap
The distance between gross pay and FIT taxable wages is built almost entirely from pre-tax deductions. Each one is an amount your employer subtracts from gross pay before running the federal income tax withholding calculation, so every dollar routed into a qualifying deduction is a dollar the IRS does not tax on that paycheck.
Traditional 401(k) and 403(b) Contributions
For most employees, traditional retirement deferrals are the largest single reason FIT wages are lower than gross pay. When you defer part of your salary into a traditional 401(k) or 403(b), that money comes out of gross pay before FIT is calculated. For 2026, the standard deferral limit is $24,500. If you’re 50 or older, you can add $8,000 in catch-up contributions. Workers aged 60 through 63 get a higher catch-up of $11,250.2Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
These deferrals are not tax-free. You’ll owe federal income tax when you withdraw the money in retirement. Your employer excludes traditional deferrals from Box 1 of your W-2 but still counts them in Social Security and Medicare wages.3Internal Revenue Service. Publication 560 (2025), Retirement Plans for Small Business
Health Insurance Premiums Under a Section 125 Plan
Most employer-sponsored health, dental, and vision premiums are deducted pre-tax through a Section 125 cafeteria plan, so the premium comes out of gross pay before federal income tax is calculated.4Office of the Law Revision Counsel. 26 USC 125 – Cafeteria Plans For many workers, health premiums are the second-largest pre-tax deduction after retirement.
The Section 125 structure has to be in place for the pre-tax treatment to work. Your employer must maintain a written plan document, and you must formally elect your benefits. Without that, the same premium would be deducted after tax and would not reduce your FIT taxable wages.4Office of the Law Revision Counsel. 26 USC 125 – Cafeteria Plans
HSAs, Health FSAs, and Dependent Care FSAs
Health Savings Account contributions made through payroll are excluded from FIT taxable wages. For 2026, the HSA contribution limit is $4,400 for self-only coverage and $8,750 for family coverage.5Internal Revenue Service. Expanded Availability of Health Savings Accounts Under the One, Big, Beautiful Bill Act You need to be enrolled in a high-deductible health plan to use one.
A health Flexible Spending Account lets you set aside pre-tax dollars for out-of-pocket medical costs. The 2026 salary reduction limit for a health FSA is $3,400. Unlike an HSA, FSA balances generally have to be spent within the plan year, though some employers offer a grace period or limited carryover.
Dependent care FSAs cover childcare or elder care with pre-tax money. For 2026, the maximum exclusion is $7,500 per household, or $3,750 if married filing separately.6Office of the Law Revision Counsel. 26 USC 129 – Dependent Care Assistance Programs
Transportation and Educational Assistance
Qualified transportation fringe benefits, including transit passes, vanpool costs, and qualified parking, can be excluded from FIT taxable wages up to $340 per month for each category in 2026.7Internal Revenue Service. Publication 15-B (2026), Employer’s Tax Guide to Fringe Benefits That’s up to $4,080 a year in transit costs alone that never shows up in taxable wages.8Office of the Law Revision Counsel. 26 USC 132 – Certain Fringe Benefits
Employer-provided educational assistance up to $5,250 per year is also excluded. The benefit now covers tuition, fees, books, and student loan repayment assistance.9Internal Revenue Service. Employers May Help With College Expenses Through Educational Assistance Programs
Roth Contributions Do Not Reduce FIT Wages
Designated Roth contributions to a 401(k) or 403(b) do not reduce your FIT taxable wages. Roth money goes in after tax, so your employer includes Roth deferrals in Box 1 of your W-2 just like regular salary.3Internal Revenue Service. Publication 560 (2025), Retirement Plans for Small Business The payoff comes later, when qualified withdrawals in retirement are tax-free.
If your pay stub shows retirement contributions but your FIT wages don’t seem to drop, check whether the plan coded them as Roth (Box 12 Code AA) rather than traditional pre-tax (Box 12 Code D). This is one of the most common reasons workers think their pre-tax deductions aren’t working when they actually are, just not the kind they thought.
What Gets Added Back: Imputed Income
A smaller number of items push in the opposite direction. Your employer may provide non-cash compensation the IRS treats as taxable, and that imputed income gets added to your FIT taxable wages even though no extra cash lands in your paycheck.
Group-term life insurance is the most common example. Your employer can provide up to $50,000 of coverage tax-free, but the imputed cost of any coverage above that has to be included in taxable wages.10Internal Revenue Service. Group-Term Life Insurance Personal use of a company vehicle and reimbursements paid under a non-accountable expense plan, meaning one that doesn’t require you to document business purposes or return excess amounts, are also added to FIT taxable wages.11Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
Why Box 1, Box 3, and Box 5 Don’t Match
Your W-2 reports different wage figures for federal income tax and for Social Security and Medicare, and the mismatch confuses people every year. The reason is specific: traditional 401(k) and 403(b) deferrals reduce your FIT taxable wages but do not reduce Social Security or Medicare wages.3Internal Revenue Service. Publication 560 (2025), Retirement Plans for Small Business If you contributed $15,000 to a traditional 401(k), your Box 3 (Social Security wages) and Box 5 (Medicare wages) will be roughly $15,000 higher than your Box 1 (FIT taxable wages).
Section 125 deductions, including health premiums, health FSA contributions, and HSA payroll deductions, reduce all three boxes. So a worker contributing heavily to a 401(k) but paying modest health premiums will see a much bigger gap between Box 1 and Box 3 than a worker doing the opposite.
Social Security wages also have a cap. In 2026, only the first $184,500 of earnings is subject to Social Security tax.12Social Security Administration. What Is the Current Maximum Amount of Taxable Earnings for Social Security High earners will see Box 3 plateau at that amount while Box 1 and Box 5 keep climbing. Medicare has no cap.
Reading Box 12 to Verify Your Deductions
Box 12 on your W-2 shows the pre-tax amounts that were excluded from Box 1. These codes tell you which deductions actually reduced your FIT taxable wages:
- Code D: Traditional 401(k) deferrals
- Code E: 403(b) salary reduction contributions
- Code W: Employer and employee HSA contributions
- Code AA: Roth 401(k) contributions (included in Box 1, not excluded)
If Boxes 1, 3, and 5 all show the same number and you thought you were making pre-tax retirement contributions, check the Box 12 codes to see whether your plan defaulted to Roth.13Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026) Box 1 is the number you carry to your Form 1040.
How the Withholding Number Comes Out
Once your employer has FIT taxable wages for the pay period, it consults your Form W-4 to figure withholding. The W-4 captures your filing status, credits you’re claiming for dependents, and any extra withholding you’ve requested.14Internal Revenue Service. Form W-4 (2026) Those inputs work together to translate your FIT taxable wages into the specific dollar amount pulled from each paycheck.
If too little was withheld during the year, you’ll owe the balance at filing. You may also face an underpayment penalty if you owe $1,000 or more and didn’t pay at least 90% of your current-year tax liability, or 100% of last year’s liability, rising to 110% if your adjusted gross income exceeded $150,000.15Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty If too much was withheld, the IRS refunds the excess after you file, but you’ve given the government an interest-free loan for the year.16Internal Revenue Service. Tax Withholding for Individuals The fix in either direction is to submit an updated Form W-4 to your employer. Review it after a new job, marriage, the birth of a child, or the start of side income.
State income tax adds another layer. Most states impose their own income tax withholding, and the definition of taxable wages varies. The pre-tax deductions that reduce federal taxable wages usually reduce state taxable wages too, but not always, so check your state’s rules if the numbers on your state wage statement look different than you expect.