How to Calculate Every Deduction on Your Paycheck

Your Pay Stub Is a Financial Model. Treat It Like One.

Your gross pay is one number. Your net pay is a completely different number. Between those two figures sit six or more distinct deduction categories, each governed by its own rules, rates, and annual caps. Most employees accept whatever appears in the net pay field. That is a significant financial error.

The 2024 IRS Data Book showed that the average federal tax refund was $3,011. That figure is not a windfall. It is evidence of chronic over-withholding. Roughly 75% of taxpayers receive a refund, meaning the majority of wage earners are lending the federal government money at 0% interest for up to 15 months.

Understanding how each deduction is calculated gives you the ability to reclaim that capital and deploy it productively throughout the year.

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The Six Deduction Categories on Every Paycheck

1. Federal Income Tax Withholding

Federal withholding is calculated using the IRS Publication 15-T tables, applied to your taxable wages for that pay period after accounting for W-4 adjustments.

The 2025 federal tax brackets for a single filer are:

• 10% on income up to $11,925
• 12% on income from $11,926 to $48,475
• 22% on income from $48,476 to $103,350
• 24% on income from $103,351 to $197,300
• 32% on income from $197,301 to $250,525
• 35% on income from $250,526 to $626,350
• 37% on income above $626,350

These brackets apply to your annualized wages, not each individual paycheck. The IRS withholding formula annualizes your per-period pay, applies the bracket math, then divides back down to your pay frequency.

A single filer earning $5,000 biweekly ($130,000 annualized) with standard W-4 elections will see approximately $1,019 withheld for federal income tax per pay period under the 2025 tables. That assumes no additional withholding, no adjustments for other income, and no deductions claimed on the W-4.

2. Social Security Tax

Social Security tax runs at 6.2% of gross wages. The 2025 wage base cap is $176,100. Once your cumulative wages cross that threshold, Social Security withholding stops entirely for the remainder of the calendar year.

For a worker earning $10,000 per biweekly paycheck, Social Security withholding is $620.00 per period. That worker hits the $176,100 cap at the end of pay period 17 (approximately late August), after which their take-home pay increases by $620 per period for the rest of the year.

3. Medicare Tax

Medicare runs at 1.45% with no wage base cap. High earners face an additional 0.9% surcharge above $200,000 in wages (for single filers), bringing the effective rate to 2.35% on wages above that threshold. Employers withhold the base 1.45% only. The additional 0.9% is reconciled at filing.

On a $5,000 biweekly paycheck, Medicare withholding is $72.50. On wages above the $200,000 threshold, each additional biweekly paycheck of $5,000 carries $117.50 in Medicare withholding.

4. State Income Tax

State withholding varies dramatically. Nine states collect no income tax on wages: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming.

California's top marginal rate is 13.3%. Oregon's is 9.9%. Illinois charges a flat 4.95%. These differences compound meaningfully over a career. A worker earning $120,000 per year in California pays approximately $7,584 more in state income tax annually than the same worker in Texas.

State withholding uses its own annualization tables and exemption structures. Always verify against your state's current employer withholding guide.

5. Pre-Tax Benefit Deductions

This category often carries the most optimization potential. Pre-tax deductions reduce your federal and state taxable income before withholding calculates.

Common pre-tax deductions:

• 401(k) contributions: The 2025 employee elective deferral limit is $23,500. Workers aged 50 or older can contribute an additional $7,500 under catch-up provisions.
• Health insurance premiums: Employer-sponsored health premiums deducted under a Section 125 cafeteria plan reduce both federal taxable wages and FICA wages.
• HSA contributions: The 2025 HSA contribution limit is $4,300 for self-only coverage and $8,550 for family coverage. Contributions via payroll are exempt from FICA.
• FSA contributions: The 2025 health FSA limit is $3,300.
• Dependent care FSA: Limit of $5,000 per household.

A worker in the 22% federal bracket who maximizes a 401(k) at $23,500 reduces their annual federal tax liability by $5,170. Add state tax savings at, say, 5%, and the total tax benefit reaches $6,345 per year.

6. Post-Tax Deductions

Post-tax deductions do not reduce taxable income. They reduce take-home pay only. Common examples include Roth 401(k) contributions, after-tax life insurance premiums above the IRS exclusion threshold, wage garnishments, and certain voluntary benefit elections.

Post-tax deductions require no special treatment in withholding calculations. They simply subtract directly from net pay after all taxes are applied.

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Worked Example 1: Single Filer, Biweekly Pay, $95,000 Salary

Gross pay per period: $3,653.85 (= $95,000 / 26)

Pre-tax deductions:

• 401(k) contribution at 6%: $219.23
• Health insurance premium: $142.00
• HSA contribution: $138.46 (= $3,600 annual / 26)

Federal taxable wages: $3,653.85 minus $499.69 = $3,154.16

Annualized federal taxable wages: $3,154.16 x 26 = $82,008.16

Federal withholding (using 2025 tables, single, standard W-4): approximately $537 per period

Social Security: $3,653.85 x 6.2% = $226.54 (applied to gross, before pre-tax deductions, except HSA through cafeteria plan)

Medicare: $3,653.85 x 1.45% = $52.98

State income tax (assuming Illinois at 4.95%): $3,154.16 x 4.95% = $156.13

Total deductions per period: $537 + $226.54 + $52.98 + $156.13 + $499.69 = $1,472.34

Net pay per period: $3,653.85 minus $1,472.34 = $2,181.51

Annualized net pay: $56,719.26

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Worked Example 2: Married Filer, Semimonthly Pay, $185,000 Salary

Gross pay per period: $7,708.33 (= $185,000 / 24)

Pre-tax deductions:

• 401(k) at maximum: $979.17 (= $23,500 / 24)
• Health insurance (family plan): $310.00
• Dependent care FSA: $208.33 (= $5,000 / 24)

Federal taxable wages: $7,708.33 minus $1,497.50 = $6,210.83

Annualized federal taxable wages: $6,210.83 x 24 = $149,059.92

Federal withholding (married, standard W-4): approximately $1,287 per period

Social Security: $7,708.33 x 6.2% = $477.92 (until $176,100 cap is reached)

Medicare: $7,708.33 x 1.45% = $111.77

State income tax (assuming Georgia at 5.49%): $6,210.83 x 5.49% = $341.00

Total deductions per period: $1,287 + $477.92 + $111.77 + $341.00 + $1,497.50 = $3,715.19

Net pay per period: $7,708.33 minus $3,715.19 = $3,993.14

Annualized net pay: $95,835.36

This worker pays $26,348 in federal income tax annually, $9,718.20 in Social Security, $2,682.50 in Medicare, and $8,184 in Georgia state income tax. Total tax burden: $46,952.70 on $185,000 gross. Effective combined rate: 25.4%.

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The W-4 Is the Lever Most Workers Never Touch

The 2020 redesigned W-4 eliminated allowances entirely. Workers now declare expected deductions, additional income, and dependent tax credits directly. Most people filed a W-4 on their first day of employment and never returned to it.

Three circumstances should trigger an immediate W-4 review:

1. Marriage, divorce, or a new dependent
2. A second job or significant investment income
3. A prior-year refund above $1,500 or a balance due above $500

The IRS Tax Withholding Estimator at irs.gov updates with each tax year. Run it. Then submit a revised W-4 to your employer's HR system. The change takes effect within one to two pay periods.

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How Pre-Tax Contributions Change Your Effective Tax Rate

Contribution sequencing matters. An employee who contributes nothing pre-tax on a $95,000 salary (single filer) carries federal taxable wages of $95,000. An employee who maximizes a 401(k) at $23,500 and an HSA at $4,300 carries federal taxable wages of $67,200.

That $27,800 reduction in taxable income produces tax savings of approximately $6,116 in federal tax (at blended effective rates across the 22% bracket), plus applicable state savings. The money did not disappear. It moved into accounts the employee controls.

This is not tax avoidance. It is correct use of the tax code as written.

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Run the Numbers Before Your Next Pay Period

A single paycheck calculation takes under five minutes with the right tool. The CalcMoney Income Tax Calculator models federal withholding, FICA, state taxes, and pre-tax benefit scenarios against your specific gross pay and filing status.

Change one input. See the result immediately. That is how you identify whether your current W-4 elections are costing you $80 per month or $300 per month in unnecessary over-withholding.

Open the CalcMoney Income Tax Calculator and model your exact paycheck deductions →

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The numbers on your pay stub are not fixed. They are outputs of inputs you can change.