Calculation Methodology & Sources

Read how the calculators work and which sources the tax rules refer to.

Tax year 2026 · Federal sources verified September 20, 2026 · Rule set federal-2026.2026-09-20.3 · Methodology and sources · State rule coverage is reviewed separately.

1. Calculation Steps

All calculations in PayNumera are deterministic and execute entirely in client memory or server rendered HTML. No financial input, salary figure, or deduction entry is recorded, transmitted, or stored on external servers.

Calculations follow the standard statutory order of operations defined by the Internal Revenue Service (IRS) and the Social Security Administration (SSA):

1. Gross Compensation Determination
2. Section 125 & Pre-Tax Benefit Reductions
3. FICA Tax Computation (Social Security & Medicare)
4. Federal Income Tax Withholding Computation
5. State & Local Income Tax Withholding Computation
6. Post-Tax Deductions
7. Net Take-Home Pay Result

2. Federal Income Tax Withholding (2026)

Federal paycheck withholding uses the percentage method in IRS Publication 15-T (Federal Income Tax Withholding Methods) for a 2020-or-later Form W-4. This payroll method is separate from annual income-tax liability and does not subtract the tax-return standard deduction.

Annualization: multiply taxable wages for the pay period by the number of payroll periods.
2026 Worksheet 1A adjustment: subtract $8,600 for Single, Married Filing Separately, or Head of Household, or $12,900 for Married Filing Jointly, unless the Step 2 checkbox schedule applies.
W-4 dollar fields: apply other income, deductions, annual credits, and extra per-paycheck withholding as directed by the worksheet.
Schedule: use the matching 2026 percentage-method table and convert the annual result back to one pay period.

The annual Income Tax Calculator separately uses the 2026 base standard deductions of $16,100 for Single or Married Filing Separately, $32,200 for Married Filing Jointly, and $24,150 for Head of Household. It models annual ordinary-income liability, not payroll withholding.

3. FICA Payroll Taxes

The Federal Insurance Contributions Act imposes two mandatory employee taxes:

Old-Age, Survivors, and Disability Insurance (OASDI): Assessed at 6.2% of covered wages up to the statutory limit of $184,500. An employer stops withholding employee Social Security after that employer’s covered wages reach the limit; Medicare continues.
Hospital Insurance (Medicare): Assessed at 1.45% on all covered wages without an annual ceiling.
Additional Medicare Tax (ACA): Employers withhold an extra 0.9% after wages they pay an employee exceed $200,000 in the year, regardless of filing status. Final return liability uses filing-status thresholds, including $250,000 for Married Filing Jointly.

Note: In accordance with IRC § 3121(v)(1), traditional pre-tax 401(k) contributions generally defer federal income tax but remain subject to Social Security and Medicare taxes. In contrast, Section 125 cafeteria plans (health, dental, vision, and HSA) are exempt from both income taxes and FICA.

4. Retirement and HSA Scenarios (2026)

The Paycheck Calculator projects the selected recurring contribution across the chosen payroll frequency. Traditional and Roth 401(k) employee deferrals share one annual elective-deferral limit across plans. Amounts entered as other-plan deferrals are added only when they are not already represented by that recurring paycheck amount.

Employee 401(k) deferrals: $24,500 before an eligible catch-up; $8,000 for the general age-50 catch-up or $11,250 for ages 60–63 when the plan permits it.
HSA contributions: $4,400 for self-only coverage or $8,750 for family coverage, plus a $1,000 age-55 catch-up when eligible. Payroll, direct, and employer HSA contributions count together.
Paycheck treatment: traditional 401(k) deferrals reduce modeled federal withholding wages but generally remain subject to FICA; Roth deferrals are post-tax. Payroll HSA contributions are modeled through an eligible cafeteria plan and reduce federal and FICA wages. A direct HSA contribution does not automatically change the current paycheck.
Limits: plans may impose lower limits. Partial-year HSA eligibility, the last-month rule, testing periods, and state conformity require separate review and are not inferred.

5. State Tax Schedules

State routes use published guidance from official taxing authorities, but only source-reviewed 2026 methods are enabled. An unavailable state formula returns a federal-only result instead of estimating a state tax.

No earned-wage income tax: Texas, Florida, Washington, Nevada, Tennessee, Wyoming, South Dakota, Alaska, and New Hampshire have verified zero state personal earned-income tax for this scope.
Verified 2026 coverage: 45 jurisdictions have resident ordinary-wage payroll withholding and 45 have limited annual wage-liability methods; 42 have both. Each page labels the supported capability separately.
Unavailable methods: Montana, New Mexico and Vermont currently return federal-only results. Idaho, Mississippi and Utah are payroll-only; District of Columbia, North Dakota and West Virginia are annual-liability-only.
Separate payroll programs: Minnesota, Oregon, Maine and Delaware public paid-leave contributions are available in Payroll with explicit employer-size and plan assumptions.
Excluded scope: most local taxes, reciprocity, nonresident allocation, disability insurance, private paid-leave plan rates and special industry rules require separate coverage.

6. Currency Rounding

Calculations are executed using decimal-safe rounding functions. Each tax and deduction component is rounded to the nearest integer cent ($0.01) on a per-line basis, eliminating floating-point rounding discrepancies.

7. Federal Rule Set and Official Sources

Rule set federal-2026.2026-09-20.3 is effective from 2026-01-01 through 2026-12-31. Its source review was completed on 2026-09-20. Passing automated tests verifies implementation behavior; it does not imply professional tax review.