Methodology: Salary / Paycheck Calculator

1. Scope

Converts gross salary to paycheck-level take-home using simplified US tax assumptions (federal brackets + FICA + flat state rate). It is not tax advice — payroll systems compute withholding differently based on W-4 elections.

2. Inputs and outputs

Inputs

annualSalary number (currency)
filingStatus enum

single | married-joint | head-of-household.
payFrequency enum

weekly | biweekly | semimonthly | monthly.
stateTaxRate percent default: 0
pretaxDeductions number (currency/year) default: 0

401(k), HSA, etc.

Outputs

federalIncomeTax

Bracketed on taxable income minus standard deduction and pre-tax.
ficaTax

7.65% up to OASDI wage base + Medicare 1.45% above.
stateTax

Flat-rate state tax on taxable income.
netAnnual

annualSalary − all taxes − pre-tax deductions.
netPerPaycheck

netAnnual / pay periods.

Engine source: src/lib/salary-calculator/engine.ts

3. Formula / scoring logic

taxable_income  = salary - standard_deduction - pretax
federal_tax     = bracket_tax(taxable_income, filing_status)
fica            = min(salary, ssa_wage_base) * 0.062 + salary * 0.0145
state_tax       = salary * state_rate
net_annual      = salary - federal - fica - state - pretax

4. Assumptions

Uses 2024 US federal tax brackets, standard deduction, and SSA wage base ($168,600).
State tax is a flat rate — does not model progressive state brackets, local taxes, or SDI/SUI.
Assumes single-job W-2 income with standard deduction, no dependents or credits.

5. Data sources

US IRS — 2024 tax brackets and standard deduction as of 2024
US SSA 2024 wage base as of 2024

6. Known limitations

Does not model itemised deductions, above-the-line adjustments, credits (CTC, EITC, education), or AMT.
FICA cap applies only to OASDI; Medicare has no cap and adds a 0.9% surtax above $200k single / $250k joint which the tool ignores.

7. Reproducibility

Input
annualSalary = $100,000, filing = single, payFrequency = biweekly, stateRate = 5%, pretax = $0.

Expected output
federal ≈ $14,260, fica = $7,650, state = $5,000, net_annual ≈ $73,090, per_paycheck ≈ $2,811 (directional, 2024).

8. Change log

2026-04-24 methodology page first published.