How to Build a Paycheck Tax Calculator

const FEDERAL_BRACKETS_2025 = {
  single: [
    { min: 0,      max: 11925,  rate: 0.10 },
    { min: 11925,  max: 48475,  rate: 0.12 },
    { min: 48475,  max: 103350, rate: 0.22 },
    { min: 103350, max: 197300, rate: 0.24 },
    { min: 197300, max: 250525, rate: 0.32 },
    { min: 250525, max: 626350, rate: 0.35 },
    { min: 626350, max: Infinity, rate: 0.37 }
  ],
  married: [
    { min: 0,      max: 23850,  rate: 0.10 },
    { min: 23850,  max: 96950,  rate: 0.12 },
    { min: 96950,  max: 206700, rate: 0.22 },
    { min: 206700, max: 394600, rate: 0.24 },
    { min: 394600, max: 501050, rate: 0.32 },
    { min: 501050, max: 751600, rate: 0.35 },
    { min: 751600, max: Infinity, rate: 0.37 }
  ]
  };

  const SS_WAGE_BASE_2025 = 168600;
  const SS_RATE = 0.062;
  const MEDICARE_RATE = 0.0145;
  const MEDICARE_SURTAX_RATE = 0.009;
  const MEDICARE_SURTAX_THRESHOLD = 200000;
  const STANDARD_DEDUCTION_2025 = { single: 15000, married: 30000 };

  function calculatePaycheckTax(annualGross, filingStatus, payFrequency, preTaxDeductions = 0) {
  const periods = { weekly: 52, biweekly: 26, semimonthly: 24, monthly: 12 };
  const periodsPerYear = periods[payFrequency];
  const grossPerPeriod = annualGross / periodsPerYear;
  const annualPreTax = preTaxDeductions * periodsPerYear;

  // Federal taxable income
  const standardDeduction = STANDARD_DEDUCTION_2025[filingStatus];
  const taxableIncome = Math.max(0, annualGross - annualPreTax - standardDeduction);

  // Federal income tax (marginal brackets)
  let federalTax = 0;
  const brackets = FEDERAL_BRACKETS_2025[filingStatus];
  for (const bracket of brackets) {
    if (taxableIncome <= bracket.min) break;
    const taxableInBracket = Math.min(taxableIncome, bracket.max) - bracket.min;
    federalTax += taxableInBracket * bracket.rate;
  }

  // FICA
  const ssWages = Math.min(annualGross - annualPreTax, SS_WAGE_BASE_2025);
  const socialSecurity = ssWages * SS_RATE;
  const medicare = (annualGross - annualPreTax) * MEDICARE_RATE;
  const medicareSurtax = Math.max(0, (annualGross - annualPreTax) - MEDICARE_SURTAX_THRESHOLD) * MEDICARE_SURTAX_RATE;

  const totalAnnualTax = federalTax + socialSecurity + medicare + medicareSurtax;
  const annualTakeHome = annualGross - totalAnnualTax - annualPreTax;

  return {
    grossPerPeriod: round(grossPerPeriod),
    federalTaxPerPeriod: round(federalTax / periodsPerYear),
    socialSecurityPerPeriod: round(socialSecurity / periodsPerYear),
    medicarePerPeriod: round((medicare + medicareSurtax) / periodsPerYear),
    preTaxDeductionsPerPeriod: round(preTaxDeductions),
    netPerPeriod: round(annualTakeHome / periodsPerYear),
    effectiveTaxRate: ((totalAnnualTax / annualGross) * 100).toFixed(2),
    marginalBracket: brackets.find(b => taxableIncome <= b.max)?.rate * 100
  };
  }

  function round(n) { return Math.round(n * 100) / 100; }