How to Build a S Corp Calculator

interface TaxScenario {
  netBusinessIncome: number;
  filingStatus: "single" | "married_joint" | "married_separate";
  otherIncome: number; // W-2 from spouse, etc.
  }

  interface SCorpInputs extends TaxScenario {
  reasonableSalary: number;
  annualSCorpCosts: number; // payroll, filing, state fees
  }

  const SS_WAGE_BASE_2025 = 168600;
  const FICA_RATE = 0.153;     // 12.4% SS + 2.9% Medicare
  const SS_RATE = 0.124;
  const MEDICARE_RATE = 0.029;
  const ADDITIONAL_MEDICARE_THRESHOLD = 200000; // single
  const ADDITIONAL_MEDICARE_RATE = 0.009;

  function calcSoleProprietorSETax(netIncome: number): {
  seTax: number;
  deductibleHalf: number;
  } {
  const taxableBase = netIncome * 0.9235; // 92.35% is subject to SE tax
  let seTax = 0;

  if (taxableBase <= SS_WAGE_BASE_2025) {
    seTax = taxableBase * FICA_RATE;
  } else {
    seTax = SS_WAGE_BASE_2025 * FICA_RATE + (taxableBase - SS_WAGE_BASE_2025) * MEDICARE_RATE;
  }

  // Additional Medicare Tax on high earners
  if (taxableBase > ADDITIONAL_MEDICARE_THRESHOLD) {
    seTax += (taxableBase - ADDITIONAL_MEDICARE_THRESHOLD) * ADDITIONAL_MEDICARE_RATE;
  }

  return { seTax, deductibleHalf: seTax / 2 };
  }

  function calcSCorpFICA(salary: number): {
  employerFICA: number;
  employeeFICA: number;
  totalFICA: number;
  } {
  let employerSS = Math.min(salary, SS_WAGE_BASE_2025) * (SS_RATE / 2);
  let employeeSS = Math.min(salary, SS_WAGE_BASE_2025) * (SS_RATE / 2);
  let employerMedicare = salary * (MEDICARE_RATE / 2);
  let employeeMedicare = salary * (MEDICARE_RATE / 2);

  return {
    employerFICA: employerSS + employerMedicare,
    employeeFICA: employeeSS + employeeMedicare,
    totalFICA: employerSS + employeeSS + employerMedicare + employeeMedicare,
  },
  }

  function compareSavings(inputs: SCorpInputs): {
  soleProprietorSETax: number;
  sCorpFICA: number;
  ficaSavings: number;
  netSavings: number;
  distribution: number;
  effectiveSavingsRate: string;
  } {
  const soleProp = calcSoleProprietorSETax(inputs.netBusinessIncome);
  const sCorp = calcSCorpFICA(inputs.reasonableSalary);
  const distribution = inputs.netBusinessIncome - inputs.reasonableSalary - sCorp.employerFICA;
  const ficaSavings = soleProp.seTax - sCorp.totalFICA;
  const netSavings = ficaSavings - inputs.annualSCorpCosts;

  return {
    soleProprietorSETax: soleProp.seTax,
    sCorpFICA: sCorp.totalFICA,
    ficaSavings,
    netSavings,
    distribution: Math.max(0, distribution),
    effectiveSavingsRate: ((netSavings / inputs.netBusinessIncome) * 100).toFixed(1) + "%",
  },
  }