UK tax calculator

UK tax calculator.js

/**
 * Calculates the employee national insurance contribution based on annual net salary.
 * @param {number} annual net salary.
 * @return The employee national insurance contribution based on annual net salary
 * @customfunction
*/
function EMPLOYEE_NATIONAL_INSURANCE(income) {
  /**
   * If you're an employee you start paying National Insurance when you earn more than £242 a week (2023/24). The National Insurance rate you pay depends on how much you earn, and is made up of: 12% of your weekly earnings between £242 and £967 (2023/24) 2% of your weekly earnings above £967.
   */

  let weeklyIncome = income / 52;
  let remaining = weeklyIncome;
  let bands = [{
    amount: 242,
    percentage: 0,
  }, {
    amount: 725,
    percentage: 0.12,
  }, {
    amount: -1,
    percentage: 0.02,
  }];

  let ni = 0;

  for (let i = 0; i < bands.length; i++) {
    let { amount, percentage } = bands[i];

    if (amount === -1) {
      ni += remaining * percentage;
      break;
    }

    let actual = Math.min(remaining, amount);
    ni += actual * percentage;
    remaining -= actual;
  }

  return ni * 52;
}

/**
 * Calculates the self employed national insurance contribution based on annual net salary. https://www.gov.uk/self-employed-national-insurance-rates
 * @param {number} annual profits
 * @return The self employeed national insurance contribution
 * @customfunction
*/
function SELF_EMPLOYEED_NATIONAL_INSURANCE(annualProfits) {
  /**
   How much you pay
  Class	Rate for tax year 2023 to 2024
  Class 2	| £3.45 a week
  Class 4	| 9% on profits between £12,570 and £50,270. 2% on profits over £50,270
   */

  let class2_weekly_NI = 3.45;

  
  let bands = [{
    amount: 12570,
    percentage: 0,
  }, {
    amount: (50270 - 12570),
    percentage: 0.09,
  }, {
    amount: -1,
    percentage: 0.02,
  }];

  let remaining = annualProfits;
  let class4_NI = 0;

  for (let i = 0; i < bands.length; i++) {
    let { amount, percentage } = bands[i];

    if (amount === -1) {
      class4_NI += remaining * percentage;
      break;
    }

    let actual = Math.min(remaining, amount);
    class4_NI += actual * percentage;
    remaining -= actual;
  }

  return (class2_weekly_NI * 52) + class4_NI;
}


/**
 * Calculates the employee taxes.
 * @param {number} annual gross salary.
 * @return The employee taxes
 * @customfunction
*/
function EMPLOYEE_INCOME_TAX(annualGrossSalary) {
  /**
  Band	Taxable income	Tax rate
  Personal Allowance	Up to £12,570	0%    (amount = 12570)
  Basic rate	£12,571 to £50,270	20%.    (amount = 37699)
  Higher rate	£50,271 to £125,140	40%.    (amount = 74869)
  Additional rate	over £125,140	45%
   */

  
  
  let remaining = annualGrossSalary;
  let bands = [{
    // Your adjusted net income is over the personal allowance threshold of £100,000. Your allowance has been reduced to £7,570 (from the default £12,570).
    amount: annualGrossSalary >= 100000 ? 7570 : 12570,
    percentage: 0,
  }, {
    amount: 50270 - 12570,
    percentage: 0.2,
  }, {
    amount: 125140 - 50270,
    percentage: 0.4,
  }, {
    amount: -1,
    percentage: 0.45,
  }];

  let taxes = 0;

  for (let i = 0; i < bands.length; i++) {
    let { amount, percentage } = bands[i];

    if (amount === -1) {
      taxes += remaining * percentage;
      break;
    }

    let actual = Math.min(remaining, amount);
    taxes += actual * percentage;
    remaining -= actual;
  }

  return taxes;
}


/**
 * Calculates the employee annual take home.
 * @param {number} annual gross salary.
 * @return The employee annual take home
 * @customfunction
*/
function EMPLOYEE_TAKE_HOME(annualGrossSalary) {
  return annualGrossSalary - EMPLOYEE_NATIONAL_INSURANCE(annualGrossSalary) - EMPLOYEE_INCOME_TAX(annualGrossSalary);
}