Mortgage and credit calculator for website

Mortgage and Credit Calculator Development on Website

A credit calculator is a mathematically rigorous tool. Mistakes in formulas cannot be made: users compare results with bank calculations, and discrepancies undermine website trust.

Annuity Payment Mathematics

Most mortgages and consumer loans use an annuity scheme — equal monthly payments:

M = P × [r(1+r)^n] / [(1+r)^n - 1]

Where:

• P — loan amount
• r — monthly interest rate = annual / 12 / 100
• n — number of months
• M — monthly payment

function calcAnnuity(principal, annualRate, months) {
  if (annualRate === 0) {
    return { monthly: principal / months, total: principal, overpayment: 0 };
  }

  const r = annualRate / 12 / 100;
  const factor = Math.pow(1 + r, months);
  const monthly = principal * (r * factor) / (factor - 1);
  const total = monthly * months;
  const overpayment = total - principal;

  return {
    monthly: Math.round(monthly * 100) / 100,
    total: Math.round(total),
    overpayment: Math.round(overpayment),
  };
}

Differentiated Payment

Early payments are larger, later ones are smaller. Some banks (especially for mortgages) offer this option:

function calcDifferentiated(principal, annualRate, months) {
  const r = annualRate / 12 / 100;
  const principalPart = principal / months;
  const schedule = [];

  let balance = principal;
  let totalInterest = 0;

  for (let i = 1; i <= months; i++) {
    const interestPart = balance * r;
    const payment = principalPart + interestPart;
    totalInterest += interestPart;
    balance -= principalPart;

    schedule.push({
      month: i,
      payment: Math.round(payment * 100) / 100,
      principal: Math.round(principalPart * 100) / 100,
      interest: Math.round(interestPart * 100) / 100,
      balance: Math.max(0, Math.round(balance * 100) / 100),
    });
  }

  return {
    schedule,
    total: Math.round((principal + totalInterest) * 100) / 100,
    overpayment: Math.round(totalInterest * 100) / 100,
    firstPayment: schedule[0].payment,
    lastPayment: schedule[months - 1].payment,
  };
}

Payment Schedule

An amortization table is a mandatory element of a mortgage calculator:

function PaymentSchedule({ schedule }) {
  const [expanded, setExpanded] = useState(false);
  const visible = expanded ? schedule : schedule.slice(0, 12);

  const fmt = (n) => new Intl.NumberFormat('en-US', {
    minimumFractionDigits: 2, maximumFractionDigits: 2,
  }).format(n);

  return (
    <div className="schedule">
      <table>
        <thead>
          <tr>
            <th>Month</th>
            <th>Payment</th>
            <th>Principal</th>
            <th>Interest</th>
            <th>Balance</th>
          </tr>
        </thead>
        <tbody>
          {visible.map(row => (
            <tr key={row.month}>
              <td>{row.month}</td>
              <td>${fmt(row.payment)}</td>
              <td>${fmt(row.principal)}</td>
              <td>${fmt(row.interest)}</td>
              <td>${fmt(row.balance)}</td>
            </tr>
          ))}
        </tbody>
      </table>
      {schedule.length > 12 && (
        <button onClick={() => setExpanded(e => !e)}>
          {expanded ? 'Collapse' : `Show all ${schedule.length} months`}
        </button>
      )}
    </div>
  );
}

Early Repayment

A real mortgage involves a series of recalculations with partial early repayment:

function applyEarlyRepayment(schedule, month, amount, mode = 'reduce_term') {
  // mode: 'reduce_term' — shorten term, 'reduce_payment' — reduce payment
  let balance = schedule[month - 1].balance - amount;
  if (balance <= 0) return { paidOff: true };

  const remainingMonths = schedule.length - month;

  if (mode === 'reduce_term') {
    // Recalculate how many months are needed at the same rate
    // (inverse problem — iteratively)
    return recalculateWithNewBalance(balance, currentRate, 'minimize_term');
  }

  if (mode === 'reduce_payment') {
    return recalculateWithNewBalance(balance, currentRate, remainingMonths);
  }
}

Visualization

A pie or bar chart "principal / overpayment" is mandatory. Implementation via Chart.js (lightweight) or Recharts (if already used in the project):

import { Doughnut } from 'react-chartjs-2';

function OverpaymentChart({ principal, overpayment }) {
  return (
    <Doughnut
      data={{
        labels: ['Principal', 'Overpayment'],
        datasets: [{
          data: [principal, overpayment],
          backgroundColor: ['#3b82f6', '#f87171'],
          borderWidth: 0,
        }],
      }}
      options={{ plugins: { legend: { position: 'bottom' } } }}
    />
  );
}

Mortgage Calculator Parameters

Add insurance to the monthly payment: insurance = balance * insuranceRate / 100 / 12.

Data Exchange with Application Form

function prefillApplicationForm(calcResult) {
  const data = {
    loan_amount: calcResult.principal,
    monthly_payment: calcResult.monthly,
    loan_term_months: calcResult.months,
    estimated_rate: calcResult.annualRate,
  };

  // Can be passed via URL to the application page
  const qs = new URLSearchParams(data).toString();
  window.open(`/mortgage-application?${qs}`, '_blank');
}

Timeframe

Annuity calculator with payment schedule and chart — 3–4 working days. Full version with differentiated calculation, early repayment, insurance, PDF export, and form integration — 7–10 days.