Mortgage Calculator: Estimate Monthly Payments & Interest

Buying a home starts with one question: can you afford the monthly payment? A free mortgage calculator turns home price, down payment, interest rate, and loan term into a clear payment estimate — plus a full amortization schedule so you see where every dollar goes over 15 or 30 years.

Even a half-point change in your rate can add tens of thousands to lifetime interest. Running scenarios before you make an offer helps you set a realistic budget, compare 15-year vs 30-year terms, and decide whether a larger down payment is worth skipping PMI.

How Monthly Mortgage Payments Are Calculated

Fixed-rate loans use a standard amortization formula. Three inputs drive the result: principal (amount borrowed), monthly interest rate (annual rate ÷ 12), and total number of payments (years × 12).

M = P * [ r(1 + r)^n ] / [ (1 + r)^n - 1 ]

Where:
- M is the total monthly payment.
- P is the principal loan amount (total amount borrowed).
- r is the monthly interest rate (annual rate divided by 12).
- n is the total number of monthly payments (loan term in years multiplied by 12).

Example: Borrow $400,000 at 6% for 30 years. Monthly rate = 0.005, payments = 360. Result: $2,398.20 per month in principal and interest. Property taxes, insurance, and PMI are usually added on top unless you enter them in the tool.

Why Use a Mortgage Calculator Before You Buy

Static estimates rarely hold up when rates shift or you adjust your down payment. A mortgage calculator lets you slide variables and see the long-term impact immediately — all in your browser, with no financial data uploaded.

Bump your down payment from 10% to 20% and you may eliminate PMI, saving hundreds each month. Compare a 15-year term against 30 years to weigh higher payments against massive interest savings. If you also invest, pair this with our trading P&L guide to balance housing costs against portfolio growth.

Reading Your Amortization Schedule

An amortization table shows how each payment splits between interest and principal. Early on, most of your payment goes to interest because the balance is highest. Over time, the principal portion grows — and extra payments accelerate that shift.

That is why early extra payments are so powerful: they reduce the balance future interest is calculated on, compounding savings for the rest of the term.

How Amortization Is Computed in Code

For developers building loan tools, the logic is a base payment formula plus a month-by-month loop:

interface AmortizationRow {
  month: number;
  payment: number;
  principal: number;
  interest: number;
  remainingBalance: number;
}

export function calculateMortgage(
  homePrice: number,
  downPayment: number,
  annualRate: number,
  years: number
) {
  const principal = homePrice - downPayment;
  const monthlyRate = annualRate / 100 / 12;
  const totalPayments = years * 12;

  const monthlyPayment =
    (principal * (monthlyRate * Math.pow(1 + monthlyRate, totalPayments))) /
    (Math.pow(1 + monthlyRate, totalPayments) - 1);

  const schedule: AmortizationRow[] = [];
  let remainingBalance = principal;

  for (let i = 1; i <= totalPayments; i++) {
    const interest = remainingBalance * monthlyRate;
    const principalPaid = monthlyPayment - interest;
    remainingBalance = Math.max(0, remainingBalance - principalPaid);

    schedule.push({
      month: i,
      payment: monthlyPayment,
      principal: principalPaid,
      interest,
      remainingBalance,
    });
  }

  return { monthlyPayment, schedule };
}

Client-side execution keeps the tool responsive — tables and charts update instantly as you adjust sliders, with no server round-trip.

Pair Mortgage Planning with Other Financial Tools

Housing is one piece of your full financial picture. Active traders can use the Trading P&L Calculator to track cost basis and returns. Income investors may also model passive cash flow with the Dividend Calculator — useful when weighing rent vs buy decisions.

Open the ToolMars Mortgage Calculator to enter your parameters, simulate extra payments, and review your full amortization table before you commit to a loan.

Conclusion

A mortgage calculator turns abstract loan terms into numbers you can actually budget around. Run your price, rate, and down payment through the tool, compare term lengths, and check how extra payments change total interest. When the monthly figure fits your plan, you can shop for a home with confidence instead of guesswork.

Frequently Asked Questions