Calculadora de Hipoteca — Fórmula

How Mortgage Payments Work

Your monthly mortgage payment stays the same every month for the life of a fixed-rate loan, but what's inside that payment shifts over time. Early on, most of your payment goes toward interest. By year 20 of a 30-year loan, the split flips and most goes toward principal.

The Formula

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

  • M = Monthly payment (principal + interest only)
  • P = Loan amount (home price minus your down payment)
  • r = Monthly interest rate (annual rate divided by 12, then by 100)
  • n = Total payments (loan term in years times 12)

    • When to Use This Calculator

    Use it when comparing homes at different price points, testing what happens if you put more down, or seeing how a rate change affects your payment. It's also useful for checking whether a 15-year term is affordable vs. a 30-year.

    What This Doesn't Include

    This calculates principal and interest only. Your actual monthly housing cost will be higher because of property taxes (typically 1-2% of home value per year), homeowner's insurance, and PMI if your down payment is under 20%. On a $300K home, taxes and insurance can add $300-$500/month on top of the P&I number shown here.

    What Changes the Payment Most

    Interest rate has the biggest impact. On a $240,000 loan, the difference between 6% and 7% is about $160/month, which adds up to $57,600 over 30 years. Loan term is next. A 15-year term roughly doubles your monthly payment but saves you more than half the total interest.

    Common Mistakes

  • Forgetting to subtract the down payment before calculating. The formula uses the loan amount, not the home price.
  • Comparing rates without checking if they include points. A lower rate with 2 points upfront might cost more than a slightly higher rate with no points.
  • Not accounting for PMI. If you put less than 20% down on a conventional loan, PMI typically adds 0.5-1% of the loan amount per year to your cost.

    • Ejemplo Resuelto

    You want to buy a $300,000 home with a $60,000 down payment (20%) at a 6.5% annual interest rate for a 30-year fixed mortgage.

    1. Calculate the loan principal: $300,000 - $60,000 = $240,000
    2. Convert annual rate to monthly: 6.5% / 12 = 0.5417% (0.005417)
    3. Calculate total payments: 30 * 12 = 360 monthly payments
    4. Apply the formula: M = $240,000 * [0.005417 * (1.005417)^360] / [(1.005417)^360 - 1]
    5. Monthly Payment = $1,517.09
    6. Total amount paid over 30 years: $1,517.09 * 360 = $546,152.40
    7. Total interest paid: $546,152.40 - $240,000 = $306,152.40