Calculadora de Interés Compuesto — Fórmula

How Compound Interest Actually Works

Compound interest means you earn interest on your interest. After month one, your earnings get added to your balance, and next month you earn interest on that larger number. Over short periods the effect is small. Over decades it gets dramatic.

The Formula

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

  • FV = What your money grows to
  • P = Starting amount
  • PMT = What you add each month
  • r = Monthly interest rate (annual rate / 12 / 100)
  • n = Total months

    • When to Use This

    Use it to compare saving vs. spending decisions. If you're wondering whether to invest $200/month or spend it, plug the numbers in and see what 20 years of compounding does. It's also useful for comparing two scenarios side by side, like 6% vs 8% returns, or starting now vs. starting in 5 years.

    What This Assumes

    This calculator uses a fixed rate of return, which doesn't happen in real markets. The S&P 500 has averaged about 10% per year historically (7% after inflation), but individual years swing from -37% to +52%. Use this for planning estimates, not predictions. It also assumes monthly compounding, which is standard for most savings accounts and investment platforms.

    The Part People Underestimate

    The contribution matters more than the starting amount when you have time. $200/month for 30 years at 7% grows to about $243,000 in contributions and $243,000 in interest. Half of your ending balance came from money you never deposited. But almost all of that interest growth happens in the last 10 years. The first decade feels slow, and that's where most people give up.

    Common Mistakes

  • Using a pre-inflation return rate and treating the result as today's purchasing power. If you use 10%, subtract 3% for inflation to get a realistic number.
  • Ignoring taxes on investment gains. In a taxable account, your actual growth is lower than the nominal rate.
  • Comparing compound interest across different compounding frequencies without adjusting. Monthly compounding at 6% is slightly more than annual compounding at 6%.

    • Ejemplo Resuelto

    You invest $10,000 and add $200 per month at 7% annual return for 20 years.

    1. Monthly rate: 7% / 12 = 0.5833% (0.005833)
    2. Total months: 20 * 12 = 240
    3. Growth of initial investment: $10,000 * (1.005833)^240 = $40,387.39
    4. Growth of contributions: $200 * [(1.005833)^240 - 1] / 0.005833 = $104,185.06
    5. Future Value = $40,387.39 + $104,185.06 = $144,572.45
    6. Total contributions: $10,000 + ($200 * 240) = $58,000
    7. Total interest earned: $144,572.45 - $58,000 = $86,572.45