Compound Interest Calculator

Compound interest is the mechanism by which invested money grows exponentially rather than linearly — each period's earnings become part of the base that generates future earnings. The difference between simple and compound interest appears modest in the first few years but becomes dramatic over decades. A $10,000 investment earning 7% simple interest grows to $17,000 in ten years; the same investment compounded monthly reaches approximately $20,097. At thirty years, the gap widens further: $31,000 versus nearly $81,000. This calculator handles two growth sources simultaneously: the lump-sum principal that compounds over the full investment period, and regular monthly contributions that each begin compounding from the moment they are added. Depositing even a modest amount each month dramatically accelerates the final balance, because every new dollar has time to earn compounding returns of its own. The compounding frequency — daily, monthly, quarterly, or annually — controls how often earned interest is added to the base. Daily compounding produces slightly more growth than monthly at the same stated rate, a difference that compounds into real money over long periods and at higher rates. Most savings accounts and brokerage accounts compound monthly or daily. This calculator is most useful for retirement planning, college savings projections, evaluating investment account options, and quantifying the long-term cost of waiting to start. The total contributions versus interest earned breakdown makes the leverage of time and rate immediately visible — and makes the case for starting sooner rather than later.

The calculator applies two separate formulas and sums the results. The first covers the lump-sum principal: FV_principal = P × (1 + r/n)^(n×t). Here P is the initial investment, r is the annual rate as a decimal, n is the compounding frequency per year (12 for monthly, 365 for daily), and t is the number of years. For $10,000 at 7% compounded monthly for 20 years: r/n = 0.005833, exponent n×t = 240, giving FV_principal = 10,000 × (1.005833)^240 ≈ $40,064. The second formula calculates the future value of all monthly contributions as an ordinary annuity, always using the monthly rate (r/12) regardless of the selected compounding frequency: FV_contributions = PMT × [(1 + r/12)^(12t) − 1] ÷ (r/12). For $500/month at 7% over 20 years, this gives approximately $131,291. The final balance is the sum of both: $40,064 + $131,291 ≈ $171,355. Total contributions equal the initial principal plus all monthly deposits (PMT × total months). Interest earned is the final balance minus total contributions. The growth multiple divides final balance by total contributions to show how much each dollar deposited has grown — a number that rises sharply with longer time horizons and higher rates, illustrating why time in market consistently outperforms timing the market.