Compound Interest Calculator
Last updated July 2, 2026
Compound interest is the mechanism by which money grows on itself — earning returns not just on the original principal but on every dollar of previously earned interest. The distinction from simple interest is profound in dollar terms over long time periods. A $10,000 investment at 7 percent simple interest grows by $700 per year to $17,000 after 10 years. At 7 percent compound interest, the same investment grows to $19,672 — an extra $2,672 produced solely by the compounding effect. Over 30 years, the gap is even more dramatic: $31,000 with simple interest versus $76,123 with compounding. The difference is entirely attributable to earning returns on accumulated returns rather than returns on just the original amount.
Compounding frequency — how often interest is calculated and added to the balance — also affects the final amount, though with diminishing returns at higher frequencies. Monthly compounding produces a meaningfully higher result than annual compounding; daily compounding produces only marginally more than monthly. The practical implication is that the compounding frequency of a savings account (daily) versus an investment account (effectively daily through price appreciation) is less important than the rate of return itself. The Rule of 72 offers a fast approximation: divide 72 by the annual return rate to estimate how many years it takes to double the investment. At 7 percent, money doubles approximately every 10.3 years; at 10 percent, every 7.2 years. That doubling math is what makes starting early so financially consequential — every decade of compounding doubles the previous decade's outcome.
Compound interest rewards two things above all: time and contribution consistency. A $10,000 investment at age 25 growing at 7 percent becomes $149,745 by age 65 — nearly 15 times the original amount with no additional contributions. Adding $500 per month to the same account produces over $1.3 million over the same period. Calculate both scenarios — lump sum and with contributions — to see how much of your projected future wealth is generated by compounding versus by what you actually put in.