Compound Interest Calculator
See how your money grows over time with the power of compound interest and regular contributions.
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Free Compound Interest Calculator
The Toolts Compound Interest Calculator shows how your savings and investments grow over time through the power of compounding. Enter your initial deposit, monthly contributions, interest rate, and time period to see a complete year-by-year breakdown of your money's growth. All calculations happen instantly in your browser.
What Is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is calculated only on the principal, compound interest makes your money grow exponentially over time. Albert Einstein reportedly called it the eighth wonder of the world. The more frequently interest compounds, the faster your balance grows.
The Compound Interest Formula
The formula for compound interest is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal (initial deposit), r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the number of years. When you add regular contributions, the formula expands to include the future value of an annuity component.
How Compounding Frequency Affects Growth
Interest can compound annually, semi-annually, quarterly, monthly, or even daily. More frequent compounding leads to slightly higher returns. For example, $10,000 at 7% compounded annually for 20 years grows to $38,697. The same amount compounded monthly grows to $40,387. The difference becomes more significant with larger amounts and longer time periods.
The Rule of 72
A quick way to estimate how long it takes to double your money is the Rule of 72. Divide 72 by the annual interest rate to get the approximate number of years. At 7% interest, your money doubles in roughly 10.3 years. At 10%, it doubles in about 7.2 years. This simple rule helps you quickly evaluate investment opportunities without a calculator.
Why Starting Early Matters
Time is the most powerful factor in compound interest. Someone who invests $500 per month starting at age 25 will have significantly more at retirement than someone who invests the same amount starting at age 35, even though the difference in contributions is only $60,000. The extra decade of compounding creates hundreds of thousands of dollars in additional growth. Starting early is the single best financial decision you can make.