See exactly how your money grows over time. Calculate future value, total interest earned, and a year-by-year growth breakdown.
Calculate Compound Interest
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⚡ Rule of 72 — Money Doubles In
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What Is Compound Interest?
Also searched as: compound interest calculator free | investment growth calculator | how much will my money grow | compound interest formula calculator Optimized for US users with American units and terminology.
Compound interest is one of the most powerful concepts in personal finance. Unlike simple interest, which is calculated only on your original principal, compound interest is calculated on both your principal and the interest you've already earned. This creates a snowball effect where your money grows faster and faster over time.
Albert Einstein reportedly called compound interest the "eighth wonder of the world" — and for good reason. Given enough time, even a modest initial investment can grow into a substantial sum without any additional contributions.
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 (starting amount), 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. This calculator handles all of that math instantly, including optional monthly contributions.
How Compounding Frequency Affects Growth
The more frequently your interest compounds, the more you earn. Daily compounding earns slightly more than monthly compounding, which earns more than annual compounding. For most practical purposes — like a savings account or index fund — the difference between daily and monthly compounding is small. What matters far more is the interest rate and time horizon.
The Power of Time
Time is the most important factor in compound interest. Starting to invest early — even with a small amount — dramatically outperforms starting later with a larger amount. For example, $10,000 invested at 8% for 30 years grows to over $100,000. The same $10,000 invested for only 20 years grows to about $46,600. Those extra 10 years more than double the outcome.
The Rule of 72
The Rule of 72 is a simple mental math shortcut to estimate how long it takes to double your money. Divide 72 by your annual interest rate. At 6% annual return, your money doubles in approximately 12 years. At 10%, it doubles in about 7.2 years. This calculator shows your Rule of 72 estimate automatically based on your entered rate.
Frequently Asked Questions
Use the Compound Interest Calculator above — enter your values and get instant results. This free online tool calculates how does compound interest work without any download or signup required. Results update in real time as you type.
Use the Compound Interest Calculator above — enter your values and get instant results. This free online tool calculates compound interest vs simple interest without any download or signup required. Results update in real time as you type.
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Over long periods, compound interest produces significantly higher returns than simple interest at the same rate.
Most US savings accounts and high-yield savings accounts compound interest daily and credit it monthly. Money market accounts typically also compound daily. CDs may compound daily, monthly, or quarterly depending on the bank.
Yes. While stocks don't pay "interest" directly, reinvesting dividends and allowing capital gains to compound produces the same exponential growth effect. The S&P 500 has historically returned around 10% annually before inflation, making it one of the most effective compounding vehicles available.
APR (Annual Percentage Rate) is the simple annual rate without compounding. APY (Annual Percentage Yield) accounts for compounding and shows the actual return you earn in a year. APY is always equal to or higher than APR. When comparing savings accounts, always compare APY.
Regular monthly contributions dramatically accelerate compound growth. Each contribution starts compounding immediately, so even small monthly additions — $100 or $200 — can add tens of thousands of dollars to your final balance over a 20–30 year period. Use the monthly contribution field to see the difference.