Estimate compound growth with an initial amount, monthly contributions, an annual rate, and your chosen compounding frequency.
Compound Interest Calculator
Compound growth builds on prior growth
The calculator converts the selected nominal annual rate and compounding frequency into an equivalent monthly growth rate so monthly contributions can be modeled consistently. Results are estimates and do not include taxes, fees, inflation, or market volatility.
How to use the compound interest calculator
Enter your initial investment, any monthly contribution you plan to add, the expected annual interest or return rate, the number of years, and how often interest is compounded. Press Calculate to see your ending balance, how much of it came from your own contributions, how much came from growth, and a year-by-year table.
What is compound interest?
With simple interest, you only earn interest on the money you put in. With compound interest, you also earn interest on the interest you have already earned. Each year, the growth is calculated on a slightly bigger balance, so the balance grows faster and faster over time.
The basic formula for a single deposit is:
- Future value = P × (1 + r ÷ n)^(n × t)
where P is the starting amount, r is the annual rate as a decimal, n is the number of times interest is compounded per year, and t is the number of years. This calculator also adds your monthly contributions and lets the formula run month by month.
Example
You invest $5,000, add $200 a month, and earn 6% a year, compounded monthly, for 10 years. You will have put in $29,000 of your own money. The ending balance is about $41,872.85, so roughly $12,873 of it is growth.
Why starting early matters
Time is the most powerful number in the formula. Because growth builds on earlier growth, the last few years of a long investment usually add more money than the first few. Even small monthly amounts can grow into a significant sum if they are left alone long enough. Try the calculator with the same monthly amount for 10, 20, and 30 years, and compare the results.
Does compounding frequency matter?
Yes, but less than most people think. In the example above, compounding annually gives about $41,449, monthly gives about $41,873, and daily gives about $41,912. The rate, the time, and the amount you contribute make a much bigger difference.
The rule of 72
Here is a quick way to estimate how long it takes money to double: divide 72 by the annual rate. At 6%, money doubles in about 12 years. At 8%, about 9 years. It’s an approximation, but it’s handy for quick comparisons.
Keep in mind
- Investment returns are not guaranteed. Savings accounts pay a stated rate, but stock and fund returns go up and down from year to year. The calculator assumes a steady rate.
- Inflation reduces buying power. $41,000 in ten years will buy less than $41,000 does today.
- Taxes and fees are not included and can lower your real return.
This tool is for education and planning only and is not financial advice. See our Financial Disclaimer.
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