Compound Interest Calculator – Investment Growth Over Time
See how your savings or investments grow with compound interest. Enter a starting amount, monthly contribution, interest rate and years to see the future value and total interest earned.
Compound Interest Calculator
Starting balance, contributions, rate and time
Assumes contributions are made at the start of each month and returns are constant. Real investment returns vary year to year.
What is compound interest?
Compound interest is interest earned on both your original money and on the interest already added to it. Because the growth builds on itself, balances grow faster the longer you leave them – Albert Einstein reportedly called it the eighth wonder of the world.
Compound interest formula
A = P(1 + r/n)nt, where P is the principal, r the annual rate as a decimal, n the number of compounding periods per year and t the years. With regular monthly contributions the calculator adds each deposit and compounds the running balance.
Example
$10,000 invested with $300 added every month at 7% per year for 20 years grows to roughly $195,000 – of which about $82,000 is your own contributions and about $113,000 is interest.
The rule of 72
Divide 72 by your interest rate to estimate how many years it takes money to double. At 7% that is about 10 years; at 10% about 7 years.
Where does 7% come from?
The US stock market (S&P 500) has returned roughly 10% per year on average over the long term, or about 7% after inflation, which is why 7% is a common planning assumption for retirement accounts such as a 401(k) or IRA. High-yield savings accounts pay far less – typically 3–5%.
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