Compound Interest Calculator

What Is Compound Interest?

Compound interest is interest calculated on both your original principal and the interest that has already accumulated on it. Simple interest, by contrast, is only ever calculated on the original principal, so it grows in a straight line. Compound interest grows on a curve that gets steeper over time, because each period's interest becomes part of the balance that earns interest in the next period.

The standard formula is A = P(1 + r/n)^(nt), where A is the final amount, P is your principal, r is the annual interest rate as a decimal, n is how many times per year interest compounds, and t is the number of years. If you want the full derivation, a worked step-by-step walkthrough, and how the formula changes once you add regular contributions, see our compound interest formula page.

As a quick illustration: $1,000 invested at a 5% annual rate, compounded monthly, for 10 years grows to $1,647.01: $647.01 of that is pure interest, a 64.7% gain on the original $1,000, without adding another dollar.

How Compounding Frequency Changes Your Result

The more often interest is added to your balance, the sooner it starts earning interest of its own, so a higher compounding frequency always produces a slightly larger result at the same nominal annual rate. The difference is real but modest, since each individual compounding period is applying a smaller slice of the annual rate. Here is $10,000 invested at a 5% annual rate for 20 years, with no further contributions, at three different compounding frequencies:

Switching from annual to daily compounding on this $10,000 investment adds $647.98 over 20 years: worth having, but far less dramatic than choosing a higher interest rate or contributing more, which is why the calculator above lets you compare all three by adjusting the compounding frequency directly.

Using This Calculator

Enter your starting amount (use 0 if you are starting from scratch), an optional monthly contribution, the number of years you plan to invest, your expected annual interest rate, and how often the return compounds. The result updates instantly as you type: your future value, your total contributions (principal plus every deposit, with no interest included), and your total interest earned.

Below the main result, the calculator also shows a three-scenario comparison: what happens if your actual rate turns out to be one percentage point lower than expected (pessimistic), exactly as entered (expected), or one percentage point higher (optimistic). Interest rates are never perfectly predictable over long horizons, so seeing the realistic spread around your estimate is more useful than trusting a single number.

Everything happens locally in your browser using plain JavaScript: nothing you type is sent to a server, logged, or stored anywhere outside your own device's local storage (which only remembers your last inputs so you don't have to retype them on your next visit).

Real-World Examples

Retirement contributions. Investing $5,000 every year from age 25 to age 65 (40 years) at a 7% average annual return grows to $998,175.56: just short of a million dollars: even though you only contributed $200,000 of your own money across those four decades. The remaining $798,175.56 is compound growth.

A college fund. $10,000 set aside for a child's education, growing at 6% compounded monthly for 8 years, becomes $16,141.43.

A high-yield savings account. $50,000 parked in an account paying 4.5% APY, compounded daily, grows to $62,615.27 after 5 years: with zero investment risk and full access to the funds.

A long-term investment portfolio. Contributing $10,000 every year for 30 years at a 10% average annual return (roughly the long-run historical average for a diversified stock portfolio) grows to $1,644,940.23. Most of that total comes from growth, not from the $300,000 actually contributed.

Debt works the same way, against you. A $5,000 credit card balance at a 20% APR, compounded daily and left completely untouched with no payments for a single year, grows to $6,106.68: $1,106.68 in interest on top of the original balance. This is the same formula working in the lender's favor, which is exactly why paying down high-interest debt quickly matters so much.

Common Misconceptions About Compound Interest

"A one-percent difference in rate barely matters." It does, enormously, over long periods. $100,000 invested at 6% for 30 years grows to $574,349.12; the same $100,000 at 5% grows to only $432,194.24: a $142,154.88 difference from a single percentage point.

"I need a large amount to start." You don't. $100 contributed every month from age 25 to age 65, at a 7% average return compounded monthly, grows to $262,481.34. Consistency and time matter far more than the size of any individual contribution. If you have a specific target amount in mind instead, try the savings goal calculator, which works backwards from a goal to tell you the monthly contribution you would actually need.

"Compounding only applies to savings accounts." The same math applies to retirement accounts, brokerage accounts, mortgages, and any interest-bearing loan: understanding it helps you evaluate all of them, not just a savings account.

Compound Interest vs. Simple Interest: A Side-by-Side Example

The gap between simple and compound interest is invisible at first and enormous later. Take $10,000 at a 6% annual rate for 30 years. Under simple interest, you would earn a flat $600 every single year, for a total of $18,000 in interest and a final balance of $28,000. Under compound interest (annual compounding), the same $10,000 at 6% grows to $57,434.91: more than double the simple-interest result, because every year's interest joins the balance and starts earning its own interest.

The two methods start close together: after year one, both produce exactly $10,600, since there has been no prior interest yet to compound. The gap only opens up over time, which is why compound interest rewards patience so disproportionately: and why the same math is worth understanding whether you are the one earning the interest, or the one paying it.

How to Maximize Compound Interest

Start earlier. Time matters more than almost any other variable. Contributing $200 a month at a 7% return starting at age 25 (40 years to age 65) produces $524,962.68. Waiting until age 35 to start the exact same $200 monthly contribution at the same rate (30 years to age 65) produces only $243,994.20: less than half, for an identical monthly amount, purely because of the ten-year delay.

Seek a higher rate, within your risk tolerance. As the misconceptions section above shows, even a single percentage point compounds into a large difference over decades.

Favor more frequent compounding when everything else is equal. Daily compounding will always slightly outperform monthly or annual compounding at the same nominal rate, see the comparison table above.

Add regular contributions. $10,000 invested once at 6%, compounded monthly, grows to $33,102.04 over 20 years with no further deposits. Add just $100 a month over the same 20 years and the total reaches $79,306.13: more than double, from a contribution of $100 a month.

Avoid early withdrawals. Every dollar withdrawn stops earning interest immediately and never catches back up to where it would have been.

Reinvest dividends and interest payouts instead of taking them as cash, so they can start compounding on their own.