Compound Interest Calculator
This compound interest calculator shows how your money grows when you earn interest on your interest. Enter a starting amount, a monthly contribution, your expected return, and a time period. The calculator above also lets you choose how often interest compounds, from yearly to daily, so you can see the effect for yourself.
How it's calculated
Compound interest is the interest you earn on both your original money and the interest it has already earned. Over time, this snowball effect can grow your balance far beyond what you put in. To use the tool, enter your starting balance, regular contribution, annual return rate, and number of years.
The calculator above includes a compounding frequency control. Compounding frequency is how often your earned interest gets added back to your balance. More frequent compounding earns slightly more. But time and your return rate matter far more, so focus there first. To project a full portfolio, try the investment growth calculator.
A worked example
Imagine you start with $5,000 and add $200 every month. You expect a 6% annual return, compounded monthly, and you leave the money invested for 30 years. After 30 years, your balance grows to $231,016. You contributed $77,000 of your own money over that time. The remaining $154,016 is pure growth, the interest earned on your interest. This shows why time in the market matters so much.
Common mistakes to avoid
- Forgetting to add regular contributions. Steady monthly deposits often drive most of your long-term growth, not the starting balance.
- Overestimating your annual return. Using an unrealistic rate inflates results. A modest, realistic figure gives a more useful estimate.
- Obsessing over compounding frequency. Daily versus annual compounding changes results only slightly compared to time and rate.
- Ignoring inflation and taxes. The calculator shows nominal growth, so your real spending power will be lower.
- Starting too late. Compounding rewards time, so delaying even a few years can cost you a large share of your final balance.
Frequently asked questions
What is a compound interest calculator?
A compound interest calculator is a tool that shows how your money grows when you earn interest on your interest. You enter a starting amount, contributions, a return rate, and a time period. It then estimates your future balance. The calculator above also lets you pick how often interest compounds.
How does compounding frequency affect my returns?
More frequent compounding earns slightly more, but the difference is small. On $10,000 at 5% over 10 years with no contributions, annual compounding grows to $16,289. Quarterly reaches $16,436, monthly $16,470, and daily $16,487. Time and your return rate matter far more than frequency.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes money to double. You divide 72 by your annual return rate. At a 6% return, your money doubles in about 12 years. It is an estimate, not an exact figure, but it helps you gauge growth fast.
Why does compound interest grow money so fast over time?
Compound interest grows money quickly because you earn returns on your past returns, not just your original deposit. Each year, the base that earns interest gets larger. This creates a snowball effect that speeds up the longer you stay invested, which is why starting early matters.
How do taxes reduce my compound interest returns?
In a taxable brokerage account, investment earnings — dividends, interest, and realized capital gains — are taxed annually, which reduces your effective compounding rate. A 22% federal income tax bracket applied to a 10% nominal return leaves roughly 7.8% after tax on ordinary income (10% × (1 − 0.22) = 7.8%). Over 30 years the difference is dramatic: $10,000 growing at 10% reaches approximately $174,500, while the same amount at 7.8% reaches only about $96,000 — nearly half as much. Tax-advantaged accounts like a Roth IRA or 401(k) eliminate this annual drag entirely, which is why most advisors recommend maxing those before investing in taxable accounts.