Compound Interest Calculator
The compound interest calculator shows how your money grows over time when interest is earned on both the principal and accumulated interest. Einstein called compound interest the eighth wonder of the world.
Advertisement
Calculator
See your Compound Interest Calculator results
Enter your email to unlock results — free forever.
No spam, ever. Unsubscribe at any time.
Advertisement
Formula
A = P(1 + r/n)^(nt)
A is the final amount, P is the principal (initial investment), 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. The power (nt) is what creates the exponential growth effect.
How to use the Compound Interest Calculator
- 1
Enter your initial investment
Value should be in $.
- 2
Enter your annual interest rate
Value should be in %.
- 3
Enter your time period
Value should be in years.
- 4
Enter your compounding frequency
- 5
Enter your monthly contribution
Value should be in $.
- 6
Read your results instantly
Results update in real time as you type.
Advertisement
What is compound interest?
Compound interest is interest calculated on both your initial principal and the accumulated interest from prior periods. Unlike simple interest — which is calculated only on the principal — compound interest grows on itself, creating an exponential curve rather than a straight line.
The more frequently interest compounds, and the longer you leave money invested, the more dramatic the effect. A $10,000 investment at 7% compounded monthly for 30 years grows to over $81,000 — without a single additional dollar contributed.
Compounding frequency matters
The number of times interest is added to your principal per year affects your final balance. Daily compounding yields slightly more than annual compounding because you earn interest on interest more frequently.
For example, $10,000 at 6% for 10 years: annually compounds to $17,908, while daily compounding reaches $18,220 — a difference of $312. The gap becomes more significant over longer time horizons.
Advertisement
The power of regular contributions
Adding regular contributions dramatically accelerates growth. Adding just $200/month to a $10,000 initial investment at 7% over 20 years turns your total contributions of $58,000 into over $151,000 — with more than $93,000 coming from interest alone.
This is the foundation of dollar-cost averaging and why financial advisors consistently recommend starting to invest as early as possible, even in small amounts.
Real-world applications
Compound interest works for you in savings accounts, certificates of deposit, index funds, and retirement accounts like 401(k)s and IRAs. It works against you in credit card debt and high-interest loans, where balances can spiral quickly if left unpaid.
The same mathematics that grows your wealth can erode it — understanding compound interest helps you harness it on the right side of the equation.
Tips & Insights
Start early, not big
A 25-year-old investing $200/month will accumulate significantly more by 65 than a 35-year-old investing $400/month, despite contributing less total money — purely due to the extra 10 years of compounding.
Tax-advantaged accounts amplify compounding
In a 401(k) or Roth IRA, your gains aren't taxed annually, so the full compounding effect applies. In a taxable account, yearly taxes reduce the effective return rate.
Inflation must be factored in
A nominal return of 7% per year with 3% inflation is a real return of about 4%. Use the real rate when estimating future purchasing power, not just nominal growth.
Worked Examples
Retirement savings scenario
Starting with $5,000 and contributing $300/month at 7% annually for 30 years results in approximately $366,000 — of which only $113,000 was contributed and $253,000 was interest.
College savings comparison
$10,000 invested today at 6% compounded monthly will grow to approximately $29,000 by the time your newborn reaches college age — nearly tripling without any additional contributions.
Advertisement
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on the initial principal as well as the accumulated interest from previous periods. This causes wealth to grow exponentially over time.
How often should interest compound?
More frequent compounding (daily vs. annually) results in slightly higher returns. For long-term investments, the difference becomes significant over decades.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it will take to double your investment. Divide 72 by the annual interest rate to get the approximate years to double. At 7%, your money doubles roughly every 10.3 years.
What is a realistic interest rate to use?
The S&P 500 has returned approximately 10% annually (7% after inflation) over the long term. For conservative planning, many advisors suggest using 6-7% nominal returns.
How does compound interest work against me?
Credit cards charge compound interest on unpaid balances. A $5,000 balance at 20% APR compounded daily costs over $1,000 in interest per year — and grows exponentially if you only make minimum payments.
Advertisement