Compound Interest Calculator
See how your investments grow over time with the power of compound interest. Enter your initial amount, monthly contributions, expected annual rate, and investment period to calculate your projected returns.
Compound interest is the process of earning interest on both your initial principal and the accumulated interest from previous periods. Unlike simple interest, which only applies to the original amount, compound interest causes your investment to grow exponentially — the longer you invest, the faster your balance accelerates.
The compounding frequency significantly affects your final balance. Monthly compounding produces higher returns than annual compounding because interest is reinvested more often. For example, $10,000 at 7% annual rate compounded monthly grows to $20,097 after 10 years, versus $19,672 with annual compounding — a difference of over $400 from the same rate.
Over long time horizons, compound interest becomes one of the most powerful forces in personal finance. Starting early matters more than contributing large amounts later. An investor who starts at age 25 and contributes $300/month at 7% will accumulate significantly more by age 65 than someone who starts at 35 with $600/month — demonstrating why time in the market is the key variable.
Popular Scenarios
- Investing $1,000 Per Month at 5% for 10 Years
- $10,000 Investment at 7% Interest Over 20 Years
- Investing $500 Per Month at 8% for 30 Years
- $50,000 Lump Sum Investment at 6% for 15 Years
- Investing $200 Per Month at 10% for 25 Years
- $25,000 Plus $300/Month at 7% for 20 Years
- $100 Per Month with Quarterly Compounding for 10 Years
- Investing $400/Month at Age 25: How Much by Age 55?
- Starting With $15,000 and Investing $300/Month at 8% for 20 Years
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. It allows your investment to grow exponentially over time.
How often should interest compound?
More frequent compounding (monthly vs. annually) produces slightly higher returns. Monthly compounding is most common for savings accounts and investment accounts.
What is a realistic annual return rate?
The S&P 500 has historically returned around 7–10% annually over long periods. A 7% rate is commonly used for conservative long-term projections after inflation adjustment.