What Is Compound Interest?
Compound interest is the process of earning interest on both your initial principal and on the interest you've already accumulated. Often called "the eighth wonder of the world," compounding is the most powerful force in personal finance — the longer your money compounds, the faster it grows.
The formula is: A = P(1 + r/n)nt — where P is principal, r is annual rate, n is compounding frequency, and t is time in years. Our calculator handles all of this instantly, including monthly contributions.
Adding monthly contributions dramatically accelerates growth. Even $100–$200/month invested consistently over 20–30 years can outperform a large lump sum. The key is consistency — start early, contribute regularly, and let compound interest do the heavy lifting.
Frequently Asked Questions
How often should interest compound for maximum growth?
The more frequently interest compounds, the more you earn. Daily compounding produces slightly more than monthly, which produces more than annually. However, the difference between daily and monthly is usually small — what matters most is starting early and maintaining a consistent contribution.
What is a realistic interest rate to use?
Historically, the S&P 500 has returned an average of about 7–10% annually (inflation-adjusted ~7%). High-yield savings accounts currently offer 4–5%. For conservative estimates, use 5–6%; for stock market projections, 7–8% is commonly used.
How does monthly contribution affect growth?
Regular contributions dramatically accelerate growth. Even small monthly additions compound over time. For example, adding $200/month at 7% for 20 years adds over $100,000 to your final balance compared to a lump sum alone.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 7%, your money doubles roughly every 10.3 years (72 ÷ 7 = 10.3).
How long to turn $10,000 into $100,000?
At 7% annual return with no contributions, $10,000 grows to $100,000 in approximately 34 years. Add $200/month and it happens in about 21 years. Higher contributions or rates speed this up significantly — use our calculator above to model your specific scenario.
Is this calculator free?
Yes, CompoundCalc.online is completely free. No sign-up, no data collection, no limits. All calculations happen instantly in your browser.