How Compound Interest Works
Compound interest means you earn interest not only on the money you deposit, but also on the interest that money has already earned. This creates a snowball effect: growth accelerates the longer you stay invested.
A = P (1 + r/n)n×t
- A — future value of the investment
- P — initial principal
- r — annual interest rate (decimal)
- n — compounding periods per year
- t — time in years
When you add monthly contributions, each contribution starts its own compounding journey — which is why consistent investing beats waiting for the "perfect" time to invest a lump sum.
The Power of Starting Early
With 8% annual returns compounded monthly, 10,000 invested plus 200/month becomes roughly 103,000 in 15 years — of which only 46,000 was money you put in. Extend it to 25 years and the balance passes 250,000. Time in the market is the single most powerful variable in this calculator: doubling your time period more than doubles your interest earned.
Comparing borrowing costs instead? Use our EMI Calculator. Need quick rate math? Try the Percentage Calculator.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and on interest already earned. Over long periods it makes investments grow exponentially rather than linearly.
What is the compound interest formula?
A = P (1 + r/n)^(n×t), where P is the principal, r the annual rate as a decimal, n the compounding periods per year and t the time in years. Monthly contributions are added period by period on top of this.
Does compounding frequency matter?
Yes. More frequent compounding (monthly or daily instead of yearly) yields slightly higher returns at the same nominal rate, because interest starts earning interest sooner. The difference is modest but real.
How do monthly contributions affect growth?
Regular monthly contributions dramatically increase your final balance because each contribution starts compounding from the moment it is added. Even small consistent amounts grow substantially over 10–20 years.