How to Calculate Compound Interest: Formula, Examples, and Calculator

Compound interest is often called the eighth wonder of the world, and for good reason. It's the mechanism that turns small, regular savings into significant wealth over time. Understanding how compound interest works is fundamental to making smart financial decisions.

What Is Compound Interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns interest on the original amount, compound interest earns interest on interest — creating an exponential growth effect.

The Compound Interest Formula

A = P(1 + r/n)^(nt)

Where:
  A = Final amount
  P = Principal (initial investment)
  r = Annual interest rate (decimal)
  n = Number of times interest compounds per year
  t = Number of years

Example:
  P = $10,000
  r = 5% (0.05)
  n = 12 (monthly compounding)
  t = 10 years

  A = 10,000(1 + 0.05/12)^(12 × 10)
  A = 10,000(1.004167)^120
  A = $16,470.09

  Interest earned: $6,470.09

Compounding Frequency Matters

The more frequently interest compounds, the more you earn. Here's how different compounding frequencies affect a $10,000 investment at 5% over 10 years:

• Annually (1x/year): $16,288.95
• Quarterly (4x/year): $16,436.19
• Monthly (12x/year): $16,470.09
• Daily (365x/year): $16,486.65
• Continuously: $16,487.21

Rule of 72

The Rule of 72 is a quick way to estimate how long it takes to double your money. Simply divide 72 by the annual interest rate. At 6% interest, your money doubles in approximately 72 ÷ 6 = 12 years. At 8%, it doubles in about 9 years.

Practical Tips for Maximizing Compound Interest

• Start early — time is the most important factor in compound growth
• Reinvest dividends and interest payments automatically
• Choose investments with higher compounding frequencies when possible
• Make regular additional contributions to accelerate growth
• Avoid withdrawing interest — let it compound
• Compare APY (Annual Percentage Yield) rather than APR for savings accounts

Compound Interest and Debt

Compound interest works both ways. While it grows your savings, it also grows your debt. Credit card interest, for example, compounds daily on most cards. A $5,000 balance at 20% APR, making only minimum payments, could take over 25 years to pay off and cost more than $8,000 in interest alone.