Compound Interest Calculator
Calculate how your investments grow with compound interest over time.
Updated April 2026 · CalcFlow Editorial
What is a Compound Interest? A compound interest calculator computes how an investment grows when interest is earned on both the original principal and the accumulated interest from prior periods, using the formula A = P(1 + r/n)^(nt), where P is principal, r is annual rate, n is compounding frequency, and t is time in years.
Rule of Thumb
The Rule of 72: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 7% annual return, money doubles in roughly 72 ÷ 7 = 10.3 years.
Example Calculation
$10,000 invested at 7% annual return, compounded monthly for 30 years = $81,165. Total contributions: $10,000. Total interest earned: $71,165. The money grew 8x with no additional contributions.
Key Facts
- •The S&P 500 has averaged approximately 10% annual return (7% after inflation) over the past 100 years.
- •Starting 10 years earlier can more than double your final investment amount due to compounding.
- •Daily compounding earns only marginally more than monthly compounding over long periods.
- •Albert Einstein reportedly called compound interest "the eighth wonder of the world."
How to Use
- Enter your initial investment (principal).
- Enter the annual interest rate.
- Set the time period in years.
- Choose how often interest is compounded.
- Optionally add monthly contributions.
- Click Calculate to see future value and total interest earned.
Formula
A = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)]