Compound Interest Calculator — Growth & Returns | ConverterPilot

Compound Interest is the interest earned on an initial principal amount plus any accumulated interest from previous periods. It creates a powerful snowball effect that is central to wealth creation and long-term financial planning.

The Compound Interest Formula

Without ongoing contributions, the standard formula to find the future value of an investment is:

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

Where:

• A: The final balance (future value of the investment).
• P: The initial investment (principal).
• r: The annual nominal interest rate (in decimal format, e.g., 8% = 0.08).
• n: The number of compounding periods per year (e.g., monthly = 12, daily = 365, annual = 1).
• t: The time period in years.

When monthly contributions are added at the end of each month, the future value of those contributions is computed as:

FV = PMT × [ (1 + R)^N - 1 ] / R

Where PMT is the monthly contribution, R is the equivalent monthly interest rate (1 + r/n)^(n/12) - 1, and N is the total number of months (12 × t).

Worked Example

Suppose you invest an initial principal of $10,000 at an annual interest rate of 8% compounded monthly (n = 12) for 10 years (t = 10), without any extra contributions:

1. Convert interest: r = 0.08, r/n = 0.08 / 12 ≈ 0.006667
2. Calculate periods: nt = 12 × 10 = 120
3. Apply formula: A = 10,000 × (1 + 0.006667)^120
4. Evaluate exponent: (1.006667)^120 ≈ 2.21964
5. Calculate final balance: A = 10,000 × 2.21964 ≈ $22,196.40
6. Your initial investment grew to $22,196.40. You earned $12,196.40 in compound interest.