Compound Interest Calculator
Calculate how your investment grows with compound interest and regular monthly contributions.
Compound Interest Formula
Compound interest earns interest on both the principal and accumulated interest.
Formula: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)]
- P: Initial principal
- PMT: Monthly contribution
- r: Annual interest rate (decimal)
- n: Compounding periods per year (monthly=12, quarterly=4, annually=1)
- t: Time in years
Rule of 72: Time to double your money ≈ 72 ÷ annual rate (%)
Example: 6% rate → doubles in ~12 years
Formula: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)]
- P: Initial principal
- PMT: Monthly contribution
- r: Annual interest rate (decimal)
- n: Compounding periods per year (monthly=12, quarterly=4, annually=1)
- t: Time in years
Rule of 72: Time to double your money ≈ 72 ÷ annual rate (%)
Example: 6% rate → doubles in ~12 years