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 Γ [((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 β 72 Γ· annual rate (%)
Example: 6% rate β doubles in ~12 years
Reading your result
- Rule of 72: 72 Γ· annual return β years to double your money. At 6% that's about 12 years; at 8%, about 9. A quick sanity check on the output.
- Nominal vs. real: the result is nominal. Subtract inflation (e.g., 2%/yr) to think in purchasing power β for long horizons the difference compounds dramatically.
- Compounding frequency: monthly beats annual at the same stated rate, but the gap is small at typical savings rates. Rate and time matter far more than frequency.
Formula: A = P(1 + r/n)^(nt) + PMT Γ [((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 β 72 Γ· annual rate (%)
Example: 6% rate β doubles in ~12 years
Reading your result
- Rule of 72: 72 Γ· annual return β years to double your money. At 6% that's about 12 years; at 8%, about 9. A quick sanity check on the output.
- Nominal vs. real: the result is nominal. Subtract inflation (e.g., 2%/yr) to think in purchasing power β for long horizons the difference compounds dramatically.
- Compounding frequency: monthly beats annual at the same stated rate, but the gap is small at typical savings rates. Rate and time matter far more than frequency.
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Content last reviewed: 2026-08-22