Calculate compound interest with monthly contributions and see how your money grows over time.
Enter your initial investment amount, the annual interest rate, and the number of years you plan to invest. Select how often interest compounds — monthly, quarterly, semi-annually, or annually.
Optionally, add a monthly contribution to see how regular deposits accelerate growth.
The results show your final balance, total contributions, and total interest earned over the investment period.
Compound interest formula with contributions:
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]Where P = initial principal, r = annual interest rate, n = compounding periods per year, t = time in years, and PMT = periodic contribution.
Example: $10,000 initial investment at 7% annual interest, compounded monthly, for 20 years with $200/month contributions:
Example 2: $5,000 at 5% compounded annually for 10 years, no contributions:
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. This “interest on interest” effect causes your money to grow exponentially over time.
More frequent compounding (e.g., monthly vs. annually) results in slightly higher returns because interest is calculated and added to the balance more often. The difference is most noticeable with larger amounts and higher rates.
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate. For example, at 8% interest, your money doubles in approximately 72 ÷ 8 = 9 years.
APY (Annual Percentage Yield) reflects the total interest earned in one year including compounding. A 5% interest rate compounded monthly has an APY of about 5.12%, which accounts for the compounding effect.