Compound Interest Calculator

Our compound interest calculator shows how money grows exponentially when interest is reinvested. Formula: A = P(1 + r/n)^(nt). Monthly compounding slightly outperforms annual compounding at the same nominal rate.

Enter a starting lump sum and monthly contribution, then set the interest rate and time period in years. The calculator shows the full compounding effect: interest is earned on the principal, then on the growing interest, then on the contributions and their accumulated interest. The chart shows how the growth curve accelerates over time — in early years the balance grows steadily; in later years it grows exponentially. Try increasing the monthly contribution by £50 and observe the long-term difference: small consistent increases in saving rate have disproportionate long-term effects through compounding. Also try changing the start date by five years to see the cost of delay — the opportunity cost of starting later is larger than most people intuit.