Investment Compound Interest Calculator
Project how an investment grows with compound interest, plus optional recurring monthly contributions, with a visual year-by-year growth chart.
This calculator provides estimates only and is not financial advice. Consult a qualified financial advisor for personalized guidance.
How to use this calculator
Enter your starting principal, annual interest rate, compounding frequency, and time period. Optionally add a monthly contribution to see how regular investing accelerates growth, then click Calculate Growth. The chart shows how much of your final balance came from contributions versus interest earned each year.
How is this calculated?
With no monthly contribution, this uses the standard compound interest formula A = P × (1 + r/n)^(n×t), where A is the final balance, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. With a monthly contribution, the calculator instead simulates growth month by month so each contribution compounds alongside your principal for the remaining time period.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and on the interest that has already accumulated. Over time this creates exponential rather than linear growth, since each period's interest is calculated on a larger balance than the last.
How often should interest compound?
More frequent compounding (monthly vs. annually) produces slightly higher returns for the same nominal interest rate, because interest starts earning interest sooner. The difference is usually small compared to the impact of the interest rate itself or regular contributions.
How much difference do monthly contributions make?
Regular contributions can dramatically increase your final balance, since each new contribution has years to compound. Try setting the monthly contribution field to 0 and comparing the result to a nonzero value to see the effect for your own numbers.
What's a realistic rate of return to use?
Historical long-term average annual returns for a diversified stock portfolio are often cited in the 7–10% range before inflation, but actual returns vary significantly year to year and are never guaranteed. Use a conservative rate if you want a cautious estimate.