Free Online Compound Interest Calculator

1. Enter the starting principal in the first field.
2. Type the annual interest rate as a percentage and pick the compounding frequency.
3. Set the term in years and any monthly contribution.
4. Read the final balance, interest earned, and the year-by-year table below.

The compound interest formula is A = P(1 + r/n)^(nt), where P is the starting principal, r is the annual rate as a decimal, n is the compounding periods per year, and t is the number of years. Adding a recurring contribution C at the same frequency turns it into A = P(1 + r/n)^(nt) + C × [((1 + r/n)^(nt) − 1) ÷ (r/n)].

Over decades the difference between compound and simple interest is enormous — it's why retirement accounts work and why credit-card debt compounds out of reach. A 7% annual return over 30 years grows $10,000 into roughly $76,000; the same rate over 10 years only reaches $20,000.

This tool accepts principal, annual rate, compounding frequency (daily / monthly / quarterly / annually), term in years, and optional monthly contributions. It shows the final balance, total contributions, total interest earned, and a year-by-year breakdown. Inflation isn't modeled — subtract your expected inflation rate from the nominal rate for a real-return view. General planning estimate only; for retirement decisions consult a qualified advisor.