Compound Interest Calculator
Calculate compound interest with flexible compounding periods and monthly contributions. Interactive chart included. Free, 100% in your browser.
Reference
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which applies only to the principal), compound interest causes your balance to grow exponentially over time. The more frequently interest compounds — monthly vs. annually, for example — the faster your investment grows. Adding regular monthly contributions accelerates growth even further, making compound interest one of the most powerful concepts in personal finance.
Compound interest formula
Without contributions: A = P(1 + r/n)^(nt)
Where P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = time in years.
With monthly contributions: A = P(1 + r/n)^(nt) + PMT × ((1 + r/n)^(nt) − 1) / (r/n)
Where PMT = monthly contribution amount. This formula assumes contributions are made at the end of each compounding period.
Common use cases
- Retirement planning — estimate how much your savings will grow over 20–40 years with regular contributions.
- Investment comparison — compare returns at different interest rates and compounding frequencies.
- Savings goals — determine how much you need to save monthly to reach a target amount.
- Education funds — plan for college savings with long-term compound growth.
- Debt awareness — understand how compound interest works against you on credit cards and loans.
Privacy
All calculations run 100% in your browser. No data is sent to any server.