Compound Interest Calculator
Calculate compound interest with optional monthly contributions.
How to Use the Compound Interest Calculator
This calculator shows how your savings or investments grow over time with compound interest — including optional recurring monthly contributions.
- Enter the initial principal — the starting amount of your investment or savings (e.g. $10,000).
- Enter the annual interest rate — the yearly rate of return you expect (e.g. 7% for a typical stock market average). Enter 0 for no growth.
- Choose a compounding frequency — how often interest is calculated and added to your balance. Options include annually, semi-annually, quarterly, monthly and daily. The more frequently interest compounds, the faster your money grows.
- Enter the time period — the number of years you plan to let your investment grow.
- Optionally enter a monthly contribution — a fixed amount you add to the investment each month (e.g. $500/month).
The calculator instantly shows your final amount, total interest earned and total contributions (principal + all monthly deposits). The growth bar visualises how much of your final balance came from contributions vs. compound interest.
The formula used is: A = P(1 + r/n)nt, where P is the principal, r is the annual interest rate, n is the compounding frequency and t is the time in years. Monthly contributions use the future value of a series formula.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (calculated only on the principal), compound interest causes your money to grow exponentially over time — often called "interest on interest."
How does compounding frequency affect my returns?
The more frequently interest compounds, the more you earn. For example, $10,000 at 7% for 10 years yields $19,671.51 with annual compounding but $20,096.61 with daily compounding. Monthly compounding is the most common for savings accounts and investment calculations.
What is a realistic annual interest rate to use?
It depends on the investment type. High-yield savings accounts offer 4–5%, bonds average 3–6%, and the US stock market (S&P 500) has historically returned about 7–10% per year after inflation. Use a conservative estimate for financial planning.
How do monthly contributions affect the final amount?
Regular monthly contributions can dramatically increase your final balance. Even small consistent deposits benefit from compound interest over time. For instance, investing $500/month at 7% for 30 years grows to over $566,000 — while your total contributions are only $180,000.
What is the Rule of 72?
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: at 7%, your money doubles in roughly 72 / 7 ≈ 10.3 years. It works best for rates between 2% and 15%.
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