Compound Interest Calculator
Calculate compound growth using principal, rate, years, and compounding frequency.
Compound interest — earning interest on your interest — is the fundamental force behind long-term investment growth and the hidden cost behind long-term debt. This calculator shows how your money grows over time with adjustable principal, rate, compounding frequency, and optional regular contributions.
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How to Calculate Compound Interest
- Enter the principal amount — the initial sum of money you are investing or saving.
- Enter the annual interest rate — the yearly rate of return, as a percentage (e.g., 7 for 7%).
- Enter the time period in years — how long the money will be invested or saved.
- Set compounding frequency — how often interest is calculated and added (annually, quarterly, monthly, or daily).
- Click Calculate — see the final amount, total interest earned, and how much compounding added compared to simple interest.
Frequently Asked Questions
What is compound interest? ▼
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. This "interest on interest" effect causes savings and investments to grow exponentially over time, which is why it is often called the most powerful force in finance.
What is the difference between simple and compound interest? ▼
Simple interest is calculated only on the original principal: Principal × Rate × Time. Compound interest adds earned interest back to the principal, so each period's interest is calculated on a larger base. Over long periods, the difference is dramatic — $10,000 at 7% for 30 years yields $31,000 with simple interest but over $76,000 with compound interest.
What is the Rule of 72? ▼
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8% annual return, money doubles in roughly 72 ÷ 8 = 9 years. It is surprisingly accurate for rates between 4% and 15%.
How does compounding frequency affect returns? ▼
More frequent compounding produces slightly higher returns. $10,000 at 10% for 10 years yields: $25,937 compounding annually, $26,851 quarterly, $27,070 monthly, and $27,179 daily. The difference between annual and daily compounding is modest but adds up over decades.
How much should I save for retirement using compound interest? ▼
A common guideline is to invest 15–20% of income starting in your 20s. Thanks to compounding, $500 per month at 8% for 35 years grows to over $1.1 million. Starting even 10 years later cuts that roughly in half, illustrating the enormous value of starting early.
Is my financial data stored? ▼
No. All calculations happen in your browser. No financial data is transmitted or stored on any server.