The compound interest calculator shows you exactly how an investment grows over time when interest is earned on both the principal and previously accumulated interest. This is the core mechanic behind savings accounts, investment portfolios, and retirement funds — and understanding it is essential for any financial decision.
How to Use the Compound Interest Calculator
- Enter your principal — the starting amount.
- Enter the annual interest rate (e.g. 7 for 7%).
- Set the compounding frequency: annually, quarterly, monthly, or daily.
- Enter the time period in years.
- See your final balance and total interest earned instantly.
Compound Interest Formula
A = P × (1 + r/n)^(nt)
Where: P = principal, r = annual rate (decimal), n = compounding periods per year, t = years.
Frequently Asked Questions
What’s the difference between simple and compound interest?
Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all previously earned interest — so your gains accelerate over time. A $10,000 investment at 7% simple interest earns $700/year forever. At 7% compound interest, it earns more each year as the balance grows.
How often should interest compound for the best return?
More frequent compounding means slightly higher returns. Daily compounding earns marginally more than monthly, which earns more than annually. In practice, the difference is small — the rate matters far more than the frequency.
What does the “Rule of 72” mean?
The Rule of 72 is a shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6% annual return, your investment doubles in roughly 12 years (72 ÷ 6 = 12).
Can I add regular contributions?
This calculator covers lump-sum investments. For regular monthly contributions (like a pension or savings plan), use a future value with contributions formula — we’ll be adding that feature soon.
Is compound interest always beneficial?
It’s beneficial when you’re earning it (investments, savings). It works against you when you’re paying it (credit card debt, loans). The same math that grows your savings accelerates your debt — which is why paying off high-interest debt quickly is so important.