Free Simple & Compound Interest Calculator
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About the Interest Calculator
Calculate simple and compound interest from a principal, rate and time — compare the two and choose the compounding frequency, all in your browser.
How to use the Interest Calculator
- 1Enter the principal amount, annual interest rate and time in years.
- 2For compound interest, pick how often it compounds (yearly, monthly, daily, etc.).
- 3See the simple and compound interest side by side, with the final totals.
- 4Everything is calculated in your browser — nothing is uploaded.
Frequently asked questions
Is this interest calculator free?
Yes — completely free, with no sign-up and no watermark.
Is my data uploaded anywhere?
No. All calculations run in your browser — nothing you enter is sent anywhere.
What's the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it's the same each year. Compound interest is calculated on the principal plus any interest already earned, so it grows faster over time.
What does compounding frequency mean?
It's how often interest is added to the balance — yearly, quarterly, monthly or daily. The more often interest compounds, the more you earn (or owe), though the difference is usually small at typical rates.
What are the formulas used?
Simple interest is P × r × t. Compound interest is P × (1 + r/n)^(n×t) − P, where P is the principal, r is the annual rate, t is the time in years and n is the number of compounding periods per year.