What Is Simple Interest?
Simple interest is calculated only on the original principal amount, unlike compound interest, which also earns interest on previously accumulated interest. It's used for some short-term loans, certain bonds, and basic interest calculations.
How to Use the Simple Interest Calculator
- Enter the principal amount.
- Enter the annual interest rate.
- Enter the time period in years.
- View the interest earned/owed and the total amount instantly.
Simple Interest Formula
Worked Example
I = $10,000 × 0.05 × 3 = $1,500
Total amount: $10,000 + $1,500 = $11,500
Understanding Your Results
Simple Interest grows linearly — it's the same dollar amount every year, unlike compound interest, which accelerates over time. Total Amount is your original principal plus all accumulated interest.
Simple Interest vs. Compound Interest
Simple interest only ever applies to the original principal. Compound interest applies to the principal plus any interest already earned, so it grows faster the longer money sits, especially over long periods. Most savings accounts, credit cards, and long-term investments use compound interest; simple interest is more common in specific short-term loans and some bonds.
Common Mistakes to Avoid
- Confusing simple interest with compound interest — they produce very different results over time.
- Using a rate expressed as a whole number (5) instead of a decimal (0.05) in manual calculations.
- Forgetting to express the time period in years to match an annual interest rate.
Frequently Asked Questions
Simple interest equals principal × rate × time (I = P × r × t), where rate is expressed as a decimal and time is in years.
Simple interest is calculated only on the original principal and grows linearly. Compound interest is calculated on the principal plus any previously earned interest, so it grows faster over time.
Some short-term personal loans, car loans, and certain bonds use simple interest, though many financial products (like savings accounts and credit cards) use compound interest instead.
Yes, with simple interest, the total interest grows in direct proportion to time — twice the time means twice the interest, assuming principal and rate stay constant.
Yes — enter a decimal value like 0.5 for 6 months or 2.5 for two and a half years.