The Simple Interest Calculator computes interest earned or owed on a principal amount using a straightforward rate — without compounding. It’s commonly used for short-term loans, bonds, and savings accounts that pay interest only on the original deposit.

How to Use This Calculator

1. Enter the Principal — your starting loan or investment amount.
2. Enter the Annual Interest Rate as a percentage.
3. Enter the Time Period in years (you can use decimals, e.g., 0.5 for 6 months).
4. Click Calculate.

Simple Interest vs. Compound Interest

With simple interest, you earn interest only on the original principal — it never compounds. With compound interest, you earn interest on both the principal and the accumulated interest from previous periods.

For short time periods (under a year), the difference is negligible. For long periods, compound interest grows dramatically faster. A $1,000 investment at 5% simple interest for 10 years earns $500. At 5% compound interest (annually), it earns $629 — 26% more.

When Is Simple Interest Used?

• Car loans and some personal loans that are “pre-computed”
• US Treasury bills and short-term government bonds
• Payday loans (which use daily simple interest rates that are very high)
• Informal loans between friends or family

Frequently Asked Questions

How do I convert monthly rate to annual rate?

Multiply the monthly rate by 12. For example, a 1.5% monthly rate = 18% annual simple interest rate.

Does simple interest apply to credit cards?

Credit cards technically use daily periodic rates, but because interest is calculated on a balance that includes prior interest, they effectively compound. Credit card interest is not simple interest.

Worked Example

Suppose you deposit $5,000 at a 4% annual simple interest rate for 3 years. The interest is 5,000 × 0.04 × 3 = $600, giving a final balance of $5,600. Unlike compound interest, the 4% is always applied to the original $5,000 — never to interest already earned.

Simple Interest vs Compound Interest

With simple interest, you earn the same amount every period because it is only ever calculated on the original principal. With compound interest, earned interest is added back to the balance and itself earns interest, so growth accelerates over time. Simple interest is common in short-term loans, car financing, and some bonds, while savings accounts and long-term investments almost always compound. For the same rate and term, compound interest always yields more.

When Simple Interest Applies

• Auto loans and many personal installment loans.
• Short-term or bridging loans between businesses.
• Treasury bills and some certificates that pay a flat coupon.
• Quick estimates where you want a conservative, easy-to-check figure.