Tiny Toolkit

Simple Interest Calculator

Calculate interest earned or owed using the simple interest formula: Principal × Rate × Time. Unlike compound interest, simple interest does not accrue on previous interest — it's used for short-term loans, some bonds, and as a baseline comparison. Enter any three of the four variables to solve for the fourth.

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Frequently Asked Questions

What is the simple interest formula?
Simple interest = Principal × Rate × Time (I = P × r × t), where rate is the annual rate as a decimal and time is in years. Total amount = Principal + Interest. For example, $10,000 at 5% for 3 years earns $1,500 in simple interest, for a total of $11,500.
When is simple interest used in real life?
Simple interest is used in short-term loans, some auto loans, certain student loans, and Treasury bills. Most savings accounts and long-term investments use compound interest, which grows faster. Knowing the difference helps you evaluate loan terms accurately.
Why does compound interest grow faster?
Compound interest is calculated on the growing balance (principal plus previously earned interest), not just the original principal. Over time, the gap widens significantly. At 5% over 20 years, $10,000 grows to $20,000 with simple interest but $26,533 with annual compounding.
How do I convert a simple interest rate to APY?
Simple interest has no direct APY equivalent since it does not compound. The effective yield depends on the term. For comparison, the equivalent compound rate that produces the same total return over the same period is: r = (Total Amount / Principal)^(1/years) − 1.

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