Simple Interest Calculator — Step-by-Step Solutions
The Simple Interest Calculator uses the formula I = P × r × t to compute interest on loans and investments where interest does not compound. It supports four modes: calculate interest earned, find the total amount, determine the required interest rate, and find the time needed to reach a target.
Enter your principal, rate, and time to get instant results with step-by-step working. Compare with our Compound Interest Calculator to see the difference compounding makes.
Your calculations will appear here
How to Use the Simple Interest Calculator
- Select a mode: Interest Earned, Total Amount, Find Rate, or Find Time.
- In Interest Earned or Total Amount mode, enter the principal, annual interest rate, and time in years.
- In Find Rate mode, enter the principal, interest earned, and time to solve for the annual rate.
- In Find Time mode, enter the principal, interest earned, and annual rate to solve for the time.
- Results appear instantly with step-by-step working and a visual breakdown of principal versus interest.
Formula and Theory
Simple interest is calculated using the formula I = P * r * t, where I is the interest earned, P is the principal (initial amount), r is the annual interest rate as a decimal, and t is the time in years.
The total amount after simple interest is A = P + I = P(1 + r * t). Unlike compound interest, simple interest is calculated only on the original principal. The interest earned each year is the same, creating linear growth.
Simple interest is commonly used for short-term loans, car loans, certificates of deposit, and some savings accounts. It is simpler to calculate than compound interest and is often used when the investment period is short.
The formula can be rearranged to solve for any variable. To find the rate: r = I / (P * t). To find the time: t = I / (P * r). To find the principal: P = I / (r * t).
Worked Examples
Example 1: Interest on a savings deposit
Problem: Calculate the interest earned on 5,000 at 4% per year for 3 years.
Solution: I = 5,000 * 0.04 * 3 = 600.
Answer: 600 in interest earned
Example 2: Total amount after investment
Problem: What is the total amount if you invest 10,000 at 6% simple interest for 5 years?
Solution: I = 10,000 * 0.06 * 5 = 3,000. A = 10,000 + 3,000 = 13,000.
Answer: Total amount of 13,000
Example 3: Finding the rate
Problem: You earned 750 interest on a 5,000 investment over 3 years. What was the rate?
Solution: r = 750 / (5,000 * 3) = 750 / 15,000 = 0.05.
Answer: 5% per year
Example 4: Finding the time
Problem: How long will it take 8,000 at 3.5% to earn 1,400 in interest?
Solution: t = 1,400 / (8,000 * 0.035) = 1,400 / 280 = 5.
Answer: 5 years
Frequently Asked Questions
What is the difference between simple and compound interest?
When is simple interest used?
Can I use simple interest for monthly calculations?
How does simple interest compare over long periods?
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