Simple interest calculator
Enter a principal, a yearly rate and a time to see the simple interest and the total. Three more tools on the page find the rate, the time or the principal when you know the other three.
Interest on a principal
This answer is for the example values in the boxes above. $5,000.00 at 6% a year for 3 years. Change the boxes to see your own answer.
Find the rate
Find the time
A year is counted as 12 months or 365 days.
Find the principal
How it works
Simple interest is the principal times the yearly rate times the time in years. The first tool gives the interest, the total of the principal and the interest, and the interest of one year. The other three solve the same formula for the number that is left, so each tool has the boxes for the three numbers that it needs.
The interest is never added to the principal, so it never earns interest of its own. That is the difference from the compound interest calculator.
A worked example
A principal of $5,000 at 6 percent for 3 years earns 5,000 × 0.06 × 3, which is $900.00. The total is $5,900.00, and each year adds $300.00.
Entered in months, 18 months is a year and a half, so the same principal earns $450.00. From the other side, $900 of interest on $5,000 in 3 years is a rate of 6 percent a year, or 0.5 percent a month. At 6 percent it takes 3 years, which is 36 months or 1,095 days, and $900 at 6 percent over 3 years comes from a principal of $5,000.
Months and days
The calculator counts a year as 12 months or as 365 days. For 90 days on $10,000 at 6 percent the interest is 10,000 × 0.06 × 90 ÷ 365, which is 147.9452 and is shown as $147.95. A time may have up to two decimals in its unit.
What it leaves out
The calculator counts a year as 365 days and does not model the way a particular lender counts days, so a figure from a lender can differ. It has no fees, no payments and no compounding. For a loan with monthly payments, use the loan calculator.
Questions
What is the formula for simple interest?
Interest is the principal times the yearly rate times the time in years. $5,000 at 6% for 3 years is 5,000 × 0.06 × 3, which is $900.
How is simple interest different from compound interest?
Simple interest is figured on the principal only, so the interest never earns interest of its own. Compound interest adds the interest to the balance, so the balance grows faster. The compound interest calculator on this site covers that case.
How do I find the interest rate?
Divide the interest by the principal and by the time in years, then multiply by 100. $900 on $5,000 over 3 years is 900 ÷ 15,000, which is 6% a year.
How do I enter months or days?
Choose the unit beside the time. The calculator counts a year as 12 months or 365 days, so 18 months is 1.5 years and 90 days is 90/365 of a year.
How long does it take to earn a given amount of interest?
Divide the interest by the principal times the yearly rate. $900 on $5,000 at 6% takes 900 ÷ 300, which is 3 years, or 36 months.
How do I find the principal from the interest?
Divide the interest by the yearly rate and the time in years. $900 at 6% over 3 years comes from 900 ÷ 0.18, which is $5,000.
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