Simple interest is the cost of using or borrowing money without compound interest or interest on interest. It's relatively easy to calculate since you only need to base it on the principal amount of money borrowed and the time period.

Simple interest works in your favor when you're a borrower because it keeps the overall amount that you pay lower than it would be with compound interest. However, it can work against you when you're an investor because you'll want your returns to compound as much as possible to get the most from your investment.

To understand how it works, it helps to look at some real-life situations in which simple interest is used.

### Key Takeaways

- Simple interest is what it costs to borrow money without compound interest— meaning there is interest on the principal and on the interest.
- Simple interest is calculated by looking at the principal amount borrowed, the rate of interest, and the time period it will cover.
- Simple interest is more advantageous for borrowers than compound interest, as it keeps overall interest payments lower.
- Car loans, amortized monthly, and retailer installment loans, also calculated monthly, are examples of simple interest; as the loan balance dips with each monthly payment, so does the interest.
- Certificates of deposit (CDs) pay a specific amount in interest on a set date, representing simple interest.

## Car Loans

Car loans are amortized monthly, which means that a portion of the loan goes to pay the outstanding loan balance every month, and the remainder goes toward the interest payment.

As the outstanding loan balance diminishes every month, the interest payable reduces, which means a greater part of the monthly payment goes toward the principal repayment.

For example, assume you have a car loan for $20,000. Your interest rate is 4%. To find the simple interest, we multiply 20000 × 0.04 × 1 year. So, by using simple interest $20,000 at 4% for 5 years is ($20,000*0.04) = $800 in interest per year. The total payment due would be $800/year * 5 years + $20,000 = $24,000. Then, the monthly interest is $800 / 12 = $66.67. You take the total monthly payment which is calculated as $24,000 / 60 months = $400 payment/month. Thus,the principal payment would be $400 - $66.67 = $333.33, each month.

Borrowers can benefit from discounts offered when they make early payments, especially when paying back simple interest loans.

## Other Consumer Loans

Department stores often offer major appliances on a simple-interest basis for periods of up to one year. So, suppose you buy a refrigerator for $2,000 and pay simple interest at an annual rate of 8%. For 12 months, your monthly payment would be $180. This means that you would end up paying a total of $2,088, for a total interest expense of $160.

This is substantially less than what you would have paid in interest expense if you had carried the $2,000 loan for the full year, instead of repaying a portion of it every month.

## Certificates of Deposit

A certificate of deposit (CD) is a type of bank investment that pays out a specific amount of money on a set date. You can't withdraw money from a CD until that set date comes.

If you invest $100,000 in a one-year CD that pays interest at 2% per annum, you would earn $2,000 in interest income (100,000 x 0.02 x 1) after a year. If the CD pays the same annual interest rate but is only for a six-month period, you would earn $1,000 in interest income after six months (100,000 x 0.02 x .5).

## Discounts on Early Payments

In the business world, suppliers often offer a discount to encourage early payment of their invoices.

For example, a $50,000 invoice may offer a 0.5% discount for payment within a month. This works out to $250 for early payment, or an annualized rate of 6%, which is quite an attractive deal for the payer.