Simple interest is paid on the principal amount borrowed and is not compounded.
Simple interest is considered the best form of interest for the borrower because it is charged only once, and is a percentage of the original principal, unlike compound interest. If you borrow $1,000 at 10 percent simple annual interest, then you pay $100 in interest per year. If you borrow it for two years, then you pay $200 in interest. This is different than the more common compound interest.
Compound interest is based not only on the original sum, but also on accrued interest. For example, many credit cards charge compound interest monthly. If you borrow $1,000 with a simple annual interest rate of 12 percent, you pay less in interest than if you borrow that same amount with a compound interest rate of 1 percent per month. If the loan with the simple interest is paid off at the end of the year, the total amount in interest paid is $120. However, with the compound interest, it costs $126.83. The reason is that, unlike simple interest, compound interest builds upon itself every month. Although the difference is only $6.83 in the above example, most people see a far more dramatic difference. Since most credit cards have compound interest instead of simple interest and many people pay only the minimum amount on their credit card payments, the interest adds up very quickly.
Because of their fixed costs, loans with simple interest are preferable than those with compound interest.
In this video, InspireMath shares the equation, along with examples, for calculating simple interest.