What is 'Interest Rate'
Interest rate is the amount charged, expressed as a percentage of principal, by a lender to a borrower for the use of assets. Interest rates are typically noted on an annual basis, known as the annual percentage rate (APR). The assets borrowed could include, cash, consumer goods, large assets, such as a vehicle or building. Interest is essentially a rental, or leasing charge to the borrower, for the asset's use. In the case of a large asset, like a vehicle or building, the interest rate is sometimes known as the "lease rate". When the borrower is a lowrisk party, they will usually be charged a low interest rate; if the borrower is considered high risk, the interest rate that they are charged will be higher.
BREAKING DOWN 'Interest Rate'
Interest is charged by lenders as compensation for the loss of the asset's use. In the case of lending money, the lender could have invested the funds instead of lending them out. With lending a large asset, the lender may have been able to generate income from the asset should they have decided to use it themselves.
Using the simple interest formula:
Simple Interest = P (principal) x I (annual interest rate) x N (years)
Borrowing $1,000 at a 6% annual interest rate for 8 months means that you would owe $40 in interest (1000 x 6% x 8/12).
Using the compound interest formula:
Compound Interest = P (principal) x [ ( 1 + I(interest rate) ^{N} (months) )  1 ]
Borrowing $1,000 at a 6% annual interest rate for 8 months means that you would owe $40.70.
The interest owed when compounding is taken into consideration is higher, because interest has been charged monthly on the principal + accrued interest from the previous months. For shorter time frames, the calculation of interest will be similar for both methods. As the lending time increases, though, the disparity between the two types of interest calculations grows.

Interest
The charge for the privilege of borrowing money, typically expressed ... 
Simple Interest
A quick method of calculating the interest charge on a loan. ... 
Periodic Interest Rate
The interest rate charged on a loan or realized on an investment ... 
AddOn Interest
A method of calculating interest whereby the interest payable ... 
Mortgage Rate Lock
An agreement between a borrower and a lender that allows the ... 
Annual Percentage Rate  APR
The annual rate that is charged for borrowing (or made by investing), ...

Economics
Explaining Interest
Interest is the price charged to borrow money, and is typically expressed as a percentage of the principal, or the amount loaned. 
Personal Finance
Dissecting the Simple Interest Formula
Simple interest ignores the effect of compounding: it's only calculated on the principal amount. This makes it easier to calculate than compound interest. 
Economics
Forces Behind Interest Rates
Interest is a cost for one party, and income for another. Regardless of the perspective, interest rates are always changing. 
Credit & Loans
All Your Questions About Loan Interest Rates
If you think interest rates don’t really matter  think again. Interest rates have a huge impact on your finances. 
Economics
Forces Behind Interest Rates
Get a deeper understanding of the importance of interest rates and what makes them change. 
Personal Finance
4 Ways Simple Interest Is Used In Real Life
Simple interest works in your favor when you're a borrower, but against you when you're an investor. 
Personal Finance
Simple Interest Loans: Do They Exist?
Yes, they do. Here is what they are – and how to use them to your advantage. 
Investing
Simple Interest
Simple interest is a quick method of calculating the interest charged on a loan. Simple interest is determined by multiplying the interest rate by the principal by the number of periods. 
Investing Basics
Learn Simple And Compound Interest
Interest is defined as the cost of borrowing money, and depending on how it is calculated, can be classified as simple interest or compound interest. 
Investing
The Effective Annual Interest Rate
The effective annual interest rate is a way of restating the annual interest rate so that it takes into account the effects of compounding.

How can I tell if a loan uses simple or compound interest?
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What is the difference between compounding interest and simple interest?
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What formula can I use to calculate interest on interest?
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What are some examples of simple interest loans?
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What is the difference between yields and interest rates?
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Are APRs different in different countries?
Learn about the term APR and how it is used in the United States and other countries. Explore why different lenders charge ... Read Answer >>