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Money and Finance

Money and Finance


What is interest?

Interest is what it costs to borrow money for a certain amount of time. If you borrow money from the bank, then you pay the bank interest. If the bank borrows money from you, then they pay you interest.

The Interest Rate

The interest rate is the amount charged for borrowing money. It is a percentage of the total money borrowed over the amount of time. For example, the interest rate might be 2% per year on $100. In this case, the interest that would be paid for one year would be $2.

Calculate Simple Interest

The most basic type of interest is simple interest. This where the interest is only based on the initial amount of the loan called the principal. There is no interest paid on past interest earned like with compound interest (explained below).

The formula for simple interest is:

Simple Interest = Principal x Interest Rate x Length of the Loan in Time

This can be written as:

I = P x r x t

where I = Interest, P = principal, r = interest rate, t = time

Compound Interest

Compound interest is more complicated. This is where you earn or pay interest on interest previously earned. For example, if you had $100 that earned 10% interest at the end of the first year you would have $110. Then, during the second year you would earn 10% of $110 which is $11. The total interest for two years would be $21. With simple interest you would not get the extra $1 in the second year. You would only earn $10 per year and the total for 2 years would be $20.

Interest can compound at different intervals. In the above example, the interest was compounded every year. In many cases, such as with a mortgage loan from the bank, interest compounds monthly.

The formula for compound interest is:

Future Value = P x (1 + rn)nt

where P = principal, r = interest rate, t = time in years, n = number of times per year interest is compounded

This will give you the "future value" of your deposit. To figure out how much interest you earned, subtract the initial value of the deposit (P) and you will have just the interest:

Compound Interest = Future Value - P

Example Problem

Now we will do an example problem calculating both simple interest and compound interest.

How much interest would you earn if you had $50,000 in the bank for 10 years at a rate of 3%. First calculate simple interest then compound interest where the interest compounds each month.

To calculate the simple interest we use the formula:

I = P x r x t

I = $50,000 x .02 x 10

I = $10,000

To calculate the compound interest we use the formula:

Compound Interest = P x (1 +rn)nt

FV = $50,000 x (1+ .0212)12x10

FV = $61, 059.97

Now I need to subtract the principal to get the compound interest earned:

I = $61, 059.97 - $50,000

I = $11, 059.97

You can see the additional gain when compounding interest monthly vs. simple interest.

Why do interest rates change?

Interest rates don't stay the same. There are many factors that can change the current interest rate. Like many things, interest rates can change with supply and demand. The more money there is available for loan, the lower the interest rate will be. If demand for money goes up, then so will interest rates. Government money policy has a huge influence on the interest rate because it determines the money supply. Long term interest rates are influenced by the rate of inflation.

Learn More about Money and Finance:

Personal Finance

Filling out a Check
Managing a Checkbook
How to Save
Credit Cards
How a Mortgage Works
How Interest Works
Insurance Basics
Identity Theft

About Money

History of Money
How Coins are Made
How Paper Money is Made
Counterfeit Money
United States Currency
World Currencies
Money Math

Counting Money
Making Change
Basic Money Math
Money Word Problems: Addition and Subtraction
Money Word Problems: Multiplication and Addition
Money Word Problems: Interest and Percent


How Banks Work
How the Stock Market Works
Supply and Demand
Supply and Demand Examples
Economic Cycle
Adam Smith
How Taxes Work
Glossary and Terms

Note: This information is not to be used for individual legal, tax, or investment advice. You should always contact a professional financial or tax advisor before making financial decisions.

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