What Is Simple & Compound Interest?

Getting to know simple and compound interest


To understand how interest affects your outstanding balance and payments, it’s important to get to know simple and compound interest. We’ve created Amy’s story to show you how it works.

Your simple interest rate is what we use to work out how much interest you’ll be charged each month. It moves in line with the Bank of England Base Rate – so when the Base Rate changes, you’ll see your simple rate change by exactly the same amount.

Your compound interest rate takes into account the interest you pay on interest as well as the outstanding amount if you don’t pay your balance off every month. Even if you do pay your balance off every month, the compound rate shows how much interest you’d pay each year if you didn’t.

Compound rates will also go up or down when the Bank of England Base Rate changes but the changes will be bigger than the changes to the simple interest rates because they take in to account the interest on interest.

We’ll always show your simple interest rate and the compound equivalent together – you’ll see them on the first page of your monthly statement and when you log into Barclaycard online servicing .


Comparing to purchase and cash interest rates

Most credit card issuers just show compound interest rates. This means that you can use the standard and cash compound rates on your Barclaycard to compare against the purchase and cash interest rates on most other credit cards.

Comparing to APR (Annual Percentage Rate)

You can also use your compound standard rate to compare against the APR of another card, unless the other card has annual or monthly fees.

An APR is a rate that is used to give people a standard way of comparing the costs of different credit cards, before they apply.  In advertising, lenders use a rate called the representative APR.  This is the APR you are most likely to get if you are successful because some card issuers give different APRs to different customers.

The APR is based on the interest rate charged on purchases and will usually be the same as the compound purchase rate.  But this is not always the case because the APR also takes into account any monthly or annual fees you pay for having the card.

Amy’s story – an example to show how compound interest works


To show you how compound interest affects a balance over time, we’re going to use a made-up example based on Amy borrowing £1,000 at a simple interest rate of 12%. For the sake of the example, let’s say that Amy doesn’t pay anything back for 12 months. Note that this example is just to show how compound interest works – it is not related to any specific financial product or service.

What happens to Amy’s balance each month?

Amy borrowed the money in December, so interest is charged for the first time in January. Her yearly simple standard rate is 12%, so her monthly simple rate is 1% (the simple standard rate divided by 12). Because Amy doesn’t pay, her £1,000 balance (plus 1% interest) is rolled over to February when interest is charged again. This time interest is charged not only on the original balance but also on the interest from January – we call this compound interest.

This example shows how interest affects Amy’s balance over 12 months. We’ve rounded the figures to the nearest 1p. You’ll see how the longer you take to pay off your balance, the more it will cost.

Amy borrows the £1,000 in December, so interest is charged for the first time in January

Amy’s balance is £1,000 with a simple interest rate of 1% per month.
Interest charged = £10
Total balance = £1,010

Amy now owes £1,010 and that’s the amount she’s charged interest on.
Interest charged = £10.10
Total balance = £1,020.10

Amy now owes £1,020 and that’s the amount she’s charged interest on.
Interest charged = £10.20
Total balance = £1,030.30

After 12 months, the interest has increased the amount Amy owes by £126.83. If Amy had been charged only a simple interest rate, she would have been charged £120 over 12 months. However, because interest is charged on interest (compound interest), Amy would be charged an extra £6.83 over 12 months. So the total amount that Amy would be charged is £126.83.

View a full yearly example showing how interest is charged month on month.

The example shows that over 12 months, Amy is charged £126.83 in interest on the £1,000 that she borrowed at a simple interest rate of 12% per year. This means that the compound interest rate was 12.7% per year (£126.83/£1,000). 

What compound interest means for you

With compound interest, interest is charged on interest from the previous month. So the longer it takes to clear your balance, the more you’ll pay in compound interest. It’s important that you try to clear your balance as quickly as you can.

With Barclaycard, you must pay the minimum amount each month

Remember that Amy’s story is just an example to show how compound interest works. With a Barclaycard, you must pay at least the minimum amount each month. If you miss payments, or pay late, you’ll be charged fees. Also, your credit rating could be affected, making it harder for you to get credit in future.