Simple interest rate: We use your simple interest rate to figure out how much interest we will charge you each month. It changes with the Bank of England base rate. So, when the base rate changes, your simple rate will go up or down by the same amount. Find out more about the Bank of England base rate. Compound interest: Your compound interest rate includes the interest you pay on interest, plus any unpaid amount if you do not pay your balance each month. Even if you pay your balance every month, the compound rate shows how much interest you would pay each year if you did not. Compound rates will also change when the Bank of England Base Rate changes, but these changes will be larger than those for simple interest rates because they include interest on interest.