In this field, the user can store a percentage value which defines the highest permissible interest limit. Interest limits can be used for all interest calculation methods that are subject to a variable interest rate. A comparison against interest rate limitations is especially sensible when the applicable interest rate is determined from an external interest rate in combination with a variable interest markup or markdown. In those cases, the interest rate cannot be checked until the determination of the interest rate has been finished. The resultant interest rate is matched against the activated interest rate limitations. A maximum interest rate defines the highest possible charge of an interest service, whereas a minimum interest defines the lowest possible interest rate. If a minimum interest rate is defined in combination with a maximum interest rate, the determined interest rate must be within these limits. When the interest rate is below the minimum interest rate, the interest rate to be matched is increased up to the value of the minimum interest rate. When the maximum interest rate is exceeded, the interest rate is cut and the maximum interest rate which is permissible for the relevant interest transaction is entered.