The premium consists of two parts: a base premium and a variable mileage premium (cost per mile). The base and cost per mile do not vary throughout the policy term unless you make changes to your policy. To calculate your monthly payment, we combine the base premium and the cost per mile multiplied by the number of miles driven in the prior billing cycle (typically the prior month). Because the variable portion of your premium is based on mileage, it will vary month to month based on the number of miles you drive.