Prorated amounts are calculated by dividing the cost of a service by the number of days in the service period, according to Lucas Hall from Landlordology. The resulting number is then multiplied by the number of days the service is used to find the prorated amount.
The term "prorate" comes from the Latin "pro rata," which means "in proportion," according to Wikipedia. Prorated amounts typically apply to rent, insurance and other services that are paid for a specified time period. When an insurance policy is canceled, the prorated amount due is calculated by dividing the number of days used in the policy period by the total number of days for the policy. The result is multiplied by the policy premium to calculate the prorated amount due.