What is the formula to use in calculating vehicle insurance premium?

An insurance premium derives from a "rate". In turn, the rate derives from a number of factors allowed by state insurance regulators. The premium is the rate multiplied by the number of dollars worth of insurance involved (usually expressed in hundreds)

Some of the factors involved in determining the rate, and hence, the premium, include the driver(s) age(s), the make and model of the vehicle, the location where the vehicle is principally garaged (sometimes, drilled down to specific zip code areas). the average number of miles driven, whether the vehicle is used primarily for personal or commercial purposes, and driving history of the driver(s). There may or may not be others depending upon the law of the state in which the policy is issued.