Premiums are based on the employer's industry classification code and payroll. Premiums for the most dangerous enterprises, such as trash hauling or logging, may be much higher than premiums for an accounting firm.

Location has also become a factor in workers comp premiums. Since the terrorist attacks of September 11, 2001, workers compensation insurers have been taking a closer look at their exposures to catastrophes, both natural and man-made. For businesses located in an area at high risk of catastrophe, premiums may be higher, regardless of the nature of the business itself.

Employers with an annual premium above a certain amount are usually eligible for experience rating, which adjusts the premium up or down depending on the claims history of the company relative to other companies in that industry category. Businesses with higher than average claims will pay a higher premium and those with lower claims will generally pay less.

Experience rating is more sensitive to the number of claims (loss frequency) than the dollar value of claims (loss severity). This is because of the insurance industry maxim, frequency breeds severity.  Insurers know from experience that where more accidents occur, there is a greater likelihood of big losses. A greater number of accidents indicates that overall in working conditions are not as safe as an environment where fewer accidents occur, even if in a given year the few accidents that occurred were more costly.