The 10 categories used by the Insurance Services Office, Inc. (ISO), to rank the fire protection in cities and towns according to the following three factors.

• Fire department quality—this includes its equipment (e.g., ladders, fire trucks), staffing (i.e., paid or volunteer), training, and geographic distribution of firefighting companies; this factor accounts for 50 percent of the total classification.
• Water supply system—this includes the condition, distribution, inspection, and maintenance of fire hydrants and a comparison of the available water to the amount needed to extinguish a fire; this accounts for 40 percent of the total classification.
• Fire alarm and communications systems—this includes telephone systems, telephone lines, staffing of these systems, and the quality of the dispatching system; this factor accounts for 10 percent of the total classification.

The Public Protection Classification program is a rating system with a range from 1 (best) to 10 (worst) that numerically ranks a community's municipal fire protection system. Class 1 represents an excellent fire protection system, and class 10 indicates virtually no protection. Actually, classes 9 and 10 are both "unprotected" classes; that is, they indicate rural areas without adequate fire hydrants or fire departments. Whether protection class 9 or 10 applies is determined by the distance from an available fire hydrant and a fire department that would respond.

Nearly all U.S. insurers use ISO's public protection classification in calculating rates for homes. Note, however, that many insurers deviate in some way from the rules of the public protection classification program through the use of independently filed rules.