International Standards Organization (ISO)

Within the Fire Suppression Rating Schedule (FSRS), a section called "Needed Fire Flow" outlines the methodology for determining the amount of water necessary for providing fire protection at representative locations throughout the community. To evaluate your community's water supply ISO calculates the needed fire flow for selected locations. We then determine the water-flow capabilities at those locations and calculate a ratio considering the need (needed fire flow) and the availability (water-flow capability). We use that ratio in calculating the credit points identified in the FSRS.

ISO calculates the needed fire flow for an individual building based on the building's area, construction, occupancy, and exposure. To get full credit, the water supply must be able to deliver water, at 20 psi residual pressure and at the specified rate of flow, for a specified period of time. Needed fire flows for individual buildings range from a minimum of 500 gpm to a maximum of 12,000 gpm. For more information, see the Guide for Determination of Needed Fire Flow.

Residential areas
For residential areas with one- and two-family dwellings, ISO determines the needed fire flows by considering the distance between buildings:

Distance Needed Fire Flow
more than 100 feet 500 gpm
31 to 100 feet 750 gpm
11 to 30 feet 1,000 gpm
10 feet or less 1,500 gpm