Portable Fire Extinguisher Coverage (NFPA 10)
Two limits that turn out to be one. NFPA 10 caps travel distance to a Class A extinguisher at 75 ft and caps coverage at 11,250 sq ft per unit -- and those are not independent numbers: on a clear floor with units on a square grid the worst point is the centre of a square, so spacing can be d x sqrt(2) and area per unit is 2 d^2, and 2 x 75^2 is exactly 11,250. The area ceiling IS the geometry of the travel distance, which tells you it already assumes a perfectly open floor. Class K at 30 ft covers only 1,800 sq ft, so travel distance governs by a mile. The caveat that outranks the arithmetic: travel distance is measured along the PATH, not straight-line, so this count is a FLOOR -- walk the routes. Extinguisher RATING is a separate table and is not shipped.
Formula and source
grid spacing = travel distance x sqrt(2); area per extinguisher = 2 x travel^2 (the worst-case point on a square grid sits at the centre of a square, sqrt(2)/2 of the spacing away); count by travel = ceil(floor area / area per extinguisher); count by the area ceiling = ceil(floor area / cap); required = the larger of the two.
NFPA 10 - a maximum travel distance to a Class A extinguisher of 75 ft at all hazard levels, 30 ft for Class K, and 30 or 50 ft for Class B depending on the extinguisher's B rating and the hazard level; and the 6.1.3.3 ceiling of 11,250 sq ft of coverage per extinguisher. The square-grid relation is geometry, not a code value.
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