Fire Alarm Initiating Device Calculator

Number of Floors moves Total Initiating Devices by an exactly proportional amount for most percentage changes -- because every per-floor device count (smoke, corridor smoke, heat, and pull stations) is simply multiplied by it, a 10% change in floor count is a 10% change in the total for any starting value of 2 floors or more. The one exception is Number of Floors's own minimum: the engine floors it at 1, so a downward change starting from 1 floor is clamped right back to 1 and produces 0% change in the total, not the expected -10%. Floor Area per Floor drives the smoke- and heat-detector counts through NFPA-72-style spacing coverage, but because the spacing math uses a ceiling function, area increases only add a detector when they cross a coverage-area boundary -- which means its measured effect isn't fixed the way Number of Floors's is: at this calculator's own defaults (smoke-only) Floor Area per Floor's effect is somewhat smaller than Number of Floors's, but in a smaller-baseline configuration like heat-only, crossing one of those coverage boundaries moves the (now smaller) total by more, proportionally, than Number of Floors does. Neither input reliably outranks the other across every detector configuration -- both matter comparably, and which one moves the total more depends on Detector Config.

Detector Config (1-3) looks like a minor picker, but it actually gates two independent boolean branches in the underlying formula: smoke detectors are counted whenever the config is NOT 3 (so both "smoke only" and "smoke + heat" get smoke coverage), and heat detectors are counted whenever the config is 2 or higher (so both "smoke + heat" and "heat only" get heat coverage). At this calculator's own default floor area and ceiling height, switching from smoke-only (1) to smoke-plus-heat (2) adds 2 heat detectors per floor without removing any smoke detectors, while switching to heat-only (3) removes all smoke detectors -- 6 regular smoke detectors plus 4 corridor smoke detectors per floor at the defaults (5,000 sq ft floor area, 100 ft corridor length), 10 devices total -- and leaves only the 2 heat detectors plus the 2 pull stations, which detector config never touches -- a real, substantial swing that a narrow ±10% nudge around any single detector-config value will not reveal, since 1, 2, and 3 are the only three values this input can take and none of the boolean comparisons flips for a small fractional change around them. Ceiling Height only matters above 10 ft, where NFPA 72's spacing-reduction table kicks in and shrinks smoke detector spacing (and therefore increases the count) in eight discrete steps up to 40 ft; below 10 ft it has zero effect on spacing. Corridor Length adds detectors independently of floor area, at 30 ft spacing, with a minimum of 1 detector whenever corridor length is greater than 0.