Smoke Detector Spacing Calculator (NFPA 72)

The layout rules

1. Spacing (S): the listed spacing of the detector on a smooth, flat ceiling. For spot smoke detectors NFPA 72 uses 30 ft as the nominal spacing. For heat detectors the listed spacing is multiplied by a ceiling-height factor: 100% up to 10 ft, 91% at 12 ft, 84% at 14 ft, 77% at 16 ft, 71% at 18 ft, 64% at 20 ft, 58% at 22 ft, 52% at 24 ft, 46% at 26 ft, 40% at 28 ft and 34% at 30 ft (Table 17.6.3.5.1, each step covering the heights above the previous one). Smoke detector spacing is not reduced by that table.
2. Detectors per row = room length ÷ S, rounded up; rows = room width ÷ S, rounded up. The detectors then sit at the center of equal cells, so none is farther than S ÷ 2 from a wall and no two are farther apart than S.
3. 0.7S: every point on the ceiling must be within 0.7 × S of a detector (21 ft for a 30 ft spacing). The farthest point in a rectangular cell is its corner, half the diagonal away from the detector; a full S × S cell puts that corner at 0.707S, which NFPA 72 rounds to 0.7S and accepts for the square layout.

These rules apply to smooth ceilings with no beams, joists or slope. Beamed ceilings, joists, peaked and shed ceilings, high air movement and partial coverage all change the spacing (17.6.3 and 17.7.3), and the detector listing, the manufacturer's instructions and the AHJ govern.

Worked example

A 60 × 40 ft room with spot smoke detectors listed at 30 ft: 60 ÷ 30 = 2 per row and 40 ÷ 30 = 1.33, so 2 rows, for 4 detectors. They sit 15 ft and 45 ft along the length and 10 ft and 30 ft across the width, so the actual spacing is 30 × 20 ft and the farthest ceiling point is 18.0 ft from a detector, inside the 21 ft (0.7S) limit. The same room with heat detectors listed at 50 ft under a 14 ft ceiling uses 50 × 84% = 42 ft, so 2 per row in a single row, 2 detectors; a 100 × 60 ft room at that spacing needs 3 per row and 2 rows, 6 detectors.