Free calculator · NFPA 72 Chapter 17
Smoke and Heat Detector Spacing Calculator
Get the detector count for a rectangular room using the grid method, with a check of the 0.7 rule. Heat detectors include the ceiling height reduction and joist or beam spacing.
Detectors required
- Spacing along length
- Spacing along width
- Farthest ceiling point
- 0.7 × spacing
Rectangular rooms on level ceilings only. Sloped ceilings, high air movement and very high ceilings need their own evaluation.
How detector spacing works
With the grid method, detectors sit no more than the spacing S apart and no more than S/2 from any wall, so the count along each direction is the room dimension divided by the spacing, rounded up. NFPA 72 also requires every point on the ceiling to be within 0.7 × S of a detector; the calculator reports the farthest point so you can check it.
count = ceil(L ÷ SL) × ceil(W ÷ SW)
Heat detector ceiling height reduction
| Ceiling height above | Up to and including | Multiply listed spacing by |
|---|---|---|
| 0 ft | 10 ft | 1.00 |
| 10 ft | 12 ft | 0.91 |
| 12 ft | 14 ft | 0.84 |
| 14 ft | 16 ft | 0.77 |
| 16 ft | 18 ft | 0.71 |
| 18 ft | 20 ft | 0.64 |
| 20 ft | 22 ft | 0.58 |
| 22 ft | 24 ft | 0.52 |
| 24 ft | 26 ft | 0.46 |
| 26 ft | 28 ft | 0.40 |
| 28 ft | 30 ft | 0.34 |
From NFPA 72 Chapter 17 (heat-sensing detectors). Spot-type smoke detectors have no mandatory height reduction.
Read more: NFPA 72 Smoke Detector Spacing: A Designer's Field Guide.
Detector Spacing Questions
What is the spacing for smoke detectors under NFPA 72?
On a smooth ceiling, spot-type smoke detectors use a nominal 30 ft spacing unless the listing says otherwise. Detectors are no more than the spacing apart, no more than half the spacing from a wall, and every point on the ceiling must be within 0.7 times the spacing of a detector.
Does ceiling height reduce smoke detector spacing?
NFPA 72 has no mandatory height-based spacing reduction for spot-type smoke detectors. Heat detector spacing is reduced as ceiling height goes above 10 ft, per the table in Chapter 17.
How do beams and joists change the spacing?
For heat detectors, solid joists cut the spacing perpendicular to the joists to half the listed spacing, and beams cut it to two-thirds. Smoke detectors on beamed or joisted ceilings follow a separate set of rules in NFPA 72 17.7.3.2.4 that depend on beam depth and ceiling height, so check your edition.
Can I use the 0.7 rule to stretch spacing in a corridor?
NFPA 72 Annex A explains how a narrow corridor can use longer spacing while keeping every point within 0.7 times the spacing. It is guidance rather than a mandatory method and not every AHJ accepts it, so confirm before using it.
More free calculators
This calculator is a planning aid and does not replace a full design review or the requirements of your authority having jurisdiction. Confirm against the NFPA 72 edition adopted in your jurisdiction. Not affiliated with NFPA.