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About this tool
Count the smoke and CO alarms a home needs under NFPA 72 and get the mounting distances for ceilings, walls and peaks.
This planner counts the smoke and carbon monoxide alarms a dwelling needs using the NFPA 72 coverage rule — one smoke alarm inside every sleeping room, one outside each separate sleeping area, and one on every level including the basement — and returns the mounting distances that decide whether each alarm actually works. Those distances are stated rather than guessed: at least 4 in (100 mm) from a sidewall on a ceiling, 4-12 in (100-300 mm) below the ceiling on a wall, within 3 ft (0.9 m) of a peak but outside the top 4 in (100 mm), and never within 10 ft (3 m) of a cooking appliance. Enter the manufacture year printed on your existing units and it dates the 10-year replacement.
Open Smoke Alarm Placement Planner on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Bedrooms, sleeping areas and levels each drive the count the way the code does.
Every rule comes with the number in both inches and millimetres, and the reason behind it.
The 10-year life runs from the manufacture date on the back of the unit, and the planner does that arithmetic.
One inside every sleeping room, one outside each separate sleeping area, and at least one on every level including the basement. For a three-bedroom home on two floors with all bedrooms upstairs, that is five: three in the bedrooms, one on the landing and one downstairs. A level that already has a sleeping-area alarm does not need a second one just to satisfy the per-level rule.
On a ceiling, keep it at least 4 in (100 mm) away from any sidewall, because air stagnates in that corner and smoke reaches it last. On a wall, the top of the alarm goes 4-12 in (100-300 mm) below the ceiling. On a peaked or cathedral ceiling, mount within 3 ft (0.9 m) horizontally of the peak but not inside the top 4 in, where a dead-air pocket forms.
At least 10 ft (3 m) from a stationary cooking appliance. Between 10 and 20 ft, use a photoelectric alarm or one with an alarm-silencing button, because ionization sensors respond strongly to cooking aerosols. Also keep alarms 3 ft (0.9 m) clear of a bathroom door with a shower, of forced-air supply registers, and of ceiling fan blade tips.
Yes, if the home has any fuel-burning appliance — a gas hob, boiler, geyser, generator, fireplace or wood stove — or an attached garage. Fit them outside each separate sleeping area and on every level, following the same pattern as smoke alarms. Carbon monoxide is colourless and odourless, so an alarm is the only warning a sleeping person gets, and its symptoms are easily mistaken for flu.
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