About Smoke & CO Detector Calculator (IRC R314/R315)
The smoke and CO detector calculator turns the IRC 2021 alarm-location rules into a minimum device count for planning a dwelling. R314.3 requires smoke alarms in each sleeping room, outside each separate sleeping area in the immediate vicinity of the bedrooms, and on each additional story of the dwelling — including basements and habitable attics but not crawl spaces or uninhabitable attics. R315.2.1 requires carbon monoxide alarms only where the dwelling contains a fuel-fired appliance or has an attached garage with an opening that communicates with the dwelling; R315.3 then places one outside each separate sleeping area and adds one inside any bedroom that has a fuel-burning appliance in the room or its attached bathroom.
Enter the bedroom count, the number of separate sleeping areas (bedroom zones each served by its own hallway alarm), the total stories, and how many of those stories contain sleeping rooms; tick the fuel-fired appliance / attached garage box and count any bedrooms with a fuel-burning appliance. The result is a smoke total and a CO total, each itemized rule by rule with its section reference. Treat the numbers as a conservative planning minimum: the placement rules — at least 3 feet from a tub or shower bathroom door, 6 to 20 feet from cooking appliances depending on sensor type, and the high-ceiling hallway rule — plus local amendments can move devices or add more.
How It Works
- Count bedrooms (sleeping rooms) — R314.3 Item 1 puts one smoke alarm inside every one of them, and that count carries straight into the total.
- Count separate sleeping areas: a sleeping area is a group of bedrooms served by one hallway, so a typical home with all bedrooms off one hall has one, while a house with a primary suite on the main floor and kids' rooms upstairs has two. R314.3 Item 2 puts a smoke alarm outside each, and R315.3 puts a CO alarm in the same vicinity when CO is required.
- Enter total stories counting basements and habitable attics (skip crawl spaces and uninhabitable attics), and how many stories contain sleeping. Stories with sleeping are already covered by the Item 1–2 alarms, so R314.3 Item 3 adds one alarm for each remaining story — the calculator computes total stories minus stories with sleeping.
- Tick the fuel-fired appliance / attached garage box if the dwelling has any gas, oil, wood, or other fuel-burning appliance (furnace, water heater, range, fireplace) or an attached garage with a door or other opening into the house — either condition triggers CO alarms under R315.2.1. If neither applies, the IRC requires no CO alarms, but many states and insurers do anyway.
- Count bedrooms with a fuel-burning appliance in the room or its attached bathroom (a gas fireplace or wall heater is the classic case) — R315.3 adds an in-bedroom CO alarm for each.
- Read the breakdown: every line shows the rule, its verified section reference, and the devices it adds, and the lines sum exactly to the smoke and CO totals. Then check the prose notes for placement restrictions, interconnection, and power — the count alone is not a compliant design.
Worked Example
A two-story house has three bedrooms, all off a single upstairs hallway (one sleeping area), and a gas furnace (fuel-fired appliance present, no fuel appliance inside any bedroom). Smoke alarms: 3 for the bedrooms (R314.3 Item 1) + 1 outside the sleeping area (Item 2) + 1 for the first floor, the one story without sleeping (Item 3) = 5 minimum. CO alarms: the furnace triggers R315.2.1, so 1 outside the sleeping area (R315.3) with no in-bedroom alarms needed = 1 minimum. All five smoke alarms must be interconnected and hardwired with battery backup in new construction.
Formulas
- Minimum smoke alarms (R314.3 Items 1–3)
N_smoke = N_bedrooms + N_sleepingAreas + (N_stories − N_storiesWithSleeping)- CO alarm trigger (R315.2.1)
CO required ⇔ fuel-fired appliance in the dwelling OR attached garage with a communicating opening- Minimum CO alarms (R315.3)
N_co = CO required ? N_sleepingAreas + N_bedroomsWithFuelAppliance : 0
Standards & References
- IRC 2021 R314.3 — smoke alarm locations: each sleeping room, outside each separate sleeping area, each additional story incl. basements and habitable attics
- IRC 2021 R314.3.1 — placement near cooking appliances: ionization ≥ 20 ft, ionization with silencing switch ≥ 10 ft, photoelectric ≥ 6 ft
- IRC 2021 R314.4 / R314.6 — interconnection (one activates all, listed wireless permitted) and hardwired power with battery backup
- IRC 2021 R315.2.1 — CO alarms required with a fuel-fired appliance or an attached garage with a communicating opening
- IRC 2021 R315.3 — CO alarm locations: outside each separate sleeping area; within bedrooms containing a fuel-burning appliance
- IRC 2021 R315.5 / R315.6 — CO interconnection and power source mirror the smoke alarm rules
Frequently Asked Questions
Where are smoke alarms NOT allowed to be installed?
The 2021 IRC restricts placement to cut nuisance alarms. R314.3 Item 4 keeps smoke alarms at least 3 feet horizontally from the door or opening of a bathroom containing a bathtub or shower, and R314.3.1 keeps them away from permanently installed cooking appliances: at least 20 feet for ionization alarms, 10 feet for ionization alarms with an alarm-silencing switch, and 6 feet for photoelectric alarms or alarms listed as reducing cooking nuisance alarms. Both restrictions yield only where they would prevent an alarm the code requires elsewhere in R314.3.
Do all the smoke alarms have to be interconnected?
Yes. Where more than one smoke alarm is required in a dwelling unit, R314.4 requires them interconnected so that actuating one alarm activates every alarm in the unit — a basement fire must wake someone sleeping two floors up. Physical wiring is not mandatory: listed wireless alarms that all sound together satisfy the rule. R315.5 applies the same interconnection requirement to carbon monoxide alarms.
Can I use battery-only smoke alarms, or must they be hardwired?
In new construction, R314.6 requires smoke alarms to draw primary power from the building wiring (where served from a commercial source) with a battery for backup, on permanent wiring without a disconnecting switch. Battery-only alarms are permitted in buildings without commercial power and in existing dwellings being altered or repaired under R314.2.2, where opening walls just to fish alarm cable is not demanded. R315.6 sets identical power rules for CO alarms.
When does the IRC require carbon monoxide alarms at all?
R315.2.1 triggers CO alarms in new construction only when the dwelling unit contains a fuel-fired appliance — gas furnace, water heater, range, or a gas or wood fireplace — or has an attached garage with an opening that communicates with the dwelling. An all-electric house with no attached garage needs none under the IRC as published. Many states, localities, and insurers require CO alarms in every dwelling regardless, so check your local rules before skipping them.
What counts as a "separate sleeping area"?
A sleeping area is a cluster of bedrooms served by a common hallway or immediate vicinity. Three bedrooms off one upstairs hall form one sleeping area — a single alarm outside them satisfies R314.3 Item 2 (and R315.3 for CO). A primary suite at one end of the house with children's bedrooms at the other, or bedrooms on different floors, are separate sleeping areas, each needing its own hallway smoke alarm and, when CO is required, its own CO alarm.
Does a basement or attic count as a story needing a smoke alarm?
Basements and habitable attics count — R314.3 Item 3 requires a smoke alarm on each additional story including them. Crawl spaces and uninhabitable attics do not. For split-level dwellings without an intervening door, an alarm on the upper level may serve the adjacent lower level if the lower level is less than one full story below. The calculator treats each story you enter as needing its own alarm, which is the conservative reading.
Is the calculated count a guaranteed pass at inspection?
No — it is a conservative planning minimum from the R314.3 and R315.3 location rules. Real layouts can shift the number either way: an inspector may accept one alarm satisfying two rules on the same story, while the placement restrictions (3 feet from tub/shower bathroom doors, 6–20 feet from cooking appliances, the high-ceiling hallway rule in R314.3 Item 5) or state and local amendments can force extra devices. Confirm final locations with your building department.