How Many Carbon Monoxide Detectors Do I Need?

You moved last week into a two-story house with a basement furnace and an attached garage, and the previous owner left one CO alarm in the upstairs hallway. Is that enough? The safety baseline is two ideas, coverage by level and coverage near sleeping areas, and for most layouts they give you the number in about a minute. Then your state can raise it, because the spacing rules run from 10 feet to 21 feet from a bedroom door and the tighter ones make a long hallway need two alarms where a loose one needs a single unit. An alarm too far from where people sleep does not wake them, which is the failure this whole count exists to prevent.

Key Takeaways

• One CO alarm per occupied level (including a finished basement) is the baseline minimum.
• At least one alarm near sleeping areas so it can wake people up at night.
• Add extra alarms for long layouts, separate sleeping wings, attached garages, or distant fuel-burning appliances.
• You do not need a detector in every room, strategic placement beats total room count.
• Your state can raise the count. Spacing rules run from 10 to 21 feet from a bedroom door, and the widest of them, the NFPA 72 figure, is measured along the path of travel rather than straight through the wall. The tighter the rule, the more units a long hallway needs.
• The per-level rule is a safety standard, not the model building code. Four states wrote it into their residential code and each worded it differently; Oregon counts bedrooms instead.

Baseline Rule: One per Level + Sleeping Areas

For most homes, the safest minimum is simple: one CO alarm on every level of the home (including a finished basement) and at least one alarm near sleeping areas so it can wake people up at night. In many layouts, the "bedroom hallway" alarm and the "level" alarm are the same unit.

A quick way to count the minimum

1. Count the levels you live on (basement, main floor, second floor, finished attic).
2. Add one for each separate sleeping wing or long hallway where an alarm might not be heard clearly.
3. Add one more if you have an attached garage or a fuel-burning appliance area that is far from the nearest alarm.

Both halves of the baseline come from NFPA 72, the National Fire Alarm and Signaling Code, which absorbed CO alarm requirements from the withdrawn NFPA 720 and now carries them in Chapter 29. Guides that still cite NFPA 720 as the live standard are working from a document whose last edition was 2015.

Where the Count Changes: Your State Sets the Spacing

Two identical houses can need a different number of alarms, and the variable is the state line rather than the floor plan. The rules that say how far an alarm may sit from a bedroom door do not agree with each other, and the arithmetic falls out of that number.

Take one upstairs hallway with bedrooms at each end and 30 feet between the two doors. Put a single alarm at the midpoint and it sits 15 feet from each door.

• Under NFPA 72's 21-foot allowance, that one alarm covers both bedrooms. The hallway needs one unit.
• Under the 15-foot rule used in Illinois and Colorado, the midpoint alarm is at the limit with nothing to spare, and any asymmetry in the hallway puts one door out of range.
• Under the 10-foot rule in Florida and New Jersey, no single position covers both doors. That hallway needs two alarms.

Two things about those numbers before you measure anything. The 21-foot allowance comes from NFPA 72 Section 29.7.1.1, and the standard measures it along the path of travel rather than straight through walls, so a hallway that turns a corner spends more of the allowance than a ruler laid across the floor plan suggests. Wisconsin adopted that same wording in ATCP 72.18(12) when its lodging rule took effect on 25 January 2026. And the 10-foot figure gets repeated online as though it were the national rule when it belongs to Florida and New Jersey, so importing it into a state that allows 21 feet adds units your own inspector will not ask for.

So the honest answer to "how many do I need" starts with a question back: where do you live, and is anyone going to inspect it? A homeowner buying for their own safety can work from the NFPA baseline. A landlord, a short-term rental host, or anyone selling a house is counting against a statute, and the statute is usually the stricter number. The full set of distance rules is laid out on our placement guide, and every jurisdiction has its own page in the state CO alarm law reference.

One warning about that 15 before you measure, because the same number is doing opposite work in two different rules. Kidde's placement guidance tells readers to keep an alarm at least 15 feet from fuel-burning appliances, a minimum set to stop the burst of CO at start-up from tripping it. Illinois at 430 ILCS 135 and Colorado both set 15 feet as a maximum from the sleeping room door. Same number, opposite direction, measured from different objects, and a reader who carries the manufacturer's 15 into the statute will push the alarm away from the bedroom to avoid a nuisance alarm and fail the law while doing it.

The two are constraints to satisfy together rather than a trade-off: within 15 feet of the door, and clear of the appliance by whatever separation your own alarm's manual specifies. Check that manual rather than the brand's general guidance, because the model sheets are not consistent with it. Kidde's own battery CO alarm manuals give 1 to 3 metres, roughly 3 to 10 feet, which leaves far more room in a small house than the 15 does. Where no position satisfies both, the statute is the constraint with a penalty attached.

One more trigger adds a unit that no floor-count method will catch. Where a fuel-burning appliance sits inside a bedroom or an attached bathroom, the alarm goes in that bedroom itself. Counted across our records rather than left as "several states", six states put it in their own instrument in those words: Arkansas, Iowa, Nevada, North Carolina, North Dakota and Ohio. Three more reach the same result by a different road: Alabama through the 2021 IBC for lodging, Illinois through municipal adoption, and New Hampshire, whose RSA 153:10-a states no location and instead requires the devices "in accordance with" a named installation standard, so the rule is real but it lives in the standard. That is why you will see this counted as anything from six to nine. A gas fireplace in a main bedroom is enough to trigger it, and so is a wall heater in the attached bathroom.

"One Per Level" Is Advice in Most States

The baseline at the top of this page comes from NFPA 72, a standard your building inspector may never open. An inspector works from the residential building code your state adopted, and the two documents disagree here. Section R315.3 of the 2021 International Residential Code, the model text most states start from, puts carbon monoxide alarms outside each separate sleeping area in the immediate vicinity of the bedrooms, and adds an alarm inside any bedroom that holds a fuel-burning appliance or has one in its attached bathroom. Levels appear nowhere in it. The smoke alarm section next door, R314.3, does require one on each additional story, so the model code uses that wording where it wants it and leaves it out here.

Five states rewrote that sentence. Four of them added a level count, and no two wrote it the same way.

Those four read as the same rule until you count a real house with them. Take a two-story home with an unfinished basement and the bedrooms upstairs. Washington's "each level of the dwelling" carries no qualifier, so the basement counts and you buy three alarms. Connecticut's "each additional habitable level" drops the unfinished basement and starts counting above the level the sleeping-area alarm already covers, and the same house needs two. North Dakota is the only one that states its own floor: "a minimum of one" tells an inspector that a single unit satisfies the level, and the word "also" stops anyone arguing that the hallway alarm doubles as the level alarm. Utah's edit is the quiet one. It adds the same clause the other three add and puts it in a different place: ahead of the sleeping-area language, which makes level coverage the primary duty and the version a builder has the hardest time reading down to a single unit.

Oregon counts bedrooms, not levels

Oregon threw the model sentence out and wrote its own. Under OAR 837-047-0130 an alarm goes inside each bedroom or within 15 feet outside each bedroom door, and bedrooms on separate floors of a building with two or more stories need separate alarms. Levels reach the count only through the bedrooms sitting on them. A single-story Oregon house with four bedrooms down a long hallway can therefore need more alarms than a three-story house in Washington, and anyone filing Oregon under "per level" will undercount it.

Check the edition before you go looking for any of this. R315 is the carbon monoxide section in the 2018 and 2021 editions of the model code, and the 2024 edition reordered the chapter, moved carbon monoxide alarms to Section R311 and gave R315 to sleeping lofts. A jurisdiction on the 2024 code will still return a section called R315, so a stale reference resolves to the wrong subject instead of failing.

For the rest of the country the sleeping-area sentence stands as the model wrote it, so the alarm you put on the basement stairs is one you chose rather than one anybody will inspect. That is not the same as saying no other state requires it. A per-level duty can arrive through a separate instrument, and North Dakota shows how: its State Electrical Board wiring standard at NDAC 24.1-06-01-40 carries an every-habitable-level rule of its own, alongside the building code amendment above. Each state's own page records which instruments reach it. We could not read the adopted text for New York, South Dakota and Wisconsin against this pattern, so four-plus-Oregon is the count we verified and the real total may be higher.

When You May Need More Than the Minimum

Long homes or split layouts

If your home is ranch-style, split-level, or has thick doors between areas, add another alarm so no sleeping room is "around the corner" from CO protection. This is the practical meaning of carbon monoxide detectors near bedrooms: keep at least one alarm close to where people sleep.

Multiple sleeping wings

Homes with bedrooms on opposite ends, multi-suite houses, or a main bedroom on the first floor plus kids upstairs often need more than one "near bedrooms" alarm. If you cannot clearly hear the test tone from the hallway alarm in a closed-bedroom situation, add coverage.

Attached garage

An attached garage is a major reason people ask whether they need a detector in every room. You do not, but you do want reliable coverage in the living space connected to the garage, plus a sleeping-area alarm as backup. The room-by-room reasoning is on our detector placement guide, and what a car in an attached garage does to the rooms on the other side of the wall is on carbon monoxide in your home.

Distant fuel-burning appliances

If your furnace, boiler, water heater, or fireplace is on a level or in a zone that is distant from the closest alarm, add coverage so a problem is detected early. This is especially helpful when appliances are in a finished basement or utility area separated by doors.

Apartment Considerations

• Small/medium apartment: 1 alarm near the sleeping area is often a reasonable minimum.
• Large apartment or long hallway: 2 alarms, one near bedrooms and one closer to the main living area.
• Unit above/next to a garage or mechanical room: consider an additional alarm on the side nearest that risk.

If you rent, check whether the building provides alarms and whether you are allowed to add battery-powered units. Even when a landlord provides the minimum, adding an extra alarm near bedrooms improves real-world safety.

Combination Smoke/CO vs Separate Units

Combination smoke/CO alarms reduce device count and work well when placed for both hazards. The practical approach: use combinations where they naturally fit your smoke-alarm plan, and use a dedicated CO unit when you need better placement flexibility for CO coverage.

Battery Backup and Power Outage Planning

Power outages are when people run generators or use alternative heat, which can increase CO risk. That is why "hardwired only" is not ideal unless it includes a working battery backup. Make sure at least one alarm near sleeping areas runs during outages. For generator-specific rules, see generator distance from the house.

Testing and Replacement

Buying the right number of alarms is step one. CO alarms have sensors that age, so they need periodic replacement. Test monthly using the test button, replace at end-of-life per the manufacturer label (commonly 5-10 years), and never ignore chirps, low battery and end-of-life signals are different, but both need action. See how long do carbon monoxide detectors last.

Before You Buy, in Four Steps

The Key Takeaways at the top are the safety baseline. This is the order to work in once you are counting units for one specific house, and it is different because the first step is the one people skip.

1. Open your own state page first. The baseline gives you a floor and the statute can raise it, so start with the spacing rule that applies where the house is. Every jurisdiction has one in the state CO alarm law reference.
2. Measure hallway door to door, then divide. With a 10-foot rule, a hallway wider than 20 feet between the outermost doors needs a second unit no matter how many levels the house has.
3. Walk the bedrooms looking for appliances. A gas fireplace, a wall heater or a water heater in an attached bathroom adds a unit inside that room in the seven states above, and adds one on any reading of the safety case elsewhere.
4. Check the dates before you buy anything. An existing alarm past its end-of-life date is not a unit you already own, and buying two while replacing three is how a house ends up under-counted.

Sources & References

• EPA: Where Should I Place a Carbon Monoxide Detector?
• CPSC: Carbon Monoxide Information Center
• NFPA: Carbon Monoxide Safety
• NFPA 72, National Fire Alarm and Signaling Code, Chapter 29. Carries the household CO alarm requirements relocated from NFPA 720, whose final edition was 2015.
• Fla. Stat. Section 553.885; 430 ILCS 135; Colorado CCR Section R315; N.J.A.C. 5:1-28.1. Sleeping-area spacing and in-bedroom triggers, as recorded in our state law reference. N.H. RSA 153:10-a is cited for the opposite point, that it states no location and names a standard instead.
• 2021 International Residential Code Section R315.3, Location, with Section R314.3 for the smoke alarm comparison. The model text places CO alarms by sleeping area and bedroom appliance, with no level requirement. The 2024 edition moves carbon monoxide alarms to Section R311.
• Utah Code Section 15A-3-202; WAC 51-51-0315; Connecticut State Building Code amendment to R315.3; North Dakota State Building Code (2023) Section R315.3 and NDAC 24.1-06-01-40; OAR 837-047-0130. The four level amendments and Oregon's per-bedroom rule.

Last updated: August 12, 2026