Smoke & CO Detector Placement Calculator

A smoke and CO detector placement calculator returns an estimated minimum count of smoke and CO alarms for a home. It uses one smoke alarm per bedroom plus one per story, and one CO alarm per story plus one more if there is a fuel-burning appliance or an attached garage.

Last reviewed September 2026Method & sources Report an error

Minimum Smoke Alarms

Breakdown
Minimum Smoke Alarms	5
Minimum CO Alarms	3

How to Calculate Smoke & CO Detector Placement

The IRC requires a smoke alarm inside each sleeping room. It also requires at least one outside each separate sleeping area (typically a hallway serving bedrooms), plus one on every additional story, including a finished basement, even without bedrooms there. This is approximated here as one alarm per bedroom plus one per story. Carbon monoxide alarms are required outside each sleeping area and on every level that has a fuel-burning appliance (like a gas furnace or water heater) or an attached garage. This is approximated here as one per story, plus one more if a fuel-burning appliance or attached garage is present.

Example: Home with 3 bedrooms, 2 stories

A home with 3 bedrooms, 2 stories, with a gas furnace (a fuel-burning appliance): 1. Minimum smoke alarms: 3 bedrooms + 2 stories = 5 2. Minimum CO alarms: 2 stories + 1 (fuel-burning appliance present) = 3 That home needs at least 5 smoke alarms and 3 CO alarms as a starting estimate.

Smoke & CO Detector Placement: Compare Scenarios

Scenario	Minimum Smoke Alarms	Minimum CO Alarms
Small (2 bed, 1 story)	3	2
Medium (3 bed, 2 story)	5	3
Large (5 bed, 3 story)	8	4

What Is Smoke & CO Detector Placement?

A smoke and CO detector placement calculator gives a minimum count estimate for smoke and carbon monoxide alarms in a home. The count is based on the IRC's general placement requirements for bedroom count and number of stories. It is a starting point for a life-safety detector plan, not a substitute for your local code official's final placement approval.

Smoke & CO Detector Placement: Pros and Cons

Pros

Based on the IRC's actual general placement principles (each sleeping room, outside each sleeping area, each additional story), not an arbitrary guess
Accounts for carbon monoxide alarm requirements separately, since they follow a different placement rule than smoke alarms
A quick starting estimate before finalizing an exact floor-plan-specific placement

Cons

This is a simplified minimum-count estimate, not an exact placement plan. The IRC's actual language is more nuanced (for example, multiple separate sleeping areas on the same story may each need their own hallway alarm) and your local code may have amendments
Doesn't account for interconnection requirements. Most current code requires all smoke alarms in a home to be interconnected so that one activating sounds all of them, which affects wiring but not the minimum count itself
Always confirm your specific home's final placement and count with your local building department or an inspector. This estimate is a planning starting point, not a compliance guarantee

Smoke & CO Detector Placement: What a Pro Checks

If a story has more than one separate sleeping area, plan a hallway alarm for each one, not just one per story. The IRC's actual rule is more granular than this calculator's simplified per-story count.
Interconnect all smoke alarms so one activating sounds all of them. Most current code requires this for newly installed alarms.
Mount CO alarms at a normal wall-outlet height rather than only on the ceiling, since carbon monoxide mixes readily with air instead of only rising like smoke.

Frequently Asked Questions

How many smoke detectors does a house need?

At minimum, one inside each bedroom, one outside each sleeping area (typically a hallway), and one on every additional story including a finished basement. A 3-bedroom, 2-story home needs at least 5 by this general rule.

How many carbon monoxide detectors do I need?

At minimum, one outside each sleeping area, plus one on every level that has a fuel-burning appliance (gas furnace, water heater, fireplace) or an attached garage. A 2-story home with a gas furnace needs at least 3 by this general rule.

Do smoke detectors need to be interconnected?

Most current building codes require newly installed smoke alarms in a home to be interconnected, so that any one alarm activating triggers all of them to sound. Check your local code for the specific requirement that applies to your situation (new construction vs. an existing home).

Where should I put a carbon monoxide detector?

Outside each sleeping area at minimum, and on any level with a fuel-burning appliance or attached garage. Unlike smoke alarms, CO alarms are commonly placed at a normal wall-outlet height rather than only on the ceiling, since carbon monoxide mixes readily with air rather than only rising like smoke.

Is this calculator a substitute for a code inspection?

No. It's a simplified planning estimate based on the IRC's general principles. Your local jurisdiction may have code amendments, and an inspector's review of your specific floor plan is the actual authority on exact placement and count.

Sources

2021 International Residential Code, Section R314 (Smoke Alarms) and Section R315 (Carbon Monoxide Alarms) (opens in a new tab)

Electrical