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Smoke and CO Detector Calculator
Put a smoke alarm in every bedroom, outside each sleeping area, and on every level including the basement. A 3-bedroom, 2-story home with a basement usually needs about 6. If the home has a fuel-burning appliance or an attached garage, add carbon monoxide (CO) alarms outside each sleeping area and on every level.
Smoke (NFPA 72): 3 in bedrooms, 1 outside sleeping areas, 2 on other levels.
CO (NFPA 720, CPSC): 1 outside sleeping areas and 2 on other levels. Combo smoke/CO alarms can fill these 3 spots. Follow the alarm maker's instructions.
Typical Northern Virginia market range (not a quote): $600 to $1,200 to replace 6 existing hardwired alarms (3 as combo smoke/CO). Alarms in new spots need new wiring and cost more.
This is an estimate for planning. Electrical work needs an expert electrician and, in most places, a permit.
How the calculator works
Smoke alarms. The tool follows the household rules in NFPA 72 and the matching rule in the International Residential Code (IRC R314.3): one in each sleeping room, one outside each separate sleeping area near the bedrooms, and one on every level, including the basement. The alarm outside a sleeping area also counts for its level, so a level with bedrooms does not get an extra one.
So the count is: bedrooms + sleeping areas + levels that have no sleeping area. A sleeping area is a group of bedrooms that share a hallway. The tool assumes each sleeping area is on its own level. If two sleeping areas are on the same level, add one more.
CO alarms. CO alarms are called for when the home has a fuel-burning appliance (gas, oil, propane or wood) or an attached garage (IRC R315). NFPA 720 and the U.S. CPSC point them outside each sleeping area and on every level. Every CO spot is also a smoke spot, so combination smoke and CO alarms can cover both. Follow the alarm maker's instructions for the exact spot.
What changes the answer
Large levels. NFPA 72 asks for more smoke alarms on a big level so that no spot is too far from one. A long ranch house or a large open basement may need more than one.
Split levels. The IRC lets one alarm on the upper level cover a lower level that is less than a full story down, when no door separates them.
A fuel appliance in a bedroom. If a gas fireplace or other fuel appliance is in a bedroom or its bathroom, the IRC calls for a CO alarm in that bedroom too.
Where not to put them. Keep smoke alarms away from cooking (NFPA advises about 10 feet from a cooktop to cut false alarms) and at least 3 feet from a bathroom door with a shower or tub (IRC R314.3). Keep CO alarms away from vents and out of kitchens.
Linked alarms and the cost range
NFPA advises linked (interconnected) alarms, so when one sounds, they all sound. New homes need hardwired, linked smoke alarms with battery backup (IRC R314.4 and R314.6). Wireless linked alarms can help in older homes.
The cost line is the number of smoke alarms x a market range of $100 to $200 per hardwired smoke or combo smoke and CO alarm, replaced on existing wiring, unit included. Alarms in new spots need new wiring, which costs more, and the tool does not price it.
When to call an expert electrician
Call an expert electrician to add hardwired alarms in new spots, to link alarms on different circuits, or when you finish a basement or add a bedroom. Replace any smoke alarm that is 10 years old (the date is on the back), and replace CO alarms by the maker's end-of-life date. New wiring needs a permit in many places.
The code basis
Smoke alarm locations: NFPA 72, National Fire Alarm and Signaling Code, household chapter, and IRC R314.3. Carbon monoxide alarms: NFPA 720 (its rules moved into NFPA 72 starting with the 2022 edition) and IRC R315. Homeowner advice: NFPA smoke alarm guidance and the CPSC. Local codes and the code edition in force decide what is required in your home.
| Home | Smoke alarms | CO alarms (fuel appliance or attached garage) | CO alarms (all electric, no attached garage) |
|---|---|---|---|
| 2 bedrooms, 1 level | 3 | 1 | 0 |
| 3 bedrooms, 1 level plus basement | 5 | 2 | 0 |
| 3 bedrooms, 2 stories plus basement | 6 | 3 | 0 |
| 4 bedrooms, 2 stories plus basement | 7 | 3 | 0 |
| 4 bedrooms in 2 sleeping areas, 2 stories | 6 | 2 | 0 |
Worked example: 3 bedrooms, 2 stories and a basement, gas furnace
- All 3 bedrooms are upstairs, off one hallway, so there is 1 sleeping area. The home has 3 levels: basement, main floor and upstairs.
- Smoke alarms in bedrooms: 3.
- Smoke alarm outside the sleeping area (upstairs hallway): 1. It also covers the upstairs level.
- Smoke alarms on the other levels (main floor and basement): 2.
- Total smoke alarms: 3 + 1 + 2 = 6.
- The gas furnace means CO alarms too: 1 outside the sleeping area + 2 on the other levels = 3. Combo smoke and CO alarms can fill those 3 spots.
- Cost to replace 6 existing hardwired alarms: 6 x $100 to 6 x $200 = $600 to $1,200 (market range, not a quote).
Smoke and CO Detector Calculator questions
Short answers to common questions.
How many smoke detectors do I need?
One in each bedroom, one outside each sleeping area, and one on every level, including the basement. A 3-bedroom, 2-story home with a basement usually needs about 6. Big levels may need more.
Do I need a smoke detector in every bedroom?
Yes. NFPA 72 and the IRC both call for a smoke alarm inside each sleeping room, plus one in the hallway or area outside the bedrooms.
Where should carbon monoxide detectors be placed?
Outside each sleeping area and on every level of the home, in homes with a fuel-burning appliance or an attached garage. CPSC advises against putting them in kitchens or right above fuel-burning appliances. Follow the alarm maker's instructions.
Can I use combination smoke and CO detectors?
Yes. Listed combination alarms can cover a spot that needs both. Every place a CO alarm goes in this guide also needs a smoke alarm, so combos can cut the number of devices.
Do I need a CO detector if my house is all electric?
If there is no fuel-burning appliance, fireplace or attached garage, the IRC does not call for one. Some people add one anyway, for example if they use a portable generator.
How often should smoke detectors be replaced?
NFPA advises replacing smoke alarms when they are 10 years old. CO alarms last about 5 to 10 years, depending on the model; most chirp or show an end-of-life signal.
Sources: NFPA: installing and maintaining smoke alarms · NFPA 72, National Fire Alarm and Signaling Code (free access) · NFPA 720, carbon monoxide detection (folded into NFPA 72 from the 2022 edition) · U.S. Consumer Product Safety Commission: carbon monoxide questions and answers · ICC, International Residential Code 2021, Chapter 3: R314 smoke alarms, R315 carbon monoxide alarms. Last updated November 7, 2026.
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