Where to put your CO₂ monitor in a bedroom

Put your CO₂ monitor at about head height, at least 2 ft (60 cm) from anyone's face, a few feet from windows, doors and vents, and out of direct sun. Then leave it in that same spot every night so one night can be compared fairly with the next.

The short version

About head height: 3 to 6 ft (1 to 2 m) off the floor.
At least 2 ft (60 cm) from anyone's face.
About 3 ft (1 m) from windows, doors and vents.
Out of direct sunlight.
The same spot every night.

Height

The UK Health and Safety Executive (HSE) recommends placing CO₂ monitors at head height. Aranet, which makes a popular home monitor, suggests "breathing height (about 3–6 feet above floor)." The aim is for the monitor to read the air at roughly the level people breathe.

In a bedroom, the top of a dresser, a bookshelf or a wall mount usually works well. Many nightstands are a bit low and sit right beside a pillow, which causes the next problem.

Distance from faces

Your breath contains CO₂. HSE advises keeping monitors more than 50 cm from people for exactly this reason, since breath close to the sensor can make readings misleadingly high.

A monitor next to your pillow can catch your breath directly and show spikes that have nothing to do with the room as a whole. Keeping it at least 2 ft (60 cm) from where anyone's head will be gives you a little margin. If the only good shelf is near the bed, put the monitor on the far side of it.

Away from windows, doors and vents

HSE advises keeping monitors away from windows, doors and air supply openings. Aranet says to avoid direct airflow from windows or vents. A monitor sitting in a draft measures the air coming in, not the air you're sleeping in.

A good rule of thumb is about 3 ft (1 m) from any window, door or vent, including heating and cooling registers.

Out of direct sun

Aranet also advises keeping monitors out of direct sunlight and away from very humid spots. A sunny windowsill breaks two rules at once, so pick somewhere that stays shaded in the morning too.

The same spot every night

Comparisons are only fair if the monitor stays put. HSE suggests trying a few positions to find the most representative one. In a bedroom, once you've found a spot that follows the rules above, stick with it. A small piece of tape on the shelf makes it easy to put the monitor back in the same place after you pick it up.

New monitors take time to settle

Many CO₂ sensors correct themselves automatically. They keep track of the lowest readings they see and treat those as fresh outdoor air. AirGradient explains that the sensor in its own monitors uses a rolling window of about 180 hours, a little over a week, to do this. As a result, a new monitor may read a little high or low for its first week or two while it settles.

This self-correction works best when the monitor sees fresh air now and then, for example when the bedroom gets aired out during the day. You can start measuring with a new monitor right away. Just run the outdoor check below, and if a number looks odd, check again after a week or so.

The outdoor-air check

Outdoor air is a good reference point. Take the monitor outside to a shady spot away from people, cars and exhaust vents. Set it down, step back, and give it 15 to 30 minutes to settle.

A well-behaved monitor should settle near 420 ppm. NOAA's global monthly average was about 428 ppm in mid-2026, but some sensors are set to treat fresh air as 400 ppm, so anything from about 400 to 450 is fine. SwitchBot treats 400 to 500 ppm outdoors as normal for its CO₂ monitor and recommends recalibrating if it reads higher.

Local conditions matter. AirGradient notes that a monitor near a busy road or an exhaust vent can sit hundreds of ppm above the background level. If your reading is high, try a different spot before you assume the monitor is off. If it still reads well above 500 in clean outdoor air, follow the maker's instructions for recalibrating.