To calibrate a CO₂ monitor, place it in fresh outdoor air (shaded, away from your breath and from traffic) for 10 to 20 minutes, then trigger the manual calibration in the menu or app. The monitor sets its current reading to about 400 to 420 ppm. Most monitors also self-correct automatically by assuming the lowest reading each week was outdoor air; manual calibration is for when that assumption fails.
Why monitors need calibration
NDIR sensors measure the ratio of infrared light transmitted through the sample chamber. Lamp output, detector sensitivity and dust on the optics all change slowly, shifting the zero point. Left alone, a sensor drifts tens of ppm per year, more in dusty or humid locations. Calibration re-establishes the zero at a known concentration.
Automatic baseline correction (ABC)
Almost every consumer monitor runs ABC. Over a rolling window (usually 7 days) it records the lowest CO₂ value seen and assumes that value was fresh air, adjusting the baseline to about 400 to 420 ppm. This works well in a home that empties during the day or has windows open at night.
ABC fails when the room never reaches outdoor levels: a bedroom with the door closed, an office occupied around the clock, a basement with no ventilation, or a greenhouse. The sensor then calls a stuffy room “fresh air” and reads low. Some monitors let you disable ABC for these situations.
Manual fresh-air calibration, step by step
- Check the manual for the exact button or app procedure and the target value (400, 410 or 420 ppm are all used).
- Take the monitor outdoors to a shaded spot away from roads, chimneys and people. A balcony or a garden works; a busy street or a parked car does not.
- Leave it for 10 to 20 minutes so the chamber fully exchanges air and the reading stabilizes.
- Do not breathe toward it. Stand a couple of meters away.
- Trigger calibration. The reading should snap to the target value within a minute.
- Bring it inside and confirm it climbs steadily in an occupied room.
Procedures by common monitor
- Aranet4: calibrate from the Aranet Home app under settings. ABC is on by default.
- Airthings View Plus: self-calibrating; the app lets you check sensor status. Manual calibration is not normally exposed.
- SwitchBot Meter Pro CO₂: calibration is triggered in the SwitchBot app while the device is in fresh air.
- Temtop and INKBIRD models: hold a button combination for a few seconds (the manual specifies which) while outdoors.
- GoveeLife H5140: calibration from the Govee Home app.
Manufacturers change app layouts, so treat the manual as authoritative.
Signs the calibration is off
- Reads below 400 ppm anywhere.
- Does not fall toward 400 to 500 ppm after an hour with windows open in an empty room.
- Two monitors in the same room differ by more than 100 ppm once warmed up and placed apart.
- The reading is flat while people come and go.
What calibration cannot fix
A monitor that estimates CO₂ from a VOC sensor cannot be calibrated into accuracy; see NDIR vs metal-oxide sensors. And a sensor that has drifted far, reads erratically or never stabilizes is failing; NDIR modules typically last 10 to 15 years but can be damaged by condensation.
For what the numbers mean once the monitor is trustworthy, see what a good indoor CO₂ level is and CO₂ monitor accuracy explained.
Frequently Asked Questions
How often should I calibrate a CO₂ monitor?
With automatic baseline correction working, rarely. Do a manual fresh-air calibration when the monitor reads below 400 ppm, fails to drop when a window opens, disagrees with another monitor by more than 100 ppm, or after several months in a room that never empties.
Can I calibrate indoors?
Only if the room has been empty with windows open long enough to reach outdoor levels. Outdoors, shaded, is more reliable.
What does ABC mean in the manual?
Automatic Baseline Correction (also called ASC, automatic self-calibration). The sensor records its lowest reading over a period, typically a week, and assumes that reading was outdoor air.