There is one rule for buying a CO2 monitor. It must have a real NDIR sensor.
Cheap monitors love to advertise "eCO2" or CO2 that is "estimated" from other gases. That is a guess, not a measurement, and it is often wildly wrong. If a listing does not clearly say NDIR, skip it. That single rule filters out most of the junk. Why does NDIR matter? Full explainer →
Everything below has a true NDIR sensor. Here is what to get, by budget.
This is what most of us actually carry. Pocket sized, e-ink screen you can read across a room, no app required, and batteries that last months. Brutally accurate. It is expensive, but it is the one every other monitor gets compared to. If you want to buy once and never think about it again, this is it.
If you mostly want to watch one room (your desk, your bedroom), a mains-powered NDIR desktop monitor from Qingping, SmartAir, or similar does the job for roughly two thirds of the Aranet price. Bigger screen, plugs in, less portable. Perfect as a fixed "is this room ok right now" gauge.
If you just want a real number without spending much, a basic INKBIRD NDIR monitor gets you there. No frills, but it measures CO2 for real. A great first monitor to prove to yourself (and your office manager) that the air is bad.
Open hardware, open data. If you want to log readings over time, self-host a dashboard, or build a small fleet across a home or office, AirGradient is the enthusiast pick. More setup, more control.
Anything that says "eCO2," "equivalent CO2," or hides the sensor type. Cheap air-quality gadgets that show a happy face but no real ppm number. And most no-name "air quality" cubes on marketplaces. They will give you a green light in a room that is actually at 2,000 ppm, which is worse than no monitor at all.
Put it in a room during a normal meeting, watch the number climb, and read it against the scale. Under 1000 is fine. Above that, the air is getting dumb.