AQI Calculator
Enter a pollutant concentration and get the corresponding U.S. EPA Air Quality Index value, category, color, and health guidance. Uses the official EPA breakpoints, including the February 2024 PM2.5 NAAQS revision.
Convert pollutant to AQI
Calculated AQI
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EPA AQI categories
| AQI range | Category | What it means |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory; little or no risk. |
| 51–100 | Moderate | Acceptable; unusually sensitive people may have symptoms. |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive groups may experience health effects. |
| 151–200 | Unhealthy | Everyone may begin to experience health effects. |
| 201–300 | Very Unhealthy | Health alert; everyone may experience more serious effects. |
| 301+ | Hazardous | Health warning of emergency conditions. |
How the AQI is calculated
The U.S. EPA Air Quality Index is a piecewise linear interpolation. For each pollutant, the EPA defines a set of "breakpoints" that map a concentration range to an AQI range. The formula is:
AQI = ((I_high − I_low) / (BP_high − BP_low)) × (C − BP_low) + I_low
where C is the measured concentration, BP_high and BP_low are the breakpoint concentrations around C, and I_high / I_low are the corresponding AQI bounds. The pollutant with the highest sub-index becomes the reported AQI for that location.
Reference: EPA Technical Assistance Document "Reporting of Daily Air Quality – the Air Quality Index (AQI)", EPA-454/B-24-002, May 2024. PM2.5 breakpoints reflect the February 2024 NAAQS revision (annual standard tightened from 12 to 9 µg/m³).
Important caveats
- This is the sub-index for one pollutant only. The AQI you see in an app like Smog Report is the maximum across all monitored pollutants at the nearest station.
- Real AQI uses time-averaged data. PM2.5 uses a 24-hour rolling average (or NowCast for live displays); ozone uses an 8-hour rolling average; SO₂ and 1-hour ozone use 1-hour values.
- Single readings can mislead. A 1-minute PM2.5 spike from a passing truck is not a 24-hour exposure. Use AirNow's NowCast or a regulatory monitor's reported AQI for decision-making.
Related guides
Understanding AQI
The full explainer on what the 0-to-500 number means, where the breakpoints come from, and how to interpret it for everyday decisions.
Read guide →PM2.5 Explained
The pollutant that drives most AQI readings in the modern era — what it is, why it matters, and how the 2024 standard change affected the numbers.
Read guide →Ground-Level Ozone
Why ozone spikes on hot afternoons, how it forms, why N95s don't help, and the diurnal pattern that changes morning vs. evening exposure.
Read guide →AQI Health Effects
What each AQI category actually does to the body, who counts as a sensitive group, and the practical thresholds for outdoor activity.
Read guide →Get real-time AQI with Smog Report
This calculator converts a single reading. Smog Report pulls live AQI worldwide for your nearest monitor, with widgets, Live Activities, and Siri voice control.
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