Air Quality Index Calculator
Converting a Raw Pollution Reading Into a Public Health Number
A PM2.5 concentration measured in micrograms per cubic meter means little on its own to most people — the EPA's Air Quality Index translates that raw number into a standardized 0–500 scale with clear health categories, the same scale used in air quality apps and news reports nationwide. This calculator applies the EPA's official piecewise linear formula to convert a 24-hour average PM2.5 reading into its AQI value and category.
The Formula
This is the EPA's standard AQI piecewise linear equation. Your PM2.5 concentration is located within one of several published breakpoint ranges, and the formula linearly interpolates between that range's AQI floor and ceiling based on where the concentration falls within its pollutant range.
EPA PM2.5 Breakpoint Table
| PM2.5 range (µg/m³) | AQI range | Category |
|---|---|---|
| 0.0–12.0 | 0–50 | Good |
| 12.1–35.4 | 51–100 | Moderate |
| 35.5–55.4 | 101–150 | Unhealthy for Sensitive Groups |
| 55.5–150.4 | 151–200 | Unhealthy |
| 150.5–250.4 | 201–300 | Very Unhealthy |
| 250.5–500.4 | 301–500 | Hazardous |
Worked Examples
| PM2.5 concentration | AQI | Category |
|---|---|---|
| 10 µg/m³ | 42 | Good |
| 40 µg/m³ | 112 | Unhealthy for Sensitive Groups |
| 100 µg/m³ | 174 | Unhealthy |
Where This Matters
- Deciding on outdoor exercise — people with asthma or respiratory conditions are advised to limit outdoor exertion once AQI enters the Unhealthy for Sensitive Groups range or above.
- Wildfire smoke tracking — PM2.5 from wildfire smoke can spike an area's AQI into Very Unhealthy or Hazardous territory quickly; converting live sensor readings to AQI gives an immediate health context.
- School and event decisions — districts and outdoor event organizers use published AQI categories to decide whether to move activities indoors.
How to Use This Calculator
- Enter the PM2.5 Concentration in µg/m³ (24-hour average).
- Select Calculate to see the AQI value and health category.
Related Calculations
Check the day's UV exposure risk with the UV Index Calculator, or see how humidity affects perceived temperature with the Heat Index Calculator.