Air Quality Index Calculator

How it works

1. 1Enter the 24-hour average PM2.5 concentration in µg/m³.
2. 2The calculator locates the matching EPA breakpoint segment for that concentration.
3. 3Piecewise-linear interpolation within the segment produces an AQI value from 0 to 500 along with the health category.

Use cases

• Check whether outdoor air quality is safe for exercise or outdoor activities.
• Convert raw sensor readings from a personal air quality monitor into the standardised AQI scale.
• Educate students and the public about how pollution levels translate to health risk categories.

Frequently asked questions

How is AQI calculated from a PM2.5 concentration?

The US EPA uses piecewise-linear interpolation: AQI = ((I_High − I_Low) / (C_High − C_Low)) × (C − C_Low) + I_Low, where C is the measured concentration and the breakpoints are defined per pollutant. For example, 35 µg/m³ falls in the 12.1–35.4 µg/m³ segment (AQI 51–100), yielding AQI ≈ 99 — just inside the Moderate category.

Does this tool work for ozone or PM10?

No. Each pollutant has its own breakpoint table. This tool uses only the 24-hour PM2.5 table. For ozone or PM10 you would need the corresponding EPA breakpoints; the interpolation formula itself is the same.

What does an AQI above 500 mean?

The standard EPA AQI scale ends at 500 (≥ 250.5 µg/m³ PM2.5 = Hazardous). Concentrations above 500.4 µg/m³ fall outside the defined scale — this tool displays “Beyond AQI scale” in that case, which can occur during severe wildfire smoke events.