AQI Calculator (US EPA)
Convert pollutant concentrations to the US EPA Air Quality Index. Enter PM2.5, PM10, ozone, CO, SO2 or NO2 to get the AQI value and health category instantly.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the AQI Calculator Works
This tool converts a measured pollutant concentration into the US EPA Air Quality Index (AQI) and its health category. Choose a pollutant (PM2.5, PM10, ozone, CO, SO2, or NO2), enter the concentration in the correct units, and it returns the AQI value plus the category from Good to Hazardous.
The Formula
The AQI uses piecewise linear interpolation between official breakpoints:
AQI = (I_Hi − I_Lo) ÷ (BP_Hi − BP_Lo) × (C − BP_Lo) + I_Lo
Where C is the truncated concentration, BP_Lo / BP_Hi are the lower and upper concentration breakpoints of the band that contains C, and I_Lo / I_Hi are the index values at those breakpoints. Each pollutant has its own breakpoint table, and the concentration is truncated to the EPA’s specified decimal places before the formula is applied.
Worked Example
Take a PM2.5 concentration of 35.5 µg/m³ (24-hour average). It falls in the band BP_Lo = 35.5, BP_Hi = 55.4, I_Lo = 101, I_Hi = 150:
- AQI = (150 − 101) ÷ (55.4 − 35.5) × (35.5 − 35.5) + 101
- AQI = (49 ÷ 19.9) × 0 + 101 = 101
- Category: Unhealthy for Sensitive Groups
A reading at the very bottom of a band lands on the band’s lower index limit, so 35.5 µg/m³ maps to exactly AQI 101.
AQI Category Reference
| AQI range | Category | Meaning |
|---|---|---|
| 0-50 | Good | Air quality is satisfactory |
| 51-100 | Moderate | Acceptable; minor risk for sensitive people |
| 101-150 | Unhealthy for Sensitive Groups | Sensitive groups may feel effects |
| 151-200 | Unhealthy | Everyone may begin to feel effects |
| 201-300 | Very Unhealthy | Health alert; serious effects possible |
| 301-500 | Hazardous | Emergency conditions for everyone |
Why AQI Matters
The AQI translates raw pollution numbers into a simple 0-500 scale that signals when to limit outdoor activity, run air purifiers, or keep sensitive people indoors. Because the index is the same nationwide, you can compare today’s air against a forecast or another city at a glance, and use the breakpoint-based math here to verify any reading from a monitor or sensor.
Frequently asked questions
How is the AQI calculated from a concentration?+
The EPA uses piecewise linear interpolation. Find the breakpoint band that contains your truncated concentration, then apply AQI = (IHi − ILo) ÷ (BPHi − BPLo) × (C − BPLo) + ILo, where BP values are concentration limits and I values are the index limits for that band.
What units does each pollutant use?+
PM2.5 and PM10 use micrograms per cubic meter (µg/m³) averaged over 24 hours. Ozone uses parts per billion (ppb) over 8 hours, CO uses parts per million (ppm) over 8 hours, and SO2 and NO2 use ppb over 1 hour. Match the averaging period to the standard for accurate results.
What do the AQI categories mean?+
0-50 is Good, 51-100 Moderate, 101-150 Unhealthy for Sensitive Groups, 151-200 Unhealthy, 201-300 Very Unhealthy, and 301-500 Hazardous. Higher values mean greater health risk, especially for children, older adults and people with heart or lung conditions.
Why is my concentration truncated before calculating?+
The EPA method truncates the input to a set number of decimals (one decimal for PM2.5 and CO, whole numbers for PM10, ozone, SO2 and NO2) before applying the formula. This calculator truncates automatically so your result matches official AQI reporting.
Which pollutant's AQI should I report?+
On any given day, the overall AQI for a location is the highest individual AQI among all monitored pollutants. This tool computes the AQI for one pollutant at a time; compute each pollutant separately and report the largest value as the site AQI.