What is the AQI formula?

AQI = ((IHi − ILo) / (BPHi − BPLo)) × (Cp − BPLo) + ILo, where Cp is the truncated concentration, BPHi/BPLo are the high/low breakpoints for that range, and IHi/ILo are the corresponding AQI index values (e.g., 51 and 100 for the Moderate range).

Why does PM2.5 use different decimal precision than other pollutants?

EPA specifies that PM2.5 concentrations are truncated to one decimal place before applying the formula, while PM10 and CO are truncated to integers, and O3 (8-hr) is truncated to three decimals. This matches the rounding rules in the EPA AQI Technical Assistance Document.

What is the highest AQI value possible?

The AQI scale runs from 0 to 500. A reading of 301–500 is Hazardous, representing air quality that poses serious health risk to the entire population. Values above 500 are sometimes reported in extreme wildfire events but are outside the standard EPA scale.

Why is O3 listed as 8-hour only?

For AQI reporting purposes, EPA uses the 8-hour ozone average. A separate 1-hour O3 standard exists but is used only when the 8-hour value cannot be calculated (e.g., a monitoring period that spans midnight); this calculator implements the standard 8-hour reporting form.

What units does each pollutant use?

PM2.5 and PM10 are in µg/m³; O3, SO2, and NO2 are in ppb (parts per billion); CO is in ppm (parts per million). These match the units in the EPA breakpoint tables.

Air Quality Index Calculator

Name: Air Quality Index Calculator
Availability: InStock
Author: Nham Vu

Enter a pollutant concentration and get the US EPA AQI value with category label using official breakpoint tables.

Pollutant & Concentration

Pollutant

Concentration (µg/m³)

Breakpoint Table — PM2.5 (µg/m³)

BPLo	BPHi	ILo	IHi	Category

Enter a concentration and click Calculate AQI

Summary

Enter a pollutant concentration and get the US EPA AQI value with category label using official breakpoint tables.

How it works

Select the pollutant type (PM2.5, PM10, O3, CO, SO2, or NO2).
Enter the measured average concentration in the appropriate units.
The calculator finds the matching EPA breakpoint range for that concentration.
AQI is computed using the EPA linear interpolation formula: AQI = ((IHi - ILo) / (BPHi - BPLo)) × (Cp - BPLo) + ILo.
The result displays the numeric AQI, the category label, and the applicable breakpoints table.

Use cases

Verify AQI values reported by environmental monitoring stations.
Convert raw sensor readings from low-cost air quality monitors into AQI.
Check air quality compliance thresholds in environmental impact assessments.
Educate students on how EPA translates pollutant concentrations into the AQI scale.
Compare AQI across multiple pollutants measured at the same monitoring site.
Cross-check automated AQI feeds when troubleshooting monitoring equipment.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu