Air Quality Index Calculator
The US EPA air quality index from a PM2.5 concentration — with the category, what it means for health, and what a reading in another scale would say.
The AQI converts a pollutant concentration into a 0–500 scale using published breakpoints, so that 100 always marks the level of the relevant health standard whatever the pollutant.
How the air quality index calculator works
The AQI converts a pollutant concentration into a 0–500 scale using published breakpoints, so that 100 always marks the level of the relevant health standard whatever the pollutant. Between breakpoints the conversion is linear, which is why the index is not proportional to the concentration: doubling PM2.5 from 12 to 24 raises the AQI from 50 to about 76, not to 100.
This page uses the US EPA PM2.5 breakpoints. Other countries use their own scales with different boundaries, so a reading of "80" means different things depending on which index produced it — one reason cross-country air quality comparisons are so often wrong.
Formula: AQI = (I_high − I_low) ÷ (C_high − C_low) × (C − C_low) + I_low
Worked examples
| Inputs | Air quality index | Note |
|---|---|---|
| 35 µg/m³ of PM2.5 | 99 | AQI 99 — moderate, just below the sensitive-group threshold |
| A clean day at 5 µg/m³ | 28 | AQI 28 — good |
| Bushfire smoke at 200 µg/m³ | 275 | AQI 275 — very unhealthy |
FAQFrequently asked questions
What is the air quality index?
A 0–500 scale converting pollutant concentrations into a common number, where 100 marks the level of the health standard.
What PM2.5 level is safe?
The WHO 24-hour guideline is 15 µg/m³. The US EPA standard is higher, which is why a reading can be "moderate" on the AQI and still several times the WHO guideline.
Why is the AQI not proportional to the concentration?
Because it is piecewize linear between breakpoints. Doubling PM2.5 from 12 to 24 raises the AQI from 50 to about 76, not to 100.
Can I compare AQI between countries?
No. Different countries use different breakpoints, so the same air gives different index numbers. Compare concentrations in µg/m³ instead.
What is PM2.5?
Particles under 2.5 micrometers across, small enough to reach deep into the lungs and enter the bloodstream. It is the pollutant most strongly linked to health harm.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- US EPA — Greenhouse Gas Equivalencies Calculator — the official US equivalency factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.