// tool 01 · air quality index
AQI Calculator & Converter
The same lungful of air gets a different score depending on which country is grading it. Enter a concentration and watch it land on four scales at once, including the EPA's 2024 breakpoints, where "Good" PM2.5 now tops out at 9 µg/m³ instead of 12.
PM2.5 · Fine particles · 24-hour mean.
// same air, four scorecards. the indices disagree by design
So what is the AQI?
The Air Quality Index is a translation layer. On its own, a pollutant reading like "35 µg/m³ of PM2.5" means nothing to most people. The AQI converts that physical concentration into a single, unitless number on a fixed scale, usually 0 to 500, where higher always means worse air and the same number carries the same health message no matter which pollutant produced it.
That number maps onto a band of colour-coded health advice. On the US scale, 0–50 is "Good" (green), 51–100 "Moderate" (yellow), 101–150 "Unhealthy for Sensitive Groups" (orange), and it climbs through red, purple and maroon as the air gets genuinely hazardous. The point of the colour, not the digits, is to tell you what to do: open the window, close it, or skip the run.
Two things the index is not. It is not a measurement unit, you can't own "42 AQI" of anything; it is a rescaling of an underlying concentration. And it is not universal: the US, EU, UK and others each drew their own breakpoints, so the same air earns different scores depending on who is grading it. That is exactly what the converter above lets you see.
How it works
The US AQI uses piecewise-linear interpolation between breakpoints. The concentration is first truncated (PM2.5 to one decimal), the band it falls in is found, and the index is read off the line between that band's endpoints:
The EU EAQI doesn't interpolate at all: a concentration is classified directly into one of six bands. The UK DAQI is a separate 1 to 10 index. And the WHO column simply shows how many times over the 2021 guideline you are. Because the systems use different breakpoints, averaging periods and even units (the EU/UK use µg/m³ for gases, the US uses ppb/ppm), the same air can read "Moderate" in one place and "Poor" in another. That disagreement is the point.
Note: gas conversions assume 25 °C and 1 atm (µg/m³ = ppb · MW ÷ 24.45). Real reporting agencies apply local temperature and pressure corrections, so treat the EU/UK gas bands here as close approximations.
Frequently asked questions
Why does the UK say 'Low' while the EU says 'Poor' for the same air?+
Because they are different indices with different breakpoints, not different measurements of the air. The UK DAQI is a 1 to 10 health-advice scale, while the EU EAQI uses its own six bands with tighter cut-offs for PM2.5. A reading near 27 µg/m³ can land in band 3 (Low) on the UK scale and in the Poor band on the EU scale at the same time. Neither is wrong; they were designed by different agencies for different purposes.
What changed in the EPA's 2024 PM2.5 breakpoints?+
In 2024 the EPA lowered the annual PM2.5 standard and updated the AQI breakpoints. The 'Good' band now tops out at 9.0 µg/m³ instead of the old 12.0 µg/m³, so a concentration that used to read as Good can now read as Moderate. This calculator uses the 2024 breakpoints.
How is the US AQI actually calculated?+
The US AQI uses piecewise-linear interpolation between published breakpoints. The concentration is first truncated (PM2.5 to one decimal place), the band it falls into is found, and the index is interpolated with I = (Ihi - Ilo) / (Chi - Clo) times (C - Clo) plus Ilo. The result is rounded to a whole number from 0 to 500.
Which air-quality standard should I trust?+
It depends on what you want. National indices like the US AQI, EU EAQI and UK DAQI translate pollution into colour-coded advice for the public in that region. The WHO 2021 guidelines are health-based targets, not an index, and they are stricter than most national standards. If your goal is long-term health, compare against the WHO guideline; if you want the official local advisory, use your country's index.
Why do the gas readings need a unit conversion?+
The US AQI defines its gas breakpoints in ppb or ppm, while the EU and UK bands are defined in µg/m³. To compare them, gases are converted using the ideal-gas molar volume at 25 °C and 1 atm: µg/m³ = ppb times molecular weight divided by 24.45. Real agencies apply local temperature and pressure corrections, so treat the converted gas bands as close approximations.
Sources
- US EPA: Technical Assistance Document for the Reporting of Daily Air Quality (AQI)
- US EPA: 2024 PM NAAQS Air Quality Index fact sheet (updated PM2.5 breakpoints)
- European Environment Agency: European Air Quality Index
- UK Defra: Daily Air Quality Index (DAQI) bands and definitions
- WHO: 2021 Global Air Quality Guidelines
- US EPA AirNow: AQI Basics
Know the number before you need the index.
An index is a label on a measurement. Partycle gives you the measurement, live PM2.5 wherever you are, so the AQI is yours to read, not a forecast for your whole city.