AQI Calculator (Air Quality Index)
Convert a PM2.5, PM10, ozone, carbon monoxide or nitrogen dioxide concentration into the US EPA Air Quality Index (2024 breakpoints) — with the AQI category, color band and health advice.
How it works
AQI = (I_hi − I_lo) ÷ (C_hi − C_lo) × (C − C_lo) + I_lo, rounded to the nearest integer · C = concentration truncated (PM2.5 and CO to 0.1, ozone to 0.001 ppm, PM10 and NO₂ to whole numbers) · C_lo, C_hi = the breakpoints that contain C; I_lo, I_hi = the matching AQI values (EPA 2024 table)
The Air Quality Index is the US Environmental Protection Agency scale (also used, with local variants, by many other countries and by apps such as AirNow) that turns a pollutant concentration into a number from 0 to 500. Each pollutant has its own breakpoints, and the AQI is interpolated linearly inside the band that contains the concentration. In 2024 the EPA lowered the PM2.5 "Good" limit from 12.0 to 9.0 µg/m³ after strengthening the annual standard. The overall AQI for a place is the highest of the individual pollutant AQIs. Official reports use the stated averaging periods; hourly sensor readings are only an approximation (the NowCast method).
Worked example
A 24-hour PM2.5 average of 35 µg/m³ falls in the 9.1–35.4 µg/m³ band (AQI 51–100). AQI = (100 − 51) ÷ (35.4 − 9.1) × (35.0 − 9.1) + 51 = 99.3, rounded to 99 — Moderate (yellow): unusually sensitive people should consider shorter, less intense outdoor activity.
Sources: US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI), 2024 · AirNow — AQI basics
Frequently asked questions
What is a good AQI?
0–50 is Good and 51–100 Moderate. Above 100 the air is Unhealthy for sensitive groups, above 150 Unhealthy for everyone, above 200 Very unhealthy and above 300 Hazardous.
How do I convert PM2.5 to AQI?
Find the EPA band containing your 24-hour PM2.5 value and interpolate: for example 0–9.0 µg/m³ maps to AQI 0–50 and 9.1–35.4 µg/m³ to 51–100.
Why did PM2.5 AQI values change in 2024?
The EPA tightened the annual PM2.5 standard to 9 µg/m³ in February 2024 and updated the AQI breakpoints, so the same concentration can now give a higher AQI: 12 µg/m³ used to be AQI 50 (Good) and is now 56 (Moderate).
Is the US AQI the same as the European or Indian index?
No. The EU European Air Quality Index, India’s National AQI and others use different breakpoints and scales, so the same air can get different numbers. This calculator uses the US EPA scale.
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This calculator is for information only and does not replace official air-quality alerts or medical advice.