AQI Calculator

Convert PM2.5, PM10, ozone, CO, and NO2 readings into the U.S.

EPA Air Quality Index using the current breakpoints, including the 2024 PM2.5 update, or look up the concentration behind any AQI value.

Last updatedHow we build & check our tools
ug/m3
ug/m3
ppm
ppm
ppm
ppb
Results will appear here once you enter your values.

How the AQI turns a concentration into one number

The Air Quality Index is a piecewise linear scale. EPA publishes a breakpoint table for each pollutant, and every row pairs a concentration range with an index range. To convert a reading, you find the row it falls in and interpolate: AQI = (Ihi - Ilo) / (Chi - Clo) x (C - Clo) + Ilo, then round to the nearest whole number.

Before any math, the concentration is truncated, not rounded: ozone to three decimals in ppm, PM2.5 and CO to one decimal, and PM10, SO2 and NO2 to whole numbers. EPA's own worked example shows why this matters. An 8-hour ozone average of 0.07853 ppm is cut to 0.078, which lands in the 0.071 to 0.085 row (AQI 101 to 150). The result is 49 / 0.014 x 0.007 + 101 = 125.5, reported as 126.

When several pollutants are measured, each gets its own sub-index and the highest one becomes the reported AQI. In the same EPA example, PM2.5 at 35.9 gives 102 and CO at 8.4 ppm gives 90, so the day is reported as AQI 126 with ozone as the main pollutant. The index is not an average, which is why one bad pollutant can push a whole city into orange.

What changed for PM2.5 in 2024

On February 7, 2024, EPA lowered the annual primary PM2.5 standard from 12.0 to 9.0 micrograms per cubic meter and revised the AQI to match. The top of the Good range moved from 12.0 to 9.0, so a 24-hour average of 12.4 that used to read 52 now reads 57. Many days that older apps called Good are now Moderate.

The upper end changed too. The Unhealthy row now runs from 55.5 to 125.4, Very Unhealthy from 125.5 to 225.4, and Hazardous starts at 225.5 (it used to start at 250.5). EPA also folded the two Hazardous rows into a single 301-and-up category for every pollutant. For extreme smoke above 325.4, EPA now continues the Hazardous slope past 500 instead of capping the scale, so 400 micrograms per cubic meter reads about 649.

  • If your air sensor or older spreadsheet still ends Good at 12.0, its PM2.5 AQI will read low.
  • Ozone, PM10, CO and NO2 kept their lower breakpoints; only their top category was merged.

Why your number may not match AirNow

The breakpoints are defined for specific averaging periods: 24 hours for particles, 8 hours for ozone and CO, and 1 hour for NO2. Hourly maps on AirNow use the NowCast, a weighted average of the last 12 hours for particles that leans on recent hours when levels are changing fast. Plugging a single one-minute sensor reading into the 24-hour table will overstate or understate the official value during a smoke plume that is arriving or clearing.

Low-cost PM2.5 sensors also tend to read high in humid air unless a correction is applied. Use this calculator to understand what a sustained concentration means on the EPA scale, and follow local advisories for decisions about schools, outdoor work, and events.

Frequently Asked Questions

Common questions about the AQI Calculator

Under the breakpoints EPA adopted in 2024, a 24-hour PM2.5 average of 0.0 to 9.0 micrograms per cubic meter is Good (AQI 0 to 50). From 9.1 to 35.4 is Moderate, and 35.5 to 55.4 is Unhealthy for Sensitive Groups.

Sources & References

Technical Assistance Document for the Reporting of Daily Air Quality - the Air Quality Index (AQI)

Table 6 breakpoints, truncation rules, equation 1, and the worked ozone, PM2.5 and CO examples used to validate this calculator.
→Source: U.S. EPA (EPA-403/B-26-003, May 2026)

Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter

February 2024 rule lowering the annual PM2.5 standard to 9.0 micrograms per cubic meter and updating the AQI.
→Source: U.S. EPA

AQI Basics

Official category names, colors, and health messaging for each AQI range.
→Source: AirNow

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