Air Quality Index (AQI) Calculator
Turn a pollutant reading into the US EPA Air Quality Index. Enter a PM2.5, PM10, ozone, carbon monoxide, sulfur dioxide or nitrogen dioxide concentration to get the AQI number, its color category and what it means for your health, or go the other way from an AQI to a concentration.
Last updated:
0 to 500
EPA units; leave any pollutant blank to skip it
How to use the Air Quality Index (AQI) Calculator
- Choose a mode: one pollutant to AQI, several pollutants for the overall AQI, or AQI back to a concentration.
- Pick the pollutant and averaging time that match your reading, for example a 24-hour PM2.5 average.
- Enter the concentration and its unit. Gas readings in µg/m³ or ppb are converted automatically.
- Press Calculate AQI to see the index, its color category and health message.
- Check the breakpoint table to see how close you are to the next category.
EPA AQI breakpoints (May 2024)
Concentrations are averaged over the period shown. Values are truncated before use: PM2.5 to 0.1 µg/m³, PM10 to 1 µg/m³, ozone to 0.001 ppm, CO to 0.1 ppm, SO₂ and NO₂ to 1 ppb.
| AQI | Category | PM2.5 24-hr (µg/m³) | PM10 24-hr (µg/m³) | O₃ 8-hr (ppm) | O₃ 1-hr (ppm) | CO 8-hr (ppm) | SO₂ 1-hr (ppb) | NO₂ 1-hr (ppb) |
|---|---|---|---|---|---|---|---|---|
| 0-50 | Good (green) | 0.0-9.0 | 0-54 | 0.000-0.054 | - | 0.0-4.4 | 0-35 | 0-53 |
| 51-100 | Moderate (yellow) | 9.1-35.4 | 55-154 | 0.055-0.070 | - | 4.5-9.4 | 36-75 | 54-100 |
| 101-150 | Unhealthy for Sensitive Groups (orange) | 35.5-55.4 | 155-254 | 0.071-0.085 | 0.125-0.164 | 9.5-12.4 | 76-185 | 101-360 |
| 151-200 | Unhealthy (red) | 55.5-125.4 | 255-354 | 0.086-0.105 | 0.165-0.204 | 12.5-15.4 | 186-304 | 361-649 |
| 201-300 | Very Unhealthy (purple) | 125.5-225.4 | 355-424 | 0.106-0.200 | 0.205-0.404 | 15.5-30.4 | 305-604* | 650-1249 |
| 301-500 | Hazardous (maroon) | 225.5-325.4 | 425-604 | - | 0.405-0.604 | 30.5-50.4 | 605-1004* | 1250-2049 |
* For AQI values above 200, SO₂ uses 24-hour averages. Source: US EPA Technical Assistance Document for the Reporting of Daily Air Quality, 2024.
Worked example
A 24-hour PM2.5 average of 35.0 µg/m³ falls in the 9.1-35.4 band (AQI 51-100). AQI = (100 − 51) / (35.4 − 9.1) × (35.0 − 9.1) + 51 = 49 / 26.3 × 25.9 + 51 = 99.3, which rounds to 99: Moderate, just one step below the orange category.
Protecting yourself on poor air days
- Plan outdoor activity for times when the AQI is lower. Ozone usually peaks in the afternoon, while smoke and particles can stay high all day.
- Take it easier. Shorter and less intense outdoor activity means you breathe in less pollution.
- Clean indoor air with a portable HEPA air cleaner and keep windows closed during smoke events.
- Know if you are sensitive. People with asthma, heart or lung disease, children, teens and older adults are affected first and should act at lower AQI values.
- Follow local advice. Air quality agencies and health departments issue alerts and forecasts that take local conditions into account.
Frequently asked questions
How is the AQI calculated?
The EPA uses a piecewise linear formula: AQI = (I_hi − I_lo) / (C_hi − C_lo) × (C − C_lo) + I_lo, where C is the pollutant concentration and C_lo, C_hi, I_lo and I_hi are the breakpoints of the band that contains it. The concentration is first truncated to the reporting precision and the result is rounded to the nearest whole number.
What PM2.5 level is a good AQI?
Under the 2024 EPA scale, a 24-hour PM2.5 average from 0 to 9.0 µg/m³ is Good (AQI 0-50), 9.1 to 35.4 is Moderate (51-100) and 35.5 to 55.4 is Unhealthy for Sensitive Groups (101-150). Above 55.4 µg/m³ the air is Unhealthy for everyone.
What do the AQI colors mean?
Green (0-50) is Good, yellow (51-100) Moderate, orange (101-150) Unhealthy for Sensitive Groups, red (151-200) Unhealthy, purple (201-300) Very Unhealthy and maroon (301-500) Hazardous.
Which number is the overall AQI?
The overall AQI for a place and time is the highest of the individual pollutant indexes. The pollutant with that highest value is reported as the main or responsible pollutant.
Why does my monitor show a different AQI?
Official AQI values use 24-hour averages for particles and 8-hour averages for ozone and CO. Air quality apps often show a NowCast value based on the last few hours, and low-cost sensors may need a correction factor, so a single short reading can give a different number.
How do I convert ppb to µg/m³?
At 25 °C and 1 atmosphere, µg/m³ = ppb × molecular weight ÷ 24.45. For example, 50 ppb of NO₂ (46.01 g/mol) is about 94 µg/m³. The calculator does this for O₃, CO, SO₂ and NO₂.
When is 1-hour ozone used?
Ozone is normally reported from 8-hour averages. The 1-hour ozone scale starts at AQI 101 and is used when it gives a more protective value, and above AQI 300 where the 8-hour scale stops.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.