Air Quality Index (AQI) Calculator

Formula used

Most AQI systems calculate a pollutant sub-index with piecewise linear interpolation. The overall index is normally the highest valid pollutant sub-index.

Here, C is the standardized concentration. The low and high values are the concentration and index breakpoints containing that reading.

How to use this calculator

1. Select the national or regional index standard.
2. Choose single, multiple, reverse, or custom calculation mode.
3. Enter measured pollutant concentrations and their original units.
4. Confirm temperature and pressure when converting gas units.
5. Calculate, review the dominant pollutant, then export results.

Example data

Understanding AQI calculations

An air quality index translates pollutant concentrations into a communication scale. A concentration is first matched to its surrounding breakpoints. Linear interpolation then estimates the pollutant sub-index within that interval.

Different authorities use different pollutants, averaging periods, breakpoints, labels, and units. Therefore, two standards can produce different numbers from identical measurements. Always select the standard used by your local environmental authority.

Particulate readings are generally expressed as micrograms per cubic metre. Gas readings may use ppm, ppb, milligrams, or micrograms per cubic metre. Gas mass-to-volume conversion depends on molecular weight, temperature, and pressure.

The controlling or dominant pollutant is the valid pollutant with the greatest sub-index. This value normally becomes the reported overall AQI. Canada’s AQHI differs because it combines the relative risks from ozone, nitrogen dioxide, and PM2.5.

Official daily AQI calculations require specific averaging and data completeness rules. A single sensor value may not represent an official index. Treat consumer sensor results as estimates unless validated by an authorized monitoring network.

Health messages are broad public guidance rather than personal medical advice. People with heart or lung conditions may need stricter precautions. Follow local alerts and advice from qualified healthcare professionals.

Frequently asked questions

What is AQI?

AQI is a dimensionless index that communicates outdoor air pollution levels and related short-term health concern.

Which pollutant determines the overall AQI?

Most systems report the maximum valid pollutant sub-index as the overall AQI.

Why do AQI values differ between countries?

Countries use different breakpoints, averaging periods, pollutants, units, categories, and public-health policies.

Can hourly PM2.5 be entered?

Yes, but a raw hourly reading is not automatically an official daily AQI. Use the required average or approved real-time method.

Does temperature affect ppm conversion?

Yes. Gas concentration conversion between ppm, ppb, and mass units depends on temperature and atmospheric pressure.

Can the calculator diagnose health effects?

No. It provides general public guidance and cannot replace clinical assessment or local emergency instructions.

What happens above the highest breakpoint?

The calculator extrapolates only where the selected standard defines or permits it, otherwise it caps the category and displays a warning.

What is a reverse AQI calculation?

It estimates the concentration associated with a chosen index value under the selected pollutant standard.

What is Canada’s AQHI?

AQHI combines ozone, nitrogen dioxide, and PM2.5 through a health-risk formula instead of simply selecting one pollutant sub-index.

Can I define my own breakpoints?

Yes. Custom mode accepts JSON definitions for pollutants, units, averaging periods, ranges, and category labels.

Are low-cost sensor values official?

Usually not. Sensor quality, placement, humidity, calibration, and averaging can substantially affect reported concentrations.