Air Quality Index Calculator

TL;DR: To calculate the Air Quality Index (AQI), input your PM2.5 concentration (in µg/m³) into the calculator, which then applies the US EPA standard breakpoint formula — \( I = \frac{I_{hi} - I_{lo}}{C_{hi} - C_{lo}} \times (C - C_{lo}) + I_{lo} \) — to instantly output both the AQI number and its corresponding category (Good, Moderate, Unhealthy, etc.).

What Is the Air Quality Index Calculator?

The Air Quality Index (AQI) is a standardized numerical scale used by environmental agencies worldwide to communicate how polluted the air currently is, or how polluted it is forecast to become. Rather than presenting complex raw data on particulate matter, the AQI translates pollutant concentrations into a single, easy-to-understand number from 0 to 500. The AQI Calculator on this page focuses specifically on PM2.5 — particulate matter with a diameter of 2.5 micrometers or less, which is among the most harmful pollutants because it can penetrate deep into the lungs and bloodstream.

This tool is essential for a broad range of users: health-conscious individuals checking daily air quality before outdoor exercise, parents deciding whether to keep children indoors during wildfire season, employers managing outdoor worker safety, and researchers or hobbyists who collect their own sensor data. For anyone who owns a low-cost PM2.5 sensor (like a Plantower or SDS011) or who receives raw data from local monitoring stations, this calculator removes the guesswork. Instead of cross-referencing complicated EPA tables or writing your own formulas, you simply enter one number and receive two immediate results: the AQI value and its health category.

In real-world terms, the AQI is also the backbone of public health alerts. When your local news says "code orange" or "code red," they are referencing the same categories this calculator uses. By understanding how the calculation works, you gain the ability to critically evaluate whether the air is safe for your specific sensitivity level, not just what the generic broadcast tells you.

How to Use the Calculator

Using this Air Quality Index Calculator is straightforward and requires only a single input. Follow these steps to get your results instantly:

1. Locate your PM2.5 concentration value. This is your raw data point. You can find it from a home sensor, a local air quality monitoring website (like AirNow or PurpleAir), or a government environmental agency report. Ensure the value is in micrograms per cubic meter (µg/m³).
2. Enter the PM2.5 value into the input field. Type the number you recorded. The calculator accepts decimal values (e.g., 35.4) for greater accuracy. Do not include the unit symbol (µg/m³) in the input box — numbers only.
3. Click the 'Calculate' button. The tool processes your input through the standard EPA breakpoint algorithm.
4. Review your two outputs. The first output is the numerical AQI Index (a whole number, usually no decimals). The second output is the Air Quality Category (e.g., Good, Moderate, Unhealthy for Sensitive Groups).

Formula and Calculation Method

The AQI is not a simple linear function. Instead, it uses a piecewise linear function based on predefined breakpoints established by the U.S. Environmental Protection Agency (EPA). Each pollutant (PM2.5, PM10, Ozone, etc.) has its own specific set of breakpoints. This calculator uses the 2012 EPA breakpoints for PM2.5, which are widely adopted as the international standard.

The general formula for converting a pollutant concentration C to an AQI value I is:

\[ I = \frac{(I_{hi} - I_{lo})}{(C_{hi} - C_{lo})} \times (C - C_{lo}) + I_{lo} \]

Where:

• C = the PM2.5 concentration you entered (in µg/m³).
• C_lo = the lower concentration breakpoint for the range that C falls into.
• C_hi = the upper concentration breakpoint for that range.
• I_lo = the AQI value corresponding to C_lo.
• I_hi = the AQI value corresponding to C_hi.

The breakpoints for PM2.5 (24-hour average) are defined in the table below. You must first find which range your concentration falls into, then apply the formula.

Worked Example: Let’s calculate the AQI for a PM2.5 concentration of 45.0 µg/m³.

First, identify the range: 45.0 falls between 35.5 and 55.4. Therefore, C_lo = 35.5, C_hi = 55.4, I_lo = 101, I_hi = 150.

Plug into the formula:

\[ I = \frac{(150 - 101)}{(55.4 - 35.5)} \times (45.0 - 35.5) + 101 \]

\[ I = \frac{49}{19.9} \times 9.5 + 101 \]

\[ I = 2.462 \times 9.5 + 101 \]

\[ I = 23.39 + 101 \]

\[ I = 124.39 \]

Rounded to the nearest whole number, the AQI is 124. This falls into the 'Unhealthy for Sensitive Groups' category (101–150).

Practical Examples

Here are three realistic scenarios showing how the same formula produces different results and what those results mean for your daily life.

Notice how the AQI scales non-linearly. A jump from 8.5 to 22.0 µg/m³ (13.5 increase) pushes the AQI from 35 to 68 (33 points). But a jump from 55 to 180 µg/m³ (125 increase) pushes the AQI from 151 to 232 (81 points). The perceived health impact accelerates as the concentration rises.

Tips for Accurate Results

To get the most reliable AQI output from this calculator, pay close attention to the following technical details. Incorrect inputs are the primary source of misleading outputs.

• Verify the averaging period. The EPA breakpoints used here are for the 24-hour average PM2.5 concentration. If you are using data from a real-time sensor that updates every minute, you will get a fluctuating AQI that may spike dramatically. You should average your PM2.5 readings over at least 12–24 hours for a stable, accurate AQI that matches official reporting standards.
• Check your units strictly. The calculator expects µg/m³ (micrograms per cubic meter). If your sensor outputs in mg/m³ (milligrams per cubic meter), you must multiply by 1000 (e.g., 0.045 mg/m³ = 45 µg/m³) before entering it. If your sensor outputs in particles per cubic meter (particle counts), you cannot use this calculator — you first need a conversion factor specific to your sensor's calibration.
• Do not round intermediate values. As demonstrated in the worked example, rounding the fraction (49/19.9 = 2.46) before multiplying will slightly skew the final result. Let the calculator handle the full precision internally. When you perform the calculation manually, carry at least four decimal places until the final step.
• Verify the range of validity. This formula is only valid for PM2.5 values between 0.0 and 500.4 µg/m³. If you enter a value above 500.4, the calculation becomes mathematically undefined (the EPA does not define breakpoints above 500). Attempting to extrapolate beyond this range produces a meaningless number. Extremely high readings (e.g., 800 µg/m³) should be reported as "Beyond Index" or simply "Hazardous +" but not assigned a specific numeric AQI.
• Note the breakpoint versions. This calculator uses the 2012 EPA breakpoints. Some international agencies (e.g., Europe's AQI or India's AQI) have different breakpoints for PM2.5. If you compare your result with a local app, they may differ by up to 20–30 points because they use a different standard, not because the math is wrong.

Frequently Asked Questions

Why does my PM2.5 sensor show a different AQI than the government's website?

There are three primary reasons for this discrepancy. First, averaging time: Government monitoring stations report a 24-hour or 12-hour running average. Your home sensor might display a real-time (1-hour or 10-minute) average, which is naturally more volatile. Second, location: Air quality can vary significantly over just a few hundred meters. The government sensor is usually placed at a rooftop or a strategic location, not near your kitchen vent or the street directly outside your window. Third, correction factors: Many low-cost sensors (like PurpleAir using Plantower sensors) read slightly higher than federal reference monitors. Some normalization or RH-correction is often applied. You can use this calculator on both numbers to see the difference, but for official health decisions, always rely on the EPA or your local environmental agency's reported AQI, not a raw home-sensor reading.

What is the highest possible AQI number, and what does it mean?

The highest defined AQI number on the EPA scale is 500. To reach 500, the PM2.5 concentration must be at least 500.4 µg/m³. There is no numeric AQI above 500. When concentrations exceed this threshold, agencies typically label the condition as "Beyond Index" or "Hazardous +". This extreme level implies that the air is severely hazardous to all people, including healthy adults, and represents life-threatening conditions. For context, a typical city during a severe wildfire smoke event might see PM2.5 values of 250–350 µg/m³ (AQI 300–400). A value exceeding 500 µg/m³ is exceptionally rare and usually only occurs in catastrophic events like a massive dust storm or near a severe industrial fire.

Can I use this AQI calculator for other pollutants like PM10 or ozone?

No, you cannot. The formula structure is the same, but the breakpoint tables are entirely different for each pollutant. PM10 (coarse particles), ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and carbon monoxide (CO) each have their own unique concentration ranges that map to the same 0–500 AQI scale. For example, a PM10 concentration of 55 µg/m³ might map to an AQI of about 50, whereas a PM2.5 concentration of 55 µg/m³ maps to an AQI of 151. Using the PM2.5 formula on PM10 data will produce entirely wrong results. If you have a sensor that outputs multiple pollutants, you must use a calculator that supports multi-pollutant selection, or use separate breakpoint tables for each. This specific calculator is strictly optimized for PM2.5 inputs.