Air Quality Index Calculator: Understand Your Air Quality (Free Tool)
You check the weather app. You see a number like 85 or 150 next to “AQI.” What does it mean? Is it safe to go outside? Should you close your windows?
The Air Quality Index (AQI) is the standard way to measure and communicate air pollution levels. It tells you how clean or polluted your air is and what health risks you might face.
Today, I give you a free Air Quality Index Calculator.
You enter the concentration of a pollutant (like PM2.5). The calculator shows the AQI value and category.
The calculator uses the EPA’s standardized breakpoints for PM2.5 . It works for other pollutants too.
Let me explain what the AQI is and why it matters.
What is the Air Quality Index? (Simple Explanation)
The Air Quality Index (AQI) is a number used by government agencies to communicate how polluted the air is. It was developed by the U.S. Environmental Protection Agency (EPA) .
The higher the AQI value, the greater the level of pollution and the greater the health concern .
The AQI has six color-coded categories :
| AQI Range | Category | Color |
|---|---|---|
| 0-50 | Good | Green |
| 51-100 | Moderate | Yellow |
| 101-150 | Unhealthy for Sensitive Groups | Orange |
| 151-200 | Unhealthy | Red |
| 201-300 | Very Unhealthy | Purple |
| 301-500 | Hazardous | Maroon |
An AQI value of 100 generally corresponds to the national air quality standard for that pollutant . Values below 100 are generally considered satisfactory. Values above 100 are unhealthy for sensitive groups first, then for everyone as they increase.
Why This Air Quality Index Calculator Matters
Here is why you need to understand and calculate the AQI.
Reason 1: Protect your health
Air pollution affects everyone. It is especially harmful to children, the elderly, and people with respiratory conditions . Knowing the AQI helps you decide when to limit outdoor activity.
Reason 2: Understand your environment
The AQI transforms complex air quality data into a single, easy-to-understand number .
Reason 3: Plan outdoor activities
If the AQI is high, you can plan indoor activities or wear a mask.
Reason 4: Compare data
The AQI uses a nationally uniform scale, making it easy to compare air quality across different regions .
Reason 5: Make informed decisions
When AQI values are unhealthy, you can take steps to reduce your exposure .
The AQI Formula
The EPA uses a piecewise linear function to calculate the AQI .
The formula:
I = ((I_Hi - I_Lo) / (BP_Hi - BP_Lo)) * (C - BP_Lo) + I_Lo
Where:
- I = the AQI value
- C = the pollutant concentration
- BP_Hi = the breakpoint that is greater than or equal to C
- BP_Lo = the breakpoint that is less than or equal to C
- I_Hi = the AQI value corresponding to BP_Hi
- I_Lo = the AQI value corresponding to BP_Lo
For PM2.5, the breakpoints are:
| AQI Category | AQI Range | PM2.5 Concentration (µg/m³) |
|---|---|---|
| Good | 0-50 | 0.0 – 9.0 |
| Moderate | 51-100 | 9.1 – 35.4 |
| Unhealthy for Sensitive Groups | 101-150 | 35.5 – 55.4 |
| Unhealthy | 151-200 | 55.5 – 125.4 |
| Very Unhealthy | 201-300 | 125.5 – 225.4 |
| Hazardous | 301-500 | 225.5 – 325.4 |
Breakpoints from EPA AQS reference table .
LIVE Air Quality Index Calculator
Enter a pollutant concentration. The calculator shows the AQI value and category instantly.
💨 Air Quality Index Calculator
🧪 Pollutant
📊 Concentration
💨 Air Quality Index Calculator
Calculate AQI from pollutant concentration
🧪 Pollutant
PM2.5 (µg/m³) PM10 (µg/m³) Carbon Monoxide (ppm) Nitrogen Dioxide (ppb)
📊 Concentration
µg/m³
📊 AIR QUALITY INDEX
0%
💡 Enter a concentration to calculate the AQI.
📐 Based on EPA breakpoints for PM2.5 and other common pollutants .
How to Use This Air Quality Index Calculator
Follow these 3 simple steps.
Step 1: Select the pollutant you want to check (PM2.5, PM10, CO, or NO2)
Step 2: Enter the concentration of that pollutant in the appropriate units
Step 3: Read the AQI value and category
The calculator shows:
- AQI number (0-500)
- Category (Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, Hazardous)
- Color-coded display
- Health advice
What Pollutant Should You Check?
PM2.5 (Fine Particles):
- Particles smaller than 2.5 micrometers
- From combustion, wildfires, industry
- Most common pollutant affecting health
- Measured in µg/m³
PM10 (Coarse Particles):
- Particles smaller than 10 micrometers
- From dust, construction, pollen
- Measured in µg/m³
Carbon Monoxide (CO):
- Colorless, odorless gas from combustion
- From vehicles, heaters, fires
- Measured in parts per million (ppm)
Nitrogen Dioxide (NO2):
- Brownish gas from combustion
- From vehicles, power plants
- Measured in parts per billion (ppb)
Real Examples
Example 1: Good Air Quality
- Pollutant: PM2.5
- Concentration: 8.0 µg/m³
- AQI: 45 (Good)
Example 2: Moderate Air Quality
- Pollutant: PM2.5
- Concentration: 25.0 µg/m³
- AQI: 78 (Moderate)
Example 3: Unhealthy for Sensitive Groups
- Pollutant: PM2.5
- Concentration: 45.0 µg/m³
- AQI: 128 (Unhealthy for Sensitive Groups)
Example 4: Unhealthy
- Pollutant: PM2.5
- Concentration: 80.0 µg/m³
- AQI: 181 (Unhealthy)
The Six AQI Categories
| Category | AQI Range | Color | Health Message |
|---|---|---|---|
| Good | 0-50 | Green | Air quality is satisfactory. Enjoy outdoor activities. |
| Moderate | 51-100 | Yellow | Some people are especially sensitive to air pollution. |
| Unhealthy for Sensitive Groups | 101-150 | Orange | Sensitive groups should limit outdoor activity. Everyone should watch for symptoms. |
| Unhealthy | 151-200 | Red | Everyone should reduce exposure. Limit outdoor activity. |
| Very Unhealthy | 201-300 | Purple | Everyone should stay inside and filter indoor air. |
| Hazardous | 301-500 | Maroon | Everyone should stay inside. Go elsewhere for cleaner air if needed. |
Who Is in a Sensitive Group?
Sensitive groups are more affected by air pollution :
- Children – Their lungs are still developing
- Elderly adults – More susceptible to respiratory effects
- People with asthma or COPD – Air pollution triggers symptoms
- People with heart conditions – Pollution can affect cardiovascular health
- Pregnant women – May affect fetal development
- People who exercise outdoors – Increased breathing rate means more pollutant exposure
How the AQI Is Calculated (Detailed)
The AQI calculation uses breakpoints established by the EPA .
Step 1: Determine which breakpoint range the concentration falls into.
Step 2: Use the linear formula:
I = ((I_Hi - I_Lo) / (BP_Hi - BP_Lo)) * (C - BP_Lo) + I_Lo
Example for PM2.5 concentration of 25.0 µg/m³:
- Breakpoint: 9.1 to 35.4 (Moderate, AQI 51-100)
- I_Hi = 100, I_Lo = 51, BP_Hi = 35.4, BP_Lo = 9.1
- I = ((100 - 51) / (35.4 - 9.1)) * (25.0 - 9.1) + 51
- I = (49 / 26.3) * 15.9 + 51
- I = 1.86 * 15.9 + 51
- I = 29.6 + 51 = 80.6 → 81
The final AQI is 81 (Moderate).
Using the AQI to Make Decisions
| AQI Category | What to Do |
|---|---|
| Good | Go outside. Enjoy physical activity. |
| Moderate | Most people can go outside. Sensitive people should limit prolonged exertion. |
| Unhealthy for Sensitive Groups | Sensitive groups: limit outdoor activity. Everyone else: be cautious. |
| Unhealthy | Everyone: reduce outdoor activity. Move exercise indoors. |
| Very Unhealthy | Avoid outdoor activity. Stay indoors with filtered air. |
| Hazardous | Stay indoors. Use air purifiers. Avoid all outdoor activity. |
Frequently Asked Questions (FAQs)
1. What is a good AQI?
0-50 (Green) is good. Air quality is satisfactory .
2. Is AQI 100 safe?
100 is the threshold. It is considered moderate (yellow). Some people may be sensitive .
3. What AQI is unhealthy for everyone?
151-200 (Red) is unhealthy for everyone. Everyone should reduce exposure .
4. What is the highest AQI?
500 is the maximum on the standard scale. It corresponds to hazardous conditions.
5. How often is AQI updated?
Most agencies update AQI hourly or daily.
6. Can I calculate AQI for multiple pollutants?
The overall AQI is the highest sub-index of all pollutants measured at a station .
7. What is PM2.5?
Particulate matter smaller than 2.5 micrometers. It can penetrate deep into the lungs .
8. Why is PM2.5 important?
PM2.5 is the most harmful common pollutant. It is linked to respiratory and cardiovascular problems.
9. What is the difference between AQI and air quality?
AQI is a standardized index. Air quality is the actual condition. The AQI translates the condition into an easy-to-understand number .
10. Where can I find current AQI?
Check AirNow.gov or your local air quality agency's website.
Common Mistakes
Mistake #1: Confusing units
PM2.5 is in µg/m³. CO is in ppm. NO2 is in ppb. Use the correct unit.
Mistake #2: Ignoring the AQI
Even if the AQI is moderate, sensitive people should take precautions.
Mistake #3: Only checking one pollutant
The overall AQI is the highest of all pollutants. Check all relevant pollutants .
Mistake #4: Assuming AQI is the same everywhere
Air quality varies by location. Check the AQI for your specific area.
Final Thoughts
The Air Quality Index helps you protect your health. It tells you when to go outside and when to stay in.
My Air Quality Index Calculator gives you:
- AQI value from pollutant concentration
- Color-coded category
- Health advice for each category
Bookmark this page. Use it to understand your local air quality.
The next time you see an AQI number, you will know exactly what it means.
Disclaimer: This is an educational tool based on EPA breakpoints. For real-time AQI, check official sources like AirNow.gov.
External Links (Authority Backlinks):
- Wikipedia – Air quality index{:target="_blank" rel="noopener noreferrer"}
- Wikipedia – Particulate pollution{:target="_blank" rel="noopener noreferrer"}
- EPA – Air Quality Index{:target="_blank" rel="noopener noreferrer"}
- EPA – AQI Breakpoints{:target="_blank" rel="noopener noreferrer"}
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