Smog doesn't announce itself. On many summer afternoons, ground-level ozone builds up quietly, and on many winter evenings, wood smoke and tailpipe exhaust settle into fine particles that you can't see or smell. A smog calculator converts the pollution numbers from a monitor into a single score, the Air Quality Index (AQI), so you know whether it's a good day to run, bike, or open the windows.

For US households, that score matters because vehicles, home heating, and wildfire smoke all feed the same pollutants. Using a smog calculator takes under a minute and replaces a confusing readout in micrograms per cubic meter with a clear category. This guide explains the math, walks through four real examples, and shows how to read your result without alarm or false comfort.

What Is a Smog Calculator and Why Does It Matter?

A smog calculator converts measured pollutant concentrations, such as PM2.5 and ground-level ozone, into an AQI value and a health category. It follows one plain-English rule: find where your reading sits inside an EPA concentration band, then scale it to that band's AQI range. Parents, runners, teachers, outdoor workers, and people with asthma or heart disease use it daily. Get it wrong and you may take a child out for a hard workout on an unhealthy day, or stay inside on a day that was perfectly fine.

What You Need Before You Start

Gather these details first so your smog calculator result matches the air outside your door.

The PM2.5 concentration in micrograms per cubic meter (µg/m³), averaged over 24 hours.
The ground-level ozone concentration in parts per million (ppm) or parts per billion (ppb), averaged over 8 hours.
The PM10 concentration in µg/m³ if your monitor reports it.
The averaging period of each reading, because the AQI uses 24-hour PM and 8-hour ozone values.
The unit your monitor reports, such as ppb for ozone or µg/m³ in European data.
The monitor location, which should be the closest official station to where you'll be.

How to Use This Smog Calculator — Step by Step

Follow these steps in order for a reliable AQI.

Find a recent reading from your nearest official monitor, such as AirNow.
Enter the PM2.5 value, using the 24-hour average if you have it.
Enter the ozone value, and check that the unit says ppm or ppb.
Add PM10 if your station reports it.
Leave the values untrimmed, because the calculator truncates them by EPA rules.
Press calculate and read the AQI for each pollutant.
Note the highest value, since that sets the overall AQI and the health category.

The Smog Calculator Formula — Explained Simply

Each pollutant has a set of concentration bands, and each band maps to a range of AQI points. The calculator finds your band, then draws a straight line between its low and high ends to place your reading. The pollutant with the highest AQI sets the overall score.

I = (Ihi − Ilo) / (BPhi − BPlo) × (C − BPlo) + Ilo

I is the AQI for one pollutant, a unitless number from 0 to 500.
C is the pollutant concentration after truncation. PM2.5 runs from 0 to 500 µg/m³, and 8-hour ozone runs from 0.000 to 0.200 ppm.
BPlo is the lowest concentration in your band, in the same unit as C.
BPhi is the highest concentration in your band, in the same unit as C.
Ilo is the AQI value at the bottom of your band, from 0 to 401.
Ihi is the AQI value at the top of your band, from 50 to 500.

This is the AQI equation from the EPA's Technical Assistance Document for the Reporting of Daily Air Quality, with PM2.5 breakpoints updated in 2024. It works because health effects rise gradually with exposure, so a straight-line scale inside each band is a fair approximation. The AQI of 100 lines up with the short-term national standard, which is 35 µg/m³ for 24-hour PM2.5 and 0.070 ppm for 8-hour ozone.

Smog Calculator — Worked Examples

Beginner Example: A Moderate PM2.5 Day in a Midwestern City

A parent checks the air before school pickup. The nearest monitor reports a 24-hour PM2.5 average of 22.4 µg/m³. The Moderate band runs from 9.1 to 35.4 µg/m³ and maps to AQI 51 to 100.

Truncate the concentration: C = 22.4.
Identify the band: BPlo = 9.1, BPhi = 35.4, Ilo = 51, Ihi = 100.
Slope: (100 − 51) / (35.4 − 9.1) = 49 / 26.3 = 1.863.
Distance into the band: 22.4 − 9.1 = 13.3.
AQI = 1.863 × 13.3 + 51 = 75.78, which rounds to 76.

An AQI of 76 falls in the Moderate category. Most people can carry on as usual, but unusually sensitive people may want to take it easier during long, hard outdoor exertion. The parent can keep regular plans and check hourly readings if conditions change.

Professional Example: An Air Quality Analyst on a Wildfire Smoke Day

An analyst at a county health department reports daily conditions. The 24-hour PM2.5 average is 88.6 µg/m³, and the 8-hour ozone value is 0.068 ppm. Her task is the overall AQI.

PM2.5 band: 55.5 to 125.4 µg/m³, AQI 151 to 200.
Slope: (200 − 151) / (125.4 − 55.5) = 49 / 69.9 = 0.701.
PM2.5 AQI: 0.701 × (88.6 − 55.5) + 151 = 0.701 × 33.1 + 151 = 174.2, or 174.
Ozone band: 0.055 to 0.070 ppm, AQI 51 to 100, slope 49 / 0.015 = 3,266.7.
Ozone AQI: 3,266.7 × (0.068 − 0.055) + 51 = 42.5 + 51 = 93.5, or 93.
Overall AQI: the higher of 174 and 93, which is 174.
Drop needed to reach the next category: 88.6 − 55.4 = 33.2 µg/m³, about 37 percent.

An AQI of 174 is Unhealthy, and smoke is the driver. She should issue a public alert advising everyone to limit prolonged outdoor exertion and recommend that sensitive groups stay indoors with filtered air.

Edge Case: Ozone Sitting Exactly on a Category Line

A runner checks an ozone reading of 0.0709 ppm. Official reports truncate ozone to three decimals, so the details matter here.

Truncate, don't round: 0.0709 becomes 0.070 ppm.
Band: Moderate, 0.055 to 0.070 ppm, AQI 51 to 100.
Slope: 49 / 0.015 = 3,266.7.
AQI = 3,266.7 × (0.070 − 0.055) + 51 = 49 + 51 = 100.
Compare with rounding to 0.071: that lands in the Unhealthy for Sensitive Groups band, where AQI = 3,500 × 0 + 101 = 101.

One thousandth of a part per million flips the result from 100 to 101, and that moves the day from Moderate to Unhealthy for Sensitive Groups. Official AQI follows truncation, so the runner should treat 100 as correct. People with asthma should still treat a reading this close to the line with care.

Different Units: A European Ozone Reading Converted for US Use

A traveler in Spain sees an 8-hour ozone value of 160 µg/m³ on a local site. She wants the US AQI. At 25°C and 1 atmosphere, 1 ppb of ozone equals about 1.963 µg/m³.

Convert to ppb: 160 / 1.963 = 81.5 ppb.
Convert to ppm and truncate: 0.0815 becomes 0.081 ppm.
Band: Unhealthy for Sensitive Groups, 0.071 to 0.085 ppm, AQI 101 to 150.
Slope: 49 / 0.014 = 3,500.
AQI = 3,500 × (0.081 − 0.071) + 101 = 35 + 101 = 136.

An AQI of 136 is Unhealthy for Sensitive Groups, and the reading is above the US 8-hour ozone standard of 0.070 ppm. The EU's 180 µg/m³ information threshold uses a one-hour average, so it measures something different. She should limit long outdoor exercise if she has asthma or other lung conditions.

Smog Calculator Results — What the Numbers Mean

Read the AQI by category. A score of 0 to 50 is Good, and air pollution poses little risk. From 51 to 100, it's Moderate, and only unusually sensitive people may notice effects. From 101 to 150, it's Unhealthy for Sensitive Groups, which includes children, older adults, pregnant people, and anyone with asthma, COPD, or heart disease. From 151 to 200, everyone may start to feel effects, and from 201 to 300 the warning applies to the whole population. Anything above 300 is Hazardous, and people should stay indoors.

Most US days land between 30 and 80, so a reading in that range is routine. Wildfire seasons push values well past 150 for days at a time.

The most common misunderstanding is treating the AQI like a percentage. An AQI of 100 doesn't mean air is 100 percent polluted. It marks the short-term national standard, and the scale isn't linear in concentration. Another frequent mistake is averaging pollutants. The overall AQI is the single highest one.

If your result is above 100, shorten hard outdoor workouts, close windows, and run a filter or HVAC on recirculate. If it's above 150, move exercise indoors and wear a well-fitted N95 respirator if you must be outside. Keep an eye on your own symptoms, since sensitivity varies.

How Your Inputs Change the Result

PM2.5 concentration changes the AQI at different speeds depending on the band. The scale is steepest at low concentrations and flattens as pollution grows.

PM2.5 (µg/m³)	AQI	Category
5.0	28	Good
12.0	56	Moderate
35.4	100	Moderate
55.4	150	Unhealthy for Sensitive Groups
88.6	174	Unhealthy
150.0	225	Very Unhealthy

In the Good band, each extra µg/m³ adds 5.6 AQI points. In the Moderate band it adds 1.9, and in the Unhealthy band only 0.7. A drop of 10 µg/m³ matters much more when you're near 20 than when you're near 100.

Ozone is even more sensitive to small changes, because its bands are narrow.

Ozone 8-hr (ppm)	AQI	Category
0.050	46	Good
0.060	67	Moderate
0.070	100	Moderate
0.075	115	Unhealthy for Sensitive Groups
0.085	150	Unhealthy for Sensitive Groups
0.095	174	Unhealthy

A rise of just 0.015 ppm, from 0.060 to 0.075, adds 48 AQI points. That's why ozone forecasts are so tightly watched on hot, sunny afternoons.

The overall score follows a "highest pollutant wins" rule. On the wildfire day in Example 2, PM2.5 scored 174 and ozone scored 93. Cleaning up ozone would do nothing for the overall number, while a 37 percent PM2.5 cut would drop the category.

Averaging time matters too. A one-hour PM2.5 spike of 90 µg/m³ doesn't equal a 24-hour average of 90. If the other 23 hours sit near 10 µg/m³, the 24-hour average is only about 13.3 µg/m³, and the AQI is 58. Always match the averaging period to the standard.

Smog Calculator — Common Mistakes to Avoid

Rounding instead of truncating. The EPA truncates PM2.5 to one decimal and ozone to three. Rounding 35.46 to 35.5 pushes you into the next category, while truncating to 35.4 keeps you at 100.
Using the wrong averaging time. PM2.5 and PM10 use 24-hour averages, and ozone uses 8-hour averages. Feeding a single hourly spike into a daily breakpoint overstates the AQI.
Mixing units. Ozone appears as ppm, ppb, or µg/m³, and a mix-up changes the result by a factor of 1,000 or about 2. Convert at 25°C and 1 atmosphere before you calculate.
Using pre-2024 PM2.5 breakpoints. EPA lowered the top of the Good band from 12.0 to 9.0 µg/m³ in 2024. Old charts will understate your AQI at low concentrations.
Averaging pollutants. The overall AQI is the maximum of the individual scores, not the mean. An average hides the pollutant that matters most.
Assuming indoor air matches outdoor air. Indoor PM2.5 may be lower with the windows closed and a filter running, or higher with gas cooking and wood fires. The calculator reflects outdoor monitor conditions.
Trusting uncorrected low-cost sensors. Consumer sensors often read high during smoke. Apply the EPA's correction for low-cost PM2.5 sensors or compare against an official monitor.
Expecting the AQI to cover every toxic. The AQI doesn't measure air toxics such as benzene or formaldehyde. A Good rating doesn't rule out a local source of those chemicals.

Key Terms Explained

Air Quality Index (AQI)

The AQI is the EPA's 0 to 500 scale that converts pollutant concentrations into a color-coded health category.

PM2.5

PM2.5 is fine particulate matter 2.5 micrometers or smaller, small enough to reach deep into the lungs and enter the bloodstream.

Ground-Level Ozone

Ground-level ozone is a lung-irritating gas formed when nitrogen oxides and volatile organic compounds react in sunlight.

Breakpoint

A breakpoint is a concentration value that separates one AQI category from the next.

National Ambient Air Quality Standards (NAAQS)

The NAAQS are EPA's legal limits for common outdoor pollutants, set to protect public health, and an AQI of 100 matches the short-term standard.

Sensitive Groups

Sensitive groups are people at higher risk from air pollution, including children, older adults, pregnant people, and those with asthma, COPD, or heart disease.

NowCast

NowCast is the EPA method that estimates current hourly AQI by weighting recent readings, giving a faster update than a 24-hour average.

How This Calculator Works — Methodology

This smog calculator applies the AQI equation and breakpoint tables from the EPA's Technical Assistance Document for the Reporting of Daily Air Quality, including the 2024 PM2.5 update. It covers 24-hour PM2.5, 24-hour PM10, and 8-hour ozone. PM2.5 truncates to one decimal, PM10 to a whole number, and ozone to three decimals. AQI results round to the nearest whole number, and the overall AQI equals the highest pollutant score. The calculator assumes your input already matches the right averaging period. It excludes one-hour ozone above 0.200 ppm, sulfur dioxide, nitrogen dioxide, carbon monoxide, and NowCast weighting, because each follows separate rules. Unit conversions assume 25°C and 1 atmosphere. Treat results as an estimate and defer to AirNow for official numbers.

Frequently Asked Questions About Smog Calculator

1. What does a smog calculator actually calculate?

A smog calculator converts pollutant readings, usually PM2.5 and ozone, into an Air Quality Index score and health category. It finds the concentration band your reading falls in, scales it to the AQI range, and reports the highest pollutant as the overall score. The result tells you how clean or polluted the air is right now.

2. How accurate is a smog calculator?

The math is exact, but the result is only as good as your input. Official monitors are accurate to within about 10 percent, while low-cost sensors can read 30 percent or more high in smoke. Distance matters too, because pollution varies block to block. Use the nearest official monitor and the correct averaging period.

3. What is the AQI formula in simple terms?

Find the concentration band your reading falls in, then place it proportionally inside that band's AQI range. For example, 22.4 µg/m³ of PM2.5 sits 13.3 above the band start of 9.1 and gives an AQI of 76. The pollutant with the highest AQI sets the overall score for the day.

4. Is the US AQI different from air quality scales in other countries?

Yes. The US AQI uses its own breakpoints and a 0 to 500 scale, while the European Air Quality Index and other national indexes use different categories and thresholds. Units differ too, since Europe reports ozone in µg/m³ and the US uses ppm. The same air can earn different labels, so convert before you compare.

5. When does a smog calculator not apply?

It doesn't apply to indoor air, to short exposure peaks inside a single hour, or to pollutants it doesn't cover, such as benzene and other air toxics. It also can't reflect a nearby source like a wood stove or a busy loading dock. If you smell smoke or fumes, trust your senses and act, whatever the number says.

6. How can I get a more precise smog reading?

Use the closest official monitor on AirNow and match the averaging period to the pollutant: 24 hours for PM2.5 and 8 hours for ozone. If you use a home sensor, apply the EPA correction for low-cost PM2.5 sensors. Check hourly NowCast values for current conditions, and compare two nearby stations to see local variation.

7. What is a typical smog calculator result?

Most US locations see AQI values between 30 and 80 on a normal day, which means Good or Moderate air. Summer ozone and winter wood smoke can lift urban readings into the 100 to 130 range. Wildfire smoke can push values past 200 for days. Annual averages in big cities usually land around 40 to 60.

8. Should I see a doctor if the AQI is high?

Yes, if you have wheezing, chest tightness, unusual shortness of breath, or heart palpitations during poor air quality, especially with asthma, COPD, or heart disease. Ask your clinician for an action plan before smoke season. Seek emergency care for severe symptoms. For persistent smog problems at home, an industrial hygienist can help.

Related Calculators Worth Bookmarking

Ozone Unit Converter: Use it to switch between ppb, ppm, and µg/m³ before you enter an ozone reading from a foreign or sensor source.
Vehicle Emissions Calculator: Use it to estimate the nitrogen oxides and particles your car adds to local smog each year.
Home Heating Emissions Calculator: Use it to compare wood, oil, gas, and electric heat by the pollution they produce.
Air Purifier Room Size Calculator: Use it to size a HEPA purifier for a room when the AQI rises above 100.
Air Changes per Hour Calculator: Use it to learn how quickly your home exchanges indoor and outdoor air, which affects how smoke gets in.
Carbon Footprint Calculator: Use it to see how the activities that create smog also add to greenhouse gas emissions.
Electric Vehicle Emissions Savings Calculator: Use it to measure how switching to an EV cuts tailpipe pollutants and carbon dioxide.
CO₂ Breathing Emission Calculator: Use it to check whether your indoor air is stale after you've sealed the house against outdoor smog.

References and Further Reading

U.S. Environmental Protection Agency. Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI). U.S. EPA, 2024.
U.S. Environmental Protection Agency. Reconsideration of the National Ambient Air Quality Standards for Particulate Matter. Federal Register, 2024.
U.S. Environmental Protection Agency. National Ambient Air Quality Standards for Ozone. Federal Register, 2015.
World Health Organization. WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide. World Health Organization, 2021.
European Parliament and Council of the European Union. Directive 2008/50/EC on Ambient Air Quality and Cleaner Air for Europe. Official Journal of the European Union, 2008.
Health Effects Institute. State of Global Air 2024. Health Effects Institute, 2024.

Important Disclaimer

This calculator is for informational and educational purposes only. Results are estimates based on the readings you enter and the EPA's published AQI method, and they may differ from official AirNow values. This tool does not constitute medical advice. Always consult a qualified physician or other licensed health professional before making real decisions about your health, especially if you have asthma, heart disease, or another condition.