AQI Calculator (US EPA & India NAQI)
Pollutant readings to AQI, category and health advice — US EPA or India CPCB.
Sub-index of each pollutant
Health advice
How it was worked out
ppb ↔ µg/m³ converter
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Breakpoint tables
US AQI (EPA)
| Category (AQI) | PM2.5 24-h µg/m³ | PM10 24-h µg/m³ | O₃ 8-h ppm | O₃ 1-h ppm | CO 8-h ppm | SO₂ 1-h ppb | SO₂ 24-h ppb | NO₂ 1-h ppb |
|---|---|---|---|---|---|---|---|---|
| Good (0–50) | 0.0–9.0 | 0–54 | 0.000–0.054 | — | 0.0–4.4 | 0–35 | — | 0–53 |
| Moderate (51–100) | 9.1–35.4 | 55–154 | 0.055–0.070 | — | 4.5–9.4 | 36–75 | — | 54–100 |
| Unhealthy for Sensitive Groups (101–150) | 35.5–55.4 | 155–254 | 0.071–0.085 | 0.125–0.164 | 9.5–12.4 | 76–185 | — | 101–360 |
| Unhealthy (151–200) | 55.5–125.4 | 255–354 | 0.086–0.105 | 0.165–0.204 | 12.5–15.4 | 186–304 | — | 361–649 |
| Very Unhealthy (201–300) | 125.5–225.4 | 355–424 | 0.106–0.200 | 0.205–0.404 | 15.5–30.4 | — | 305–604 | 650–1249 |
| Hazardous (301 and above) | 225.5+ | 425+ | — | 0.405+ | 30.5+ | — | 605+ | 1250+ |
8-hour ozone above 0.200 ppm: use the 1-hour value. 1-hour SO₂ from 305 ppb: use the 24-hour average. AQI 500 = PM2.5 325.4 µg/m³, PM10 604 µg/m³, O₃ 1-h 0.604 ppm, CO 50.4 ppm, SO₂ 1004 ppb, NO₂ 2049 ppb.
India National AQI (CPCB)
| Category (AQI) | PM10 µg/m³ | PM2.5 µg/m³ | NO₂ µg/m³ | O₃ 8-h µg/m³ | CO 8-h mg/m³ | SO₂ µg/m³ | NH₃ µg/m³ | Pb µg/m³ |
|---|---|---|---|---|---|---|---|---|
| Good (0–50) | 0–50 | 0–30 | 0–40 | 0–50 | 0–1 | 0–40 | 0–200 | 0–0.5 |
| Satisfactory (51–100) | 50–100 | 30–60 | 40–80 | 50–100 | 1–2 | 40–80 | 200–400 | 0.5–1 |
| Moderate (101–200) | 100–250 | 60–90 | 80–180 | 100–168 | 2–10 | 80–380 | 400–800 | 1–2 |
| Poor (201–300) | 250–350 | 90–120 | 180–280 | 168–208 | 10–17 | 380–800 | 800–1200 | 2–3 |
| Very Poor (301–400) | 350–430 | 120–250 | 280–400 | 208–748 | 17–34 | 800–1600 | 1200–1800 | 3–3.5 |
| Severe (401–500) | 430+ | 250+ | 400+ | 748+ | 34+ | 1600+ | 1800+ | 3.5+ |
24-hour averages; O₃ and CO 8-hour. Each band starts where the one below ends, as in CPCB’s calculator.
For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
About the AQI Calculator (US EPA & India NAQI)
An Air Quality Index turns measured pollutant concentrations into one number with a colour and plain-language advice. This calculator works out the US AQI exactly as the US Environmental Protection Agency specifies in its Technical Assistance Document (EPA-403/B-26-003) — including the stricter PM2.5 breakpoints introduced in 2024, the 8-hour and 1-hour ozone rules, the SO₂ rule and the PM NowCast for hourly data — and India’s National AQI as defined by the Central Pollution Control Board (CPCB) for eight pollutants: PM10, PM2.5, NO₂, O₃, CO, SO₂, NH₃ and lead.
For every pollutant you enter you get its sub-index, the breakpoints used and the arithmetic, then the overall AQI (the highest sub-index), its category and colour, and the official health messages: EPA’s cautionary statements and sensitive groups, or CPCB’s health statement. Gas readings can be typed in ppb, ppm, µg/m³ or mg/m³ — they are converted at 25 °C and 1 atm, or at the temperature and pressure you set. Everything runs in your browser.
How to use it
- Choose US AQI, India AQI or NowCast (hourly PM2.5 or PM10 data).
- Type the concentrations you have and leave the rest empty. Pick the unit next to each gas if your data are not in the standard’s unit.
- Use daily values: 24-hour averages for particles (and, in India, NO₂, SO₂, NH₃ and lead), the day’s highest 8-hour average for ozone and CO, and for the US the day’s highest 1-hour value for NO₂ and SO₂.
- Read the AQI, category and main pollutant, the sub-index of each pollutant and the health advice. Open “How it was worked out” to see each calculation.
- For hourly readings, switch to NowCast and paste the last 12 hourly averages, oldest first.
- Copy the summary or download the sub-indices as CSV.
Examples
0.07853333 ppm
Truncated to 0.078 ppm → AQI 126, Unhealthy for Sensitive Groups
(150 − 101) ÷ (0.085 − 0.071) × (0.078 − 0.071) + 101 = 125.5, rounded to 126.
O₃ 0.078 ppm, PM2.5 35.9 µg/m³, CO 8.4 ppm
Sub-indices 126, 102 and 90 → AQI 126, main pollutant ozone
8-hour 0.078 ppm, 1-hour 0.162 ppm
AQI 147 (the higher, 1-hour index)
Equation 1 gives 147.49, which rounds to 147 — the same as AirNow’s own concentration calculator. The worked example in the TAD prints 148.
PM10 121, PM2.5 34, NO₂ 8, O₃ 57, NH₃ 34 µg/m³
Sub-indices 114, 56, 10, 57 and 8 → AQI 114, Moderate, main pollutant PM10
PM2.5 24-hour average 100 µg/m³
US AQI 182 (Unhealthy) · India AQI 233 (Poor)
The scales use different breakpoints and category names, so their numbers are not interchangeable.
Hourly PM2.5 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 µg/m³ (oldest first)
Weight factor 0.5 → NowCast 55.0 µg/m³ → AQI 149
A plain 12-hour average (33.0 µg/m³) would give 96 — the NowCast follows the latest hours.
40 ppb NO₂ at 25 °C and 101.325 kPa
75.2 µg/m³ (1 ppb NO₂ = 1.880 µg/m³)
Common uses
- Checking what a reading from a CPCB station, AirNow, a government bulletin or a lab report means for health.
- Converting a sensor’s ppb or µg/m³ readings so they can be compared with the breakpoints.
- Comparing the US and Indian scales for the same air — useful when apps disagree.
- Environmental science, public-health and engineering coursework on air quality indices.
- Turning a day of hourly PM readings into the current NowCast AQI.
How a sub-index is calculated
Both systems draw a straight line between two breakpoints: I = (I_Hi − I_Lo) ÷ (BP_Hi − BP_Lo) × (C − BP_Lo) + I_Lo, where BP are the concentration breakpoints around the concentration C and I the index values that belong to them. The AQI is the highest sub-index; that pollutant is the “main” or “responsible” pollutant.
- US (EPA): first truncate — ozone to 0.001 ppm, PM2.5 to 0.1 µg/m³, PM10 to 1 µg/m³, CO to 0.1 ppm, SO₂ and NO₂ to 1 ppb — then apply the equation and round to the nearest whole number. Above 500 the slope of the 301–500 band continues; everything above 300 is Hazardous.
- India (CPCB): each band runs from the top of the band below (PM2.5 30 → 60 µg/m³ for index 50 → 100), as in CPCB’s AQI calculator; the whole-number part is reported, which reproduces CPCB’s published examples (PM2.5 of 31, 45 and 60 µg/m³ give 51, 75 and 100). Above the top breakpoint the Very Poor slope continues; the scale itself ends at 500.
US AQI breakpoints (EPA)
- PM2.5, 24-hour (µg/m³): 0–9.0 Good · 9.1–35.4 Moderate · 35.5–55.4 Unhealthy for Sensitive Groups · 55.5–125.4 Unhealthy · 125.5–225.4 Very Unhealthy · 225.5 and above Hazardous (500 = 325.4). The 2024 update lowered the top of “Good” from 12.0 to 9.0.
- PM10, 24-hour (µg/m³): 0–54 · 55–154 · 155–254 · 255–354 · 355–424 · 425+ (500 = 604).
- Ozone, 8-hour (ppm): 0–0.054 · 0.055–0.070 · 0.071–0.085 · 0.086–0.105 · 0.106–0.200; above 0.200 only the 1-hour value (0.125–0.164 · 0.165–0.204 · 0.205–0.404 · 0.405+, 500 = 0.604) sets the AQI. The 1-hour value counts only from 0.125 ppm, and the higher ozone index is reported.
- CO, 8-hour (ppm): 0–4.4 · 4.5–9.4 · 9.5–12.4 · 12.5–15.4 · 15.5–30.4 · 30.5+ (500 = 50.4).
- SO₂ (ppb): the day’s highest 1-hour value 0–35 · 36–75 · 76–185 · 186–304; from 305 ppb the 24-hour average decides (305–604 → 201–300, 605+ → 301+, 500 = 1004). If the 1-hour value is 305 or more but the 24-hour average is below 305, the index is exactly 200.
- NO₂, 1-hour (ppb): 0–53 · 54–100 · 101–360 · 361–649 · 650–1249 · 1250+ (500 = 2049).
India National AQI breakpoints (CPCB)
Categories: Good 0–50, Satisfactory 51–100, Moderate 101–200, Poor 201–300, Very Poor 301–400, Severe 401–500. Upper concentration of each band from Good to Very Poor (µg/m³; CO mg/m³; 24-hour averages, 8-hour for O₃ and CO):
- PM10: 50 · 100 · 250 · 350 · 430
- PM2.5: 30 · 60 · 90 · 120 · 250
- NO₂: 40 · 80 · 180 · 280 · 400
- O₃: 50 · 100 · 168 · 208 · 748
- CO: 1.0 · 2.0 · 10 · 17 · 34
- SO₂: 40 · 80 · 380 · 800 · 1600
- NH₃: 200 · 400 · 800 · 1200 · 1800
- Pb: 0.5 · 1.0 · 2.0 · 3.0 · 3.5
CPCB calculates the overall AQI only when at least three pollutants are available, one of them PM2.5 or PM10, and needs at least 16 hours of data for a 24-hour average. With fewer pollutants this calculator shows the highest sub-index and says so.
CPCB’s table marks the two highest ozone bands (209–748 and above 748 µg/m³) as one-hour values used for calculation only; like CPCB’s own calculator spreadsheet, this tool applies them to the 8-hour value you enter.
The NowCast
The AQI is defined for daily values, so EPA does not allow plugging a single hourly reading into it. For real-time reports AirNow uses the NowCast: take the last 12 hourly averages, find the minimum and maximum, and set the weight factor w = 1 − (max − min) ÷ max, but never below 0.5. The NowCast is Σ wⁱ cᵢ ÷ Σ wⁱ, where i is how many hours ago the value was measured (0 for the current hour). Steady air gives w near 1 — close to a 12-hour average; changing air gives w = 0.5 — close to a 3-hour average. At least 2 of the 3 most recent hours must have data. PM2.5 and PM10 use the same method. Ozone has its own NowCast (since 2019), based on two weeks of 1-hour and 8-hour data; it is not included here.
ppb, ppm, µg/m³ and mg/m³
ppb and ppm are mixing ratios (parts per billion or million by volume); µg/m³ and mg/m³ are mass per volume. They are linked by the molar volume of air, Vm = RT ÷ P: µg/m³ = ppb × M ÷ Vm, with M the gas’s molar mass. At 25 °C and 101.325 kPa (the reference conditions of US air-quality data, 40 CFR 50.3) Vm = 24.465 L/mol, so 1 ppb is 1.880 µg/m³ of NO₂, 1.962 µg/m³ of O₃, 2.618 µg/m³ of SO₂ and 0.696 µg/m³ of NH₃, and 1 ppm of CO is 1.145 mg/m³. Molar masses come from the IUPAC abridged standard atomic weights (H 1.008, C 12.011, N 14.007, O 15.999, S 32.06). Particles (PM2.5, PM10) and lead are measured only by mass.
Sources
- US EPA, Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI), EPA-403/B-26-003: breakpoints (Table 6), Equation 1, truncation, SO₂ and ozone rules, colours (Tables 1–3), sensitive groups (Table 4), cautionary statements (Table 5), the NowCast and its answer on real-time sensor data.
- US EPA, Final Updates to the Air Quality Index (AQI) for Particulate Matter (fact sheet): the previous and new PM2.5 breakpoints.
- AirNow, AQI calculator: concentration to AQI — used to cross-check rounding.
- US Code of Federal Regulations, 40 CFR 50.3: reference conditions for gases, 25 °C and 760 mm Hg.
- Central Pollution Control Board, National Air Quality Index: the report National Air Quality Index (Control of Urban Pollution Series CUPS/82/2014-15; breakpoints Table 3.11, health statements Table 3.12, colours, guidelines for Very Poor and Severe days in section 3.4), the notes “About NAQI” and “How AQI is calculated” (with the PM2.5 examples 31 → 51, 45 → 75 and 60 → 100) and the CPCB AQI calculator spreadsheet.
- IUPAC Commission on Isotopic Abundances and Atomic Weights, abridged standard atomic weights.
Limitations
- This is a calculator, not a live air-quality feed: it does not fetch readings. Use official sources such as CPCB’s AQI portal or AirNow for current data.
- An AQI describes daily averages. A single hourly or one-minute reading (for example from a home sensor) is not an AQI: EPA does not advise applying AQI values to real-time sensor data, especially one-minute readings. Use the NowCast for hourly data.
- Low-cost PM sensors often read differently from regulatory monitors, especially in humid air; their numbers may need a correction before they mean the same thing.
- The ozone NowCast (two weeks of data and a fitted relationship) is not implemented.
- Health messages are the agencies’ general statements. They do not replace advice from a doctor, and people with asthma, heart or lung disease should follow their own care plan.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How do I calculate the AQI from PM2.5?
Find the band that contains the 24-hour average and interpolate. For the US AQI and 12 µg/m³: the band is 9.1–35.4 µg/m³ for 51–100, so AQI = (100 − 51) ÷ (35.4 − 9.1) × (12.0 − 9.1) + 51 = 56.4, rounded to 56 (Moderate). On India’s scale 12 µg/m³ is in 0–30 for 0–50, giving 20 (Good).
Why do two apps show different AQI numbers for the same city?
They may use different scales (the US AQI and India’s National AQI have different breakpoints and categories), different averaging (a NowCast or a single hour instead of a 24-hour average), different monitors or sensors, or a different main pollutant. The same 24-hour PM2.5 of 100 µg/m³ is 182 on the US scale and 233 on India’s.
What changed for PM2.5 in 2024?
EPA tightened the annual PM2.5 standard to 9.0 µg/m³ in February 2024 and changed the AQI breakpoints with it: “Good” now ends at 9.0 µg/m³ instead of 12.0, Unhealthy runs 55.5–125.4 and Very Unhealthy 125.5–225.4, and Hazardous starts at 225.5 µg/m³. The technical document this calculator follows (EPA-403/B-26-003) keeps these values.
What is the NowCast?
EPA’s way to report current air quality from hourly data. It weights the last 12 hours, giving the most recent hours more weight when the air is changing fast, so the AQI responds to smoke or traffic peaks within a few hours.
Which pollutants does India’s AQI include?
Eight with short-term (up to 24-hour) national standards: PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃ and lead. CPCB needs at least three of them, including PM2.5 or PM10, to report an overall AQI.
How do I convert ppb to µg/m³?
Multiply by the molar mass and divide by the molar volume: µg/m³ = ppb × M ÷ 24.465 at 25 °C and 1 atm. For NO₂ (46.005 g/mol), 40 ppb × 46.005 ÷ 24.465 = 75.2 µg/m³. The converter in the tool does this for any temperature and pressure.
Is an AQI under 50 always safe?
It means the pollutants are below the levels the agency considers a concern for short-term exposure. Long-term exposure to particles can still affect health, and very sensitive people may react at lower levels — follow your doctor’s advice if you have a heart or lung condition.