This AQI calculator converts a PM2.5 concentration (µg/m³ — the number most air sensors report) into the US EPA Air Quality Index, with its color category and the health guidance that goes with it.
Breakpoints, Not a Formula from Physics
AQI is a policy scale: the EPA assigns concentration breakpoints per health category, and the index interpolates linearly inside each band — AQI = (I_high − I_low) ÷ (C_high − C_low) × (C − C_low) + I_low. PM2.5 drives most bad-air days in North America (wildfire smoke above all), which is why this calculator is built on it. Part of our Ecology Calculators collection.
Worked Example (Verified)
- PM2.5 = 40 µg/m³ (defaults): inside the 35.5–55.4 band → (150−101) ÷ (55.4−35.5) × (40−35.5) + 101 = AQI 112 — Unhealthy for Sensitive Groups (orange)
- Clean mountain air at 5 µg/m³: AQI ≈ 21, green — the level most weather apps bury because there’s nothing to warn about
- Wildfire smoke day at 180 µg/m³: band 150.5–250.4 → AQI ≈ 230, purple — the 2023 East-Coast smoke events peaked in this territory and beyond
- The scale’s honesty: a doubling of concentration inside a band does not double the AQI slope changes between bands (the 55.5–150.4 band is deliberately wide)
Using AQI Well
- Sensitive groups are broader than they sound: asthma (including “only as a kid”), pregnancy, outdoor workers, anyone training hard — exertion multiplies dose by ventilation rate.
- Indoor PM2.5 without filtration runs 50–80% of outdoor on smoke days; a HEPA cleaner sized to the room is the difference between shelter and slower exposure.
- Other countries scale differently (India’s AQI runs PM2.5 bands to 500 at much higher concentrations; the EU uses its own index) — the same air can wear three different numbers.
- Dust masks ≠ respirators: only fit-tested-style N95/KN95 masks materially cut PM2.5 dose.
Related Tools
- Carbon Footprint Calculator — the emissions side of air
- Wind Chill Calculator — the other outdoor go/no-go number
- Ecology Calculators — the full collection
Frequently Asked Questions
How is AQI calculated from PM2.5?
By linear interpolation inside EPA breakpoint bands: 0–12.0 µg/m³ maps to AQI 0–50 (Good), 12.1–35.4 to 51–100, 35.5–55.4 to 101–150, 55.5–150.4 to 151–200, 150.5–250.4 to 201–300, and 250.5–500.4 to 301–500. PM2.5 of 40 µg/m³ gives AQI 112, as worked on this page.
What AQI is dangerous?
Above 100 (orange) sensitive groups should adapt; above 150 (red) the air is unhealthy for everyone; above 200 (purple) is a health alert where outdoor exertion should stop; above 300 (maroon) is hazardous/emergency territory. There is no perfectly “safe” PM2.5 level epidemiologically — the bands are risk-management, not a cliff edge.
Why is my app showing a different AQI than my sensor?
Three usual suspects: averaging windows (your sensor shows minutes; official AQI uses 24-hour averages or NowCast weighting), different pollutants winning the day (ozone often leads on hot afternoons), and different national scales abroad. The calculator converts your concentration on the US EPA PM2.5 scale directly.
What is PM2.5 exactly?
Particulate matter ≤ 2.5 micrometers across — about 1/30th a hair’s width. Small enough to cross from lungs into bloodstream, which is why it’s the index’s heavyweight: combustion (engines, wood smoke, wildfires) is its dominant source.
Does an air purifier help on high-AQI days?
Yes — a true-HEPA purifier correctly sized to the room (CADR matched to square footage) can hold indoor PM2.5 at a small fraction of outdoor levels with doors and windows closed. Carbon-only or “ionic” units do far less for particles; the filter media is the mechanism.
Is this AQI calculator free?
Yes — free, instant, no sign-up.