PM2.5 to AQI Calculator

Convert fine-particle concentration to the US Air Quality Index—or work backwards from AQI to an approximate PM2.5 concentration. Every breakpoint and calculation step remains visible.

2024 US EPA breakpointsBidirectionalNo live-data claim

This is a conversion, not a measurement.Enter a value from a defined data source. The calculator does not detect smoke, retrieve local conditions or issue health guidance.

Calculation direction

PM2.5 → AQI AQI → PM2.5

PM2.5 concentration

µg/m³

Input basisClosest match to the 24-hour PM2.5 breakpoints used here; official AQI reporting involves prescribed monitoring and data handling.

AQI INTERPOLATIONI = (I_hi − I_lo) ÷ (BP_hi − BP_lo) × (C − BP_lo) + I_loPM2.5 concentration is truncated to 0.1 µg/m³ before conversion; AQI is rounded to the nearest integer.

CALCULATED US AQI

100 AQI

Moderate

35.4 µg/m³ PM2.5 · truncated to one decimal place

GoodModerateSensitiveUnhealthyVery unhealthyHazardous

Concentration band9.1–35.4 µg/m³

AQI band51–100

Method statusWithin AQI scale

Use official AirNow or local-authority information for current conditions and protective guidance.

CURRENT BREAKPOINTS

The full conversion table.

These PM2.5 breakpoints took effect with the 2024 US EPA AQI update. Other countries and indices may use different pollutants, averaging periods, breakpoints and category language.

Category	US AQI	PM2.5, 24-hour
Good	0–50	0.0–9.0 µg/m³
Moderate	51–100	9.1–35.4 µg/m³
Unhealthy for Sensitive Groups	101–150	35.5–55.4 µg/m³
Unhealthy	151–200	55.5–125.4 µg/m³
Very Unhealthy	201–300	125.5–225.4 µg/m³
Hazardous	301–500	225.5–325.4 µg/m³

DO NOT CONFUSE THE INPUTS

A sensor number is not automatically an official AQI.

Official AQI values depend on the pollutant, averaging period, monitoring method, quality assurance and reporting procedure. A short-term indoor sensor reading can be converted mathematically, but it should be labelled as a screening equivalent—not an official local AQI.

This page does not determine whether indoor or outdoor air is safe. Follow official public-health and emergency information during smoke events.

REPRODUCIBLE METHOD

From concentration to index.

Find the breakpoint interval containing the truncated concentration, interpolate between its concentration and AQI limits, then round the result to the nearest integer.

Define

Confirm pollutant, unit, location, time and averaging basis.

Truncate

Reduce PM2.5 concentration to one decimal place without rounding.

Interpolate

Apply the EPA equation within the matching breakpoint band.

Label

Report both concentration and category with the input basis.

Sources and limits

The calculator implements the US EPA PM2.5 AQI breakpoints effective May 6, 2024. Values above 325.4 µg/m³ are displayed as beyond the standard 0–500 AQI scale rather than extrapolated as ordinary AQI.

EPA: AQI breakpoints · EPA: 2024 AQI update · AirNow: AQI information

Quick answers

Does this show live wildfire smoke?

No. It converts only the number entered. Use AirNow or the responsible local authority for current conditions.

Why truncate PM2.5 instead of rounding it?

EPA AQI calculation instructions specify truncating PM2.5 concentration to one decimal place before applying the interpolation equation.

Is US AQI used worldwide?

No. Air-quality indices differ among countries. Always identify the index system before comparing values.