EPA particle NowCast calculator
PM2.5 & PM10 NowCast Calculator
Turn the latest 12 hourly particle measurements into an EPA-weighted NowCast concentration and current AQI sub-index. The result reacts faster when smoke or dust conditions change quickly.
Weight the latest 12 particle readings
Enter the current hourly concentration first. Leave unavailable hours blank; the EPA completeness rule still requires at least two of the latest three hours.
PM2.5 NowCast AQI
- NowCast concentration
- 32.1 µg/m³
- Weight factor
- 0.500
- Simple average
- 22.3 µg/m³
- Valid hours
- 12 of 12
U.S. AQI scale
A factor near 1 means stable conditions. Fast-changing concentrations lower the factor toward 0.5, giving recent hours more influence. This calculator does not apply NowCast to ozone.
How the EPA PM NowCast weighting works
- The calculator needs at least two valid measurements among the current hour and previous two hours; missing older hours may be left blank.
- The weight factor is one minus the range divided by the maximum concentration, with an EPA floor of 0.5 and ceiling of 1.
- The current hour has weight 1; each older valid hour is multiplied by another power of the weight factor before the weighted mean is calculated.
- The NowCast concentration is truncated using the matching PM2.5 or PM10 convention, then converted with the current U.S. AQI breakpoints.
Air quality tools for the decision in front of you
Start with the number or question you already have. Every tool leads to a concrete calculation, comparison, or action plan instead of another generic article.
PurpleAir EPA correction calculator
Correct a PurpleAir CF=1 PM2.5 reading for humidity, including extreme-smoke conditions.
U.S. multi-pollutant AQI calculator
Calculate AQI sub-indices for ozone, particles, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
Indoor PM2.5 calculator
Estimate steady-state indoor PM2.5 from outdoor air, room volume, infiltration, sources, and purifier CADR.
Trust Signals
Keep hourly timestamps aligned and preserve missing values
Review policy
August 8, 2026. We update supporting pages when calculator logic, live data behavior, or source framing changes.
Primary sources
AQI uses EPA / AirNow breakpoint logic. Cigarette-equivalent framing follows Berkeley Earth’s PM2.5 interpretation.
Live data layer
City pages use EPA AirNow where supported, with cached fallbacks and planning baselines if the API fails or coverage is unavailable.
PM NowCast questions
Why not use a simple 12-hour average?
NowCast weights newer hours more heavily when concentrations are changing quickly, while stable conditions receive nearly equal weights.
Can an hourly value be zero?
Yes. Zero is a valid measurement. Leave a field blank only when the measurement is missing.
Can I calculate NowCast with fewer than 12 readings?
Yes, if at least two of the latest three hours are valid. The available valid hours are weighted according to their age.
Does this method apply to ozone?
No. This calculator implements the particle NowCast method for PM2.5 and PM10 only.