Indoor Air Quality Score Calculator

This calculator turns five indoor-air readings into one composite score by scoring each parameter on its own 0–100 scale against ASHRAE and EPA reference ranges, then combining them with fixed weights. CO2 scores 100 at or below 600 ppm and linearly declines to 0 at 2,000 ppm, reflecting ASHRAE 62.1's ventilation-adequacy target of roughly 700 ppm above outdoor ambient. PM2.5 scores 100 at 0 µg/m³ and falls linearly to 0 at 50 µg/m³, well above the EPA's current annual NAAQS standard of 9.0 µg/m³ (tightened from 12.0 µg/m³ in EPA's February 2024 reconsideration of the PM standard) — so a "good" PM2.5 sub-score still permits levels above the strict annual health standard, since this scale is tuned for a momentary reading rather than annual compliance. TVOC scores 100 up to 200 µg/m³ and reaches 0 at 3,000 µg/m³.

Humidity and temperature both use a peaked scoring shape rather than a linear ramp — humidity scores 100 within the ASHRAE-recommended 40–50% band and falls off in both directions, and temperature peaks at 21–24°C per ASHRAE 55 comfort guidance, also falling off outside that range. The five sub-scores combine as a weighted average — 30% CO2, 30% PM2.5, 20% TVOC, 10% humidity, 10% temperature — reflecting that health-relevant pollutants outweigh comfort parameters, and the overall score maps to a four-tier rating: Excellent (80+), Good (60–79), Fair (40–59), Poor (below 40). Because this is a weighted composite, a single bad reading can be masked by good scores elsewhere — TVOC carries only a 20% weight, so a room with dangerously high TVOC (a TVOC sub-score of 0) but excellent CO2, PM2.5, humidity, and temperature (100 each) still computes to an overall score of 80, which lands squarely in the "Excellent" tier despite one pollutant being off the charts — so check the individual sub-scores rather than relying on the headline number if any one pollutant is a specific concern. The scoring bands are reasonable interpretations of ASHRAE/EPA guidance rather than a codified formula, so this is a screening tool for spotting problem areas, not a substitute for a certified IAQ assessment or compliance testing.