You set a temperature and a humidity. It tells you how comfortable that air would be for most people — and why — by answering two separate questions:
Comfort is a function of both. Air can be the perfect temperature and still be miserable because it's bone-dry or muggy, and "feels-like" numbers like Heat Index can't see that — they're built to measure heat stress, and to them dry air is simply cooler. That's why this tool uses a map instead of a single number.
Temperature runs left→right (the map shows –; outside that it's uniformly too cold or too hot, and the marker pins to the edge with an arrow), humidity bottom→top. The green island is where most people are comfortable. Color tells you the cause when you're outside it: blue = too cold, orange/red = too hot, tan/brown = too dry, purple = too humid. Darker means worse. Drag anywhere on the map to set both values at once.
We use the PMV/PPD model (Fanger), the standard HVAC engineers use worldwide (ISO 7730, ASHRAE 55). It predicts the percentage of people who would say they're uncomfortable at a given condition. ASHRAE calls a space compliant when that's 10% or less.
There is no single comfortable temperature — it depends on clothing. The tool assumes people dress sensibly for indoors (0.5–1.0 clo, i.e. shorts-and-tee up to trousers-and-sweater) and takes the best case, which is how ASHRAE defines its comfort zone. Even so, leaves about 1 in 4 people cold.
At room temperature, humidity changes thermal comfort by only a few percent. At it matters a lot — sweat can't evaporate. At it barely registers. So dryness has to be judged on its own terms, not through a feels-like temperature.
Research on indoor air puts the sweet spot at 40–60% relative humidity. Below ~30% people report dry eyes, skin and airways, and static shocks; the air also lets flu virus survive longer. Above the line, dust mites and mold take off, and the air starts to feel sticky.
On the muggy side, dew point is the better measure — it's the actual amount of moisture in the air, independent of temperature. ASHRAE's upper comfort limit works out to a dew point of about . Meteorologists call + muggy and + oppressive.
These are different models that disagree with each other — that's expected, and it's why all four are shown.