Relative Humidity vs Heat Index
Both calculators describe the same muggy air, but they answer different questions. The relative humidity calculator reports how close the air is to saturation: the percent of the maximum water vapor the air could hold at its current temperature. The heat index calculator reports how hot the air feels: an NWS apparent temperature in degrees that combines heat and humidity. The relative humidity page itself points to the heat index calculator for hot-weather stress, and the heat index page accepts relative humidity as an input — so the two tools chain naturally rather than compete.
What each calculator does
The Relative Humidity Calculator takes air temperature and dew point in Celsius or Fahrenheit. Dew point is the temperature at which the current vapor would saturate the air, so the calculator applies the Magnus-Tetens saturation vapor pressure relationship to both values, divides actual by saturation vapor pressure, multiplies by 100, and clamps the display between 0 and 100 percent. It rejects a dew point above air temperature. The result is rounded to two decimals and labeled comfortable only from 30 through 60 percent. In the worked example, 24 °C air with a 15 °C dew point gives 57.15 percent. Because the same dew point produces lower relative humidity in warmer air, the metric changes with daytime heating even when moisture content stays constant.
The Heat Index Calculator takes air temperature and either relative humidity or dew point. It follows the official NWS branches: a simple formula below the 80 °F decision average, the nine-term Rothfusz regression at or above it, and adjustments for very low (below 13 percent) and very high (above 85 percent) humidity. The result is rounded to the nearest whole degree Fahrenheit. In the worked example, 90 °F air at 60 percent relative humidity gives 100 °F. The page notes the index assumes shaded, light-wind conditions, and the heat index equation page documents the procedure’s official applicability limits.
Side-by-side comparison
| Feature | Relative Humidity Calculator | Heat Index Calculator |
|---|---|---|
| Question | How close is the air to saturation? | How hot does it feel? |
| Inputs | Air temperature and dew point | Air temperature and relative humidity or dew point |
| Formula | Magnus-Tetens vapor pressure ratio | NWS simple formula / Rothfusz regression with humidity adjustments |
| Output | Percent, two decimals | Whole degrees Fahrenheit |
| Comfort band | 30–60 percent labeled comfortable | Degrees, with NWS interpretation for risk |
| Typical use | Condensation, mold, wood, archives, greenhouses | Heat stress planning in shaded, light-wind conditions |
| Chaining | Result can feed the heat index’s humidity input | Accepts the dew point directly, skipping the RH step |
When to use which
Use the relative humidity calculator when your question is about moisture management — whether a basement, crawl space, greenhouse, or archive is at risk of condensation or mold, or why a window fogs before a room feels wet. Its percent-saturation view is the one that responds to the temperature of surfaces and room air, and the 30-to-60 percent band is the practical indoor target.
Use the heat index calculator when your question is about heat stress — how a humid 90 °F afternoon actually feels, or how to interpret a forecast’s “feels like” number. It is the tool for the human comfort and risk side of the same air, within the official shaded, light-wind assumptions. The two are best used together: run the heat index for how the day feels, and the relative humidity calculator for what that same moisture means for surfaces and structures. For the moisture content that stays stable across the day, the dew point calculator is the complementary tool, and either page’s guidance should be treated as educational rather than emergency or health advice.
Where to start
- Relative Humidity Calculator — percent saturation from air temperature and dew point, with a 30–60 percent comfort band.
- Heat Index Calculator — NWS apparent temperature from temperature and relative humidity or dew point.