Heat index and dew point both involve humidity, but they answer different questions. Dew point reports the actual amount of water vapor in the air — the temperature to which the air would have to cool to become saturated. Heat index converts temperature plus humidity into an apparent “feels like” temperature for hot conditions. One tells you how much moisture is present; the other tells you how that moisture affects perceived heat.
What each calculator does
The dew point calculator takes air temperature and relative humidity and applies the Magnus approximation with constants a = 17.27 and b = 237.7. After converting the temperature to Celsius it computes gamma = aT/(b + T) + ln(RH/100) and then solves Td = b × gamma / (a − gamma). The result is rounded to two decimals and shown in the selected unit plus Celsius. The page pairs the number with comfort rules of thumb — dew points below about 50°F (10°C) feel dry to many people, the low 60s are noticeable, and the low 70s are often described as oppressive — and with condensation checks: a surface at or below the dew point can collect moisture.
The heat index calculator takes air temperature and either relative humidity or dew point, converting a dew point to relative humidity with a separate NWS approximation. It runs the NWS simple formula, averages it with air temperature, and applies the nine-term Rothfusz regression when that average is 80°F or above, with narrow low- and high-humidity adjustments. The result is the apparent temperature rounded to the nearest whole degree Fahrenheit, valid for shaded, light-wind conditions.
Side-by-side
| Heat index calculator | Dew point calculator | |
|---|---|---|
| Inputs | Air temperature and relative humidity (or dew point) | Air temperature and relative humidity |
| Primary output | Apparent temperature in whole degrees Fahrenheit | Dew point rounded to two decimals, in the selected unit plus Celsius |
| What the number means | How hot the air feels in the shade with light wind | The temperature at which the air becomes saturated at constant moisture |
| Behavior through the day | Tracks temperature swings | Tracks the actual water-vapor content, so it changes less as temperature rises and falls |
| Formula basis | NWS simple formula plus Rothfusz regression with adjustments | Magnus approximation (a = 17.27, b = 237.7) |
| Typical uses | Hot-weather comfort and heat-safety context | Mugginess, overnight dew, condensation, building moisture, greenhouse planning |
| Output precision | Whole degrees Fahrenheit | Two decimal places |
When to use which
Use the dew point calculator when your question is about moisture content: why a warm afternoon feels dry or muggy, whether a window or basement wall will condense, or whether leaves will stay wet overnight. Relative humidity rises and falls as air warms and cools, while dew point stays closer to the amount of water vapor present, which is why forecasters use it to describe mugginess.
Use the heat index calculator when you want the combined temperature-and-humidity “feels like” number for hot conditions. It can even accept a dew point as an input and derive relative humidity for you. In hot weather, the heat index is the figure to pair with heat-safety decisions, since heat-illness risk depends on the combined environmental load rather than moisture alone.
Limits and disclaimer
Both pages are educational only and are not medical advice. The dew point is a Magnus approximation, accurate enough for common weather and indoor conditions but not for precision psychrometrics; the form also accepts zero percent humidity even though the logarithm is undefined there, so extremely dry air needs a small positive value. The heat index assumes shade and light wind and is not valid for extreme temperature and humidity combinations outside the underlying data. Comfort descriptions are rules of thumb, not official medical limits, and heat illness depends on exertion, clothing, sun, hydration, and cooling access in addition to any index.