What Is Dew Point and How Is It Calculated?
Dew point is the temperature air must be cooled to, at constant pressure, for water vapor to condense into liquid dew. Unlike relative humidity, dew point is an absolute measure of moisture in the air, which is why meteorologists rely on it as the standard gauge of how muggy conditions actually feel.
This calculator uses the Magnus formula (also called the August-Roche-Magnus approximation), the method used by the National Weather Service and nearly every consumer weather station: α = (17.625 × T) / (243.04 + T) + ln(RH/100), then Td = 243.04 × α / (17.625 − α), where T is air temperature in °C and RH is relative humidity as a percentage. The formula is accurate to within about 0.35°C for temperatures between -40°C and 60°C.
According to the National Weather Service, dew points at or below 55°F (13°C) feel dry and comfortable, while dew points in the 60s°F start to feel sticky and anything above 70°F (21°C) is considered oppressively humid by most people.
How to Use This Dew Point Calculator
Enter the air temperature and relative humidity and the dew point updates instantly. Here's what each field means:
- Air Temperature: The actual thermometer reading, in Fahrenheit or Celsius. Switch units with the toggle.
- Relative Humidity: The percentage of moisture in the air relative to what it could hold at the current temperature — check a local weather report or hygrometer for a live reading.
This calculator works in both Fahrenheit and Celsius, converting internally to Celsius for the Magnus formula and back to your selected units for display, so results are accurate anywhere in the world. It also reports the "dew point spread" — the gap between air temperature and dew point — which pilots and gardeners use to estimate fog and frost risk: a spread under about 5°F (3°C) at night signals a high chance of fog or frost forming.
What Your Dew Point Result Means
The higher the dew point, the more moisture is actually in the air, regardless of what the relative humidity percentage reads. This comfort scale is derived from National Weather Service guidance on how dew point correlates with perceived mugginess.
| Category | Dew Point Range (°F) | How It Feels |
|---|---|---|
| Dry | Below 50°F | Dry, even crisp air |
| Very Comfortable | 50°F to 54°F | Ideal humidity for most people |
| Comfortable | 55°F to 59°F | Pleasant for the vast majority |
| Somewhat Humid | 60°F to 64°F | Noticeable moisture, still tolerable |
| Somewhat Uncomfortable | 65°F to 69°F | Sticky, muggy for most people |
| Very Humid | 70°F to 74°F | Heavy, oppressive air |
| Oppressive | 75°F and above | Extremely uncomfortable, saturated air |
For example, a summer afternoon at 90°F (32°C) with 75% relative humidity produces a dew point of roughly 80°F (27°C) — solidly in the "Oppressive" range, even though the relative humidity number alone wouldn't reveal how uncomfortable it will feel.
Frequently Asked Questions
What is a good dew point for outdoor comfort?
According to National Weather Service guidance, a dew point at or below 55°F (13°C) feels dry and comfortable to most people, while dew points from 55°F to 60°F (13°C to 16°C) still feel pleasant. Once dew point climbs past 65°F (18°C), most people describe the air as sticky to oppressive.
How accurate is this dew point calculator?
This calculator implements the Magnus (August-Roche-Magnus) formula, which the National Weather Service and most consumer weather stations use, and is accurate to within about 0.35°C across the -40°C to 60°C range. Results assume standard sea-level atmospheric pressure.
What is the difference between dew point and relative humidity?
Relative humidity measures moisture as a percentage of what the air could hold at its current temperature, so it rises and falls with temperature even when the actual moisture content is unchanged. Dew point is an absolute measure of moisture and stays constant as temperature changes, which is why meteorologists consider it the more reliable comfort indicator.
How do I lower the dew point indoors?
Run a dehumidifier or air conditioner, improve ventilation in humid rooms like bathrooms and kitchens, and fix any sources of excess moisture such as leaks or standing water. Check our Heat Index Calculator to see how that same humidity affects perceived outdoor temperature.
