01 / Meet the dew point
What is a dew point calculator?
A dew point calculator estimates the temperature at which moist air becomes saturated when cooled without changing its water-vapor content, at approximately constant pressure.
Dew point helps explain fog, dew, condensation on windows, and why some warm days feel noticeably more humid than others.
Unlike relative humidity, which depends strongly on the current air temperature, dew point provides a useful indication of actual atmospheric moisture. At comparable pressure, a higher dew point generally means more water vapor in the air.
UtilityGlen's free online calculator uses air temperature and relative humidity to estimate dew point in Celsius and Fahrenheit, along with a temperature-to-dew-point difference and general moisture comfort information.
Instant Dew Point Results
Enter temperature and relative humidity to calculate dew point immediately. Results update as you adjust your measurements.
Celsius & Fahrenheit
Switch between °C and °F, with automatic conversion and a convenient result in the alternate temperature unit.
Interactive Humidity Controls
Adjust temperature and humidity sliders to explore how changing conditions affect dew point and atmospheric moisture.
Moisture Comfort Indicator
Explore an easy-to-read moisture comfort scale to understand the difference between dry, moderate, and humid air.
Condensation Insights
View the temperature-to-dew-point difference and learn how cooling surfaces can lead to condensation or dew formation.
Private Browser Calculations
Dew point calculations run locally in JavaScript. No weather API, account, or server-side calculation is required.
03 / The condensation mystery
Why do droplets appear on a cold glass?
Imagine taking a chilled drink outside on a warm, humid day. Within minutes, its outer surface may become covered with tiny water droplets.
The droplets generally aren't leaking through the glass. They're coming from water vapor in the surrounding air.
Step 1
Warm, moist air
The air contains invisible water vapor. Its dew point depends on its temperature and moisture content.
Step 2
Contact with a cold surface
Air immediately next to the glass cools. If it reaches saturation, some water vapor can condense.
Step 3
Liquid droplets form
Water vapor changes into liquid water on the glass, creating visible condensation droplets.
Important distinction: Dew point helps identify conditions favorable for condensation, but it does not predict exactly how much water will form. Surface temperature, airflow, pressure, and moisture transfer all matter.
04 / A surprisingly useful number
What is the temperature–dew point spread?
The dew point spread is the difference between the current air temperature and dew point. It is sometimes called the dew point depression.
The simple formula
Dew Point Spread = Air Temperature − Dew Point
Large spread
Air temperature is well above dew point, so the air is relatively far from saturation.
25°C air − 10°C dew point = 15°C spread
Small spread
Air temperature is closer to dew point, indicating conditions nearer saturation.
20°C air − 18°C dew point = 2°C spread
A small spread can be relevant to fog, low clouds, and condensation, but these phenomena also depend on cooling, atmospheric stability, mixing, and other conditions.
05 / Reading the moisture in the air
Dew point comfort chart: dry to very humid
Dew point can be a more intuitive indicator of outdoor mugginess than relative humidity alone. These approximate ranges describe commonly reported comfort impressions, not fixed medical or meteorological safety thresholds.
Dry
Comfortable
Noticeable
Humid
Very humid
Approximate dew point comfort reference table
| Dew point °C |
Dew point °F |
General feeling |
| Below 10°C | Below 50°F | Generally dry-feeling air |
| 10–15°C | 50–59°F | Often comfortable |
| 16–18°C | 61–64°F | Moisture becomes more noticeable |
| 19–21°C | 66–70°F | Often feels humid or muggy |
| 22°C and above | 72°F and above | Very humid or oppressive to many people |
Perceived comfort is subjective and depends on air temperature, sunlight, wind, activity, clothing, and individual factors. High dew point alone does not determine heat-stress risk.
06 / Behind the numbers
How is dew point calculated?
One common method for estimating dew point from air temperature and relative humidity is the Magnus approximation. It models the relationship between temperature and saturation vapor pressure.
Magnus dew point approximation
γ = ln(RH / 100) + aT / (b + T)
Td = bγ / (a − γ)
T — Air temperature
Measured in degrees Celsius when using the Celsius form of the equation.
RH — Relative humidity
Expressed as a percentage from greater than 0 up to 100.
Td — Dew point
The estimated dew point temperature in degrees Celsius.
a and b — Empirical constants
Constants vary between published Magnus formulations and affect the approximation slightly.
The calculator uses a Magnus-type relationship. Results are approximations and depend on the accuracy of the input measurements and the temperature range. At very low temperatures, different vapor-pressure formulations can give different estimates.
07 / Where dew point becomes useful
Everyday places where dew point matters
Dew point isn't just for meteorologists. It can help explain familiar situations around homes, gardens, greenhouses, and outdoor environments.
Condensation on windows
Indoor air touching a sufficiently cold window may cool below its dew point, producing water droplets on the glass. Surface insulation, ventilation, and indoor moisture levels influence the result.
Gardens and morning dew
Grass and plant surfaces can cool overnight. When their surface temperature falls to or below the local dew point, condensation may develop.
Fog and cloud formation
As air approaches saturation, condensation becomes possible. Fog and clouds also depend on lifting, cooling, particles, mixing, and atmospheric structure.
Indoor moisture awareness
Dew point helps explain why moisture collects on cooler building surfaces. It can complement indoor humidity monitoring, although it cannot diagnose dampness or mold problems by itself.