Dew Point Calculator — Free Online Dew Point Temperature Tool

How to Use the Dew Point Calculator

1. Enter the air temperature: Type the current air temperature in degrees Celsius into the Temperature field. If you only have the temperature in Fahrenheit, convert it first by subtracting 32 and multiplying by 5/9. The calculator accepts any temperature from -40°C to 60°C, covering virtually all inhabited climates on Earth. Use a reading from a shaded, ventilated thermometer for the most accurate result.
2. Enter the relative humidity: Input the current relative humidity as a percentage. Relative humidity measures how close the air is to being fully saturated with water vapor. You can find this reading from weather apps, local weather service reports, or a personal hygrometer. The calculator accepts values from 1% to 100%. At 100% relative humidity, the dew point equals the air temperature and condensation is occurring.
3. Review the dew point temperature: The calculator instantly shows the dew point in both Celsius and Fahrenheit. A higher dew point means more moisture in the air and greater discomfort. The dew point is always equal to or lower than the air temperature because the air cannot hold more moisture than its saturation point at the current temperature.
4. Check comfort level and condensation risk: The results include a qualitative comfort assessment ranging from Dry and Comfortable to Dangerously Humid, plus a condensation risk indicator. High condensation risk means fog, dew, or moisture problems are likely. Use these assessments for outdoor planning, HVAC decisions, or evaluating whether a dehumidifier is needed in your home.

All results update in real time. Adjust the humidity slider to see how moisture content affects the dew point and comfort level at any given temperature.

Dew Point Formula (Magnus Approximation)

Variables Explained

• T (Temperature): The air temperature in degrees Celsius. This represents the current thermal state of the atmosphere. The Magnus formula is accurate for temperatures between -45°C and 60°C, which covers the full range of temperatures experienced in populated areas worldwide.
• RH (Relative Humidity): The percentage of water vapor currently in the air relative to the maximum amount the air can hold at the given temperature. At 100%, the air is fully saturated and the dew point equals the air temperature. At lower humidity values, the dew point drops below the air temperature.
• a (Magnus constant): A dimensionless empirical constant equal to 17.27. This value was determined through laboratory experiments relating temperature to saturation vapor pressure and has been widely validated across meteorological applications since the early 20th century.
• b (Magnus constant): An empirical constant equal to 237.7°C. Together with the constant a, it defines the shape of the saturation vapor pressure curve as a function of temperature. These constants provide accuracy within 0.4°C across the normal atmospheric temperature range.
• ln (Natural Logarithm): The natural logarithm function accounts for the nonlinear relationship between relative humidity and the saturation vapor pressure. This mathematical transformation ensures the formula correctly maps humidity percentages to dew point temperatures.

Step-by-Step Example

Suppose the air temperature is 30°C with 55% relative humidity:

1. Compute the first part of alpha: (17.27 x 30) / (237.7 + 30) = 518.1 / 267.7 = 1.9355
2. Compute ln(55 / 100) = ln(0.55) = -0.5978
3. Alpha = 1.9355 + (-0.5978) = 1.3377
4. Dew point = (237.7 x 1.3377) / (17.27 - 1.3377) = 317.97 / 15.93 = 19.96°C
5. Convert to Fahrenheit: 19.96 x 9/5 + 32 = 67.9°F

At 30°C with 55% humidity, the dew point is approximately 20°C (68°F). This falls in the Oppressive Humidity category, meaning most people will feel noticeably uncomfortable outdoors, especially during physical activity. The spread between temperature and dew point is 10°C, indicating moderate condensation risk.

Practical Examples

Example 1: Sarah's Basement Moisture Problem

Sarah notices moisture forming on her basement walls during summer. The basement temperature is 18°C (64°F) and the outdoor air entering through vents is 28°C with 70% humidity. She calculates the outdoor dew point:

• Outdoor temperature: 28°C
• Outdoor humidity: 70%
• Calculated dew point: approximately 22.1°C (71.8°F)
• Condensation risk: High

The outdoor dew point of 22°C exceeds the basement wall temperature of 18°C, which means warm humid air entering the basement will condense on the cool walls. Sarah installs a dehumidifier to lower the indoor humidity and closes the basement vents during hot humid days to prevent the moisture-laden outdoor air from reaching the cool basement surfaces.

Example 2: Carlos's Morning Run Decision

Carlos runs every morning in Miami and uses the dew point to decide his pace and hydration strategy. The morning reading shows 26°C (79°F) with 85% humidity:

• Temperature: 26°C
• Humidity: 85%
• Calculated dew point: approximately 23.2°C (73.8°F)
• Comfort level: Very oppressive

With a dew point above 23°C, Carlos knows his body will struggle to cool itself through sweating. He slows his pace by 30 seconds per mile, carries extra water, and plans a shorter route of 4 miles instead of his usual 7. He also moves his run to 5:30 am instead of 6:30 am to take advantage of slightly cooler pre-dawn conditions.

Example 3: Tom's HVAC System Evaluation

Tom is an HVAC technician evaluating a client's air conditioning system in Atlanta. The indoor conditions are 24°C with 65% humidity, and the client complains the house feels clammy despite the AC running:

• Indoor temperature: 24°C
• Indoor humidity: 65%
• Calculated dew point: approximately 17.1°C (62.8°F)
• Comfort level: Humid, starting to feel sticky

The indoor dew point of 17°C confirms the client's discomfort. For optimal comfort, the indoor dew point should be between 7°C and 13°C. Tom discovers the AC unit is oversized, cooling the air quickly but cycling off before removing enough moisture. He recommends a variable-speed unit or adding a whole-house dehumidifier to bring the indoor dew point down to the comfortable range.

Example 4: Emily's Aviation Weather Check

Emily is a private pilot preparing for a morning departure from a small airport. The METAR report shows 15°C temperature with 90% humidity. She calculates the dew point to assess fog risk:

• Temperature: 15°C
• Humidity: 90%
• Calculated dew point: approximately 13.4°C (56.1°F)
• Temperature-dew point spread: 1.6°C
• Condensation risk: High

With only a 1.6°C spread between temperature and dew point, fog formation is highly likely if the temperature drops even slightly during the early morning hours. Emily delays her departure by 2 hours to allow morning warming to increase the temperature-dew point spread to a safer 5°C or more, reducing the risk of encountering fog during takeoff and initial climb.

Dew Point Comfort Reference Table

Comfort thresholds vary by individual. Acclimated residents of tropical climates may tolerate higher dew points than those from dry regions.

Tips and Complete Guide

Using Dew Point for Weather Prediction

Dew point is one of the most practical tools for amateur weather prediction. When the temperature-dew point spread (the difference between air temperature and dew point) is small, typically less than 3°C, fog, low clouds, or drizzle become likely. As the spread increases to 5°C or more, conditions are generally clear. Rising dew points over several days indicate an approaching warm front or tropical air mass, which often brings rain. Falling dew points signal drier air moving in, typically behind a cold front. Pilots, sailors, and outdoor event planners regularly monitor dew point trends to anticipate weather changes hours before they arrive.

Dew Point and Home Maintenance

Understanding dew point helps prevent costly moisture damage in your home. When warm, humid air contacts cool surfaces like basement walls, cold water pipes, or poorly insulated windows, condensation forms if the surface temperature is at or below the dew point. Over time, this moisture promotes mold growth, wood rot, and rust. In winter, keeping indoor humidity between 30% and 50% prevents window condensation while maintaining comfort. In summer, running air conditioning naturally lowers the dew point by removing moisture from the air. If your AC cannot keep indoor humidity comfortable, a standalone dehumidifier can bring the indoor dew point into the ideal 7°C to 13°C range. Use our heat index calculator to understand how humidity and temperature combine to affect perceived heat during summer months.

Dew Point in Agriculture

Farmers and gardeners use dew point to make critical crop management decisions. High dew points promote fungal diseases in crops like wheat, corn, and soybeans because prolonged leaf wetness from dew provides ideal conditions for pathogen growth. When dew points are consistently above 18°C during the growing season, fungicide applications become more important. Conversely, very low dew points combined with high temperatures create drought stress conditions. Irrigation scheduling often considers dew point alongside temperature and wind to calculate evapotranspiration rates. Frost prediction also relies on dew point because when the dew point is below freezing (0°C), frost will form if the air temperature drops to the dew point overnight.

Common Mistakes to Avoid

• Confusing dew point with relative humidity: A relative humidity of 50% does not indicate the same moisture level at different temperatures. At 35°C with 50% humidity, the dew point is about 23°C (very humid). At 10°C with 50% humidity, the dew point is about 0°C (very dry). Always use dew point to compare moisture levels across different temperatures.
• Ignoring dew point for indoor comfort: Many people set their thermostat based on temperature alone. A room at 22°C with a dew point of 17°C feels much less comfortable than the same 22°C with a dew point of 10°C. If you feel clammy despite a comfortable temperature, the dew point is too high and you need dehumidification.
• Assuming fog clears based on temperature alone: Fog lifts when the temperature rises above the dew point by at least 3°C. On humid mornings where both temperature and dew point are high, fog can persist well into the late morning even as the sun warms the air because the dew point is also rising with evaporation from wet surfaces.
• Not accounting for surface temperature differences: Condensation forms based on surface temperature, not air temperature. A cold beverage, a window, or a basement wall can be below the dew point even when the air temperature is well above it. Always check surface temperatures against the dew point when assessing condensation risk.
• Overlooking dew point trends: A single dew point reading is useful, but tracking dew point trends over hours and days reveals approaching weather systems. Steadily rising dew points often precede storms, while sharply falling dew points indicate drier air arriving behind a frontal passage.

Frequently Asked Questions