Dew Point Calculator
Calculate the dew point temperature from air temperature and relative humidity using the Magnus formula — for weather, HVAC, and condensation analysis.
Enter the air temperature in °C and relative humidity percentage to calculate the dew point and absolute humidity.
Dew Point Calculator
Calculate the dew point temperature from air temperature and relative humidity using the Magnus formula — for weather, HVAC, and condensation analysis.
About the Dew Point Calculator
The dew point is the temperature to which air must be cooled at constant pressure and water vapour content for condensation to begin. When a surface is at or below the dew point, water vapour in the adjacent air condenses into liquid water — the dew that appears on grass, windows, and cold pipes. Understanding the dew point is fundamental to meteorology, HVAC engineering, materials preservation, aviation, and everyday comfort assessment.
The dew point is calculated using the Magnus formula, one of the most accurate and widely used approximations for the saturation vapour pressure of water as a function of temperature. The saturation vapour pressure es (in hPa) at temperature T (°C) is: es = 6.1078 × exp(17.625 × T / (243.04 + T)). The actual vapour pressure e = (RH/100) × es. The dew point Td is then the temperature at which es(Td) = e, which can be solved analytically as: Td = 243.04 × γ / (17.625 − γ), where γ = ln(e/6.1078) = ln(RH/100) + 17.625 × T / (243.04 + T).
Relative humidity and dew point convey related but different information about atmospheric moisture. Relative humidity is the ratio of actual water vapour pressure to saturation pressure at the current air temperature — it tells you how close the air is to being fully saturated. Dew point is an absolute measure of moisture content: air with a dew point of 15°C contains the same amount of water vapour whether the air temperature is 20°C (RH ≈ 74%) or 30°C (RH ≈ 42%). This makes dew point a more stable and informative comfort index than relative humidity alone.
Humid discomfort is strongly related to dew point. A dew point below 10°C feels dry and comfortable; 10–16°C is comfortable for most people; 16–18°C begins to feel muggy; 18–21°C feels humid; 21–24°C is very uncomfortable; above 24°C is oppressive. These thresholds are why tropical regions with dew points regularly above 24°C feel so taxing — the body struggles to cool itself by perspiration when the air already contains so much moisture.
For aviation, the dew point is part of the METAR weather report and is used to estimate cloud base height. The approximate rule is: cloud base (ft) ≈ 400 × (T − Td), where T is temperature and Td is dew point in °C. When temperature equals dew point (spread = 0), fog or stratus forms. Pilots use the temperature–dew point spread to anticipate visibility and ceiling changes during approach and departure.
Absolute humidity — the mass of water vapour per unit volume of air — is derived from the vapour pressure using the ideal gas law: AH (g/m³) = 1000 × e × M_w / (R × T_K), where e is in Pa, M_w = 0.018015 kg/mol, R = 8.314 J/(mol·K), and T_K is temperature in Kelvin. This calculator provides absolute humidity alongside dew point so you have a complete picture of atmospheric moisture content.
Dew Point Calculation Examples
Dew point for common weather and indoor conditions using the Magnus formula.
| Conditions | Dew Point | Interpretation |
|---|---|---|
| T = 22°C, RH = 45% | Td ≈ 9.5°C | Comfortable indoor environment. Dew point below 10°C feels dry. No condensation risk on surfaces above 9.5°C. |
| T = 30°C, RH = 75% | Td ≈ 25.1°C | Hot and humid summer day. Dew point above 24°C is considered oppressive. High condensation risk on cold surfaces and cold beverages. |
| T = 5°C, RH = 30% | Td ≈ −11.2°C | Cold, dry winter conditions. Very low dew point means little moisture in the air. No condensation risk unless surfaces are chilled well below freezing. |
| T = 18°C, RH = 95% | Td ≈ 17.2°C | Near-saturation conditions. Temperature–dew point spread of only 0.8°C means fog formation is imminent if temperature drops even slightly. |
How to Use the Dew Point Calculator
- Enter the current air (dry-bulb) temperature in degrees Celsius. This is the standard air temperature measured by a thermometer in the shade.
- Enter the relative humidity as a percentage between 1 and 100. This value is available from weather stations, hygrometers, and smartphone weather apps.
- Click Calculate. The results show the dew point in °C and °F, absolute humidity in g/m³, and both saturation and actual vapour pressure in hPa.
- Interpret the dew point: values below 10°C are dry and comfortable; 16–18°C starts to feel muggy; above 21°C is humid; above 24°C is oppressive.
- Click Reset to clear all values and enter a new set of conditions.
Dew Point Calculator FAQ
What is the difference between dew point and relative humidity?
Relative humidity is a ratio — it tells you what percentage of the maximum possible moisture the air holds at its current temperature. Dew point is an absolute temperature — it tells you the actual moisture content regardless of air temperature. As air warms, relative humidity drops even though the moisture content stays the same; dew point stays constant. This is why dew point is a better comfort indicator.
What dew point indicates uncomfortable humidity?
Below 10°C is dry and pleasant. Between 13 and 16°C is comfortable for most people. Between 18 and 21°C starts to feel muggy. Above 21°C is noticeably humid and uncomfortable. Above 24°C is considered oppressive, and above 27°C — common in tropical coastal areas — is dangerously humid because the body struggles to cool itself through sweat evaporation.
What is the Magnus formula and how accurate is it?
The Magnus formula (also called the Magnus-Tetens formula) approximates the saturation vapour pressure of water as an exponential function of temperature. The version used here, with constants b = 17.625 and c = 243.04°C, has a maximum error of about 0.1% over the range −40°C to 60°C — more than sufficient for meteorological and HVAC applications. More complex polynomial formulas (e.g., the Buck equation) offer slightly higher accuracy at extreme temperatures.
How is dew point used in aviation?
Pilots use the temperature–dew point spread to estimate cloud base: approximately 400 ft per degree Celsius of spread. A spread of 5°C predicts a cloud base around 2,000 ft AGL. When the spread falls below 2°C, fog or low stratus is likely. METAR reports always include both temperature and dew point, and when they are equal the report often notes mist (BR) or fog (FG) in the weather fields.
What is absolute humidity and how does it differ from relative humidity?
Absolute humidity is the mass of water vapour per cubic metre of air (g/m³). It is a direct, concentration-based measure that does not depend on temperature. Relative humidity compares actual vapour pressure to the saturation pressure at the current temperature, producing a percentage. Warm air can hold far more water vapour than cold air, so the same absolute humidity can correspond to very different relative humidity values depending on air temperature.
At what temperature does water condense on a cold surface?
Water begins to condense on a surface when that surface temperature falls to or below the dew point of the surrounding air. For example, if the room air has a dew point of 12°C, any surface cooler than 12°C will collect condensation. This is why cold water pipes sweat in summer, windows fog up when the indoor temperature drops, and electronic equipment must be kept above the dew point to avoid moisture damage.