Calculate dew point temperature, relative humidity, or air temperature. Fill in any two of the three values and leave the third blank — the calculator will compute it for you.
The dew point is the temperature at which air must be cooled — at constant pressure and water vapor content — for water vapor to condense into liquid water (dew). When the air temperature drops to the dew point, the relative humidity reaches 100% and water vapor begins to condense onto surfaces, forming dew, frost (if below 0°C), or fog. The closer the dew point is to the actual air temperature, the more humid and uncomfortable the air feels.
Dew point is measured in degrees (°C, °F, or Kelvin) and is considered one of the most reliable indicators of human comfort in hot and humid weather. Unlike relative humidity, which changes as the temperature changes, dew point remains relatively stable throughout the day, making it a more useful indicator of actual moisture content in the air.
This calculator lets you solve for any one of three interrelated atmospheric variables:
Simply enter any two values and leave the third field blank. The calculator will determine the missing value using the Magnus formula, the standard method used by meteorologists worldwide. It also calculates the absolute humidity (the actual mass of water vapor per cubic meter of air) and displays a comfort level based on the dew point.
All three temperature inputs support Celsius (°C), Fahrenheit (°F), and Kelvin (K) — you can mix and match units freely.
The calculator uses the August-Roche-Magnus approximation, which is the standard formula in operational meteorology:
γ(T, RH) = (a × T) / (b + T) + ln(RH / 100)
Td = b × γ / (a − γ)
Where: a = 17.27, b = 237.7°C
In this formula, T is the air temperature in Celsius, RH is relative humidity as a percentage (0–100), and ln is the natural logarithm. The constants a = 17.27 and b = 237.7 are empirically determined values that give excellent accuracy over the temperature range most commonly encountered in weather observations (approximately −40°C to +60°C).
If you know the air temperature and dew point, relative humidity is calculated as:
RH = 100 × exp[ (a × Td) / (b + Td) − (a × T) / (b + T) ]
To find the air temperature when you know the dew point and relative humidity:
β = (a × Td) / (b + Td) − ln(RH / 100)
T = b × β / (a − β)
Absolute humidity is the actual mass of water vapor present in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity, which is a ratio, absolute humidity tells you the exact amount of moisture in the air regardless of temperature. The calculator uses the following formula:
AH = 6.112 × exp(17.67 × T / (T + 243.5)) × RH × 2.1674 / (273.15 + T)
where T is in Celsius, RH is in percent, and the result is in g/m³. At 25°C and 60% relative humidity, for example, the absolute humidity is approximately 13.8 g/m³. At 0°C and 100% relative humidity, it is only about 4.8 g/m³ — cold air simply cannot hold as much moisture as warm air.
Relative humidity is the measure most people are familiar with, and it appears on almost every weather app. However, dew point is actually a more useful comfort indicator, especially in warm weather.
Here is why: relative humidity depends on temperature. On a hot summer day (35°C), a relative humidity of 50% contains far more moisture than 50% relative humidity on a cool spring day (15°C). The dew point, by contrast, is an absolute measure of moisture that does not change with temperature. A dew point of 20°C feels oppressively humid whether the air temperature is 25°C (RH ≈ 78%) or 35°C (RH ≈ 46%).
Experienced meteorologists and heat-index researchers prefer dew point over relative humidity precisely because it more accurately reflects how much moisture the air can absorb from sweating skin — which is the body's primary cooling mechanism.
Air with a dew point below 10°C has very little moisture. This is typical of desert climates, high-altitude environments, and indoor air during winter heating. Very dry air can cause chapped lips, dry skin, and irritated sinuses. Outdoor conditions feel crisp and arid. In cold weather, this corresponds to extremely low humidity that can lead to static electricity buildup and increased susceptibility to respiratory illness.
This range is widely considered the ideal dew point for human comfort. The air contains enough moisture to feel fresh without being clammy. Spring and autumn days in temperate climates often fall in this range. Most people experience no discomfort and can be active outdoors without any weather-related fatigue.
Moisture in the air becomes perceptible in this range. The air may feel "sticky" in the morning or evening. Summer days in inland temperate regions often have dew points in this range. Most people are still comfortable at rest but may notice increased sweating during physical activity.
Dew points in this range are considered muggy. Sweating becomes less effective as a cooling mechanism because the humid air limits evaporation from the skin. Coastal areas and regions near large bodies of water frequently experience dew points in this range during summer. Physical exertion becomes more taxing, and sensitive individuals may feel fatigued more quickly.
This range is associated with what most people describe as "oppressive" summer humidity. The air feels heavy and wet. Sweat does not evaporate efficiently, causing the body to struggle to regulate its core temperature. Air conditioning becomes a significant comfort factor. Tropical and subtropical regions — including the Gulf Coast of the United States, Southeast Asia, and the Caribbean — frequently experience dew points in this range.
Dew points above 21°C are extremely uncomfortable and can pose a health risk during prolonged exposure, especially for the elderly, young children, and people with cardiovascular or respiratory conditions. At dew points above 24°C, the human body's cooling system becomes severely compromised. The highest dew points ever recorded on Earth have reached 35°C (95°F) in the Persian Gulf region, creating conditions where outdoor exposure can be rapidly dangerous. In the United States, dew points above 24°C are rare but do occur during extreme heat events in the Midwest and South.
Meteorologists use dew point extensively because it remains stable across temperature changes throughout the day, making it a reliable indicator of the actual moisture content of an air mass. A rising dew point signals increasing humidity and the possibility of fog, clouds, or precipitation. A dew point at or near the air temperature means fog or dew formation is likely. The spread between air temperature and dew point (the "temperature-dewpoint spread") tells forecasters how far the air is from saturation: a spread of less than 2–3°C typically indicates fog or low cloud cover.
In heating, ventilation, and air conditioning (HVAC) engineering, dew point is critical for preventing condensation on surfaces. When a surface temperature drops below the dew point of the surrounding air, moisture condenses on that surface. This is why cold water pipes "sweat" in summer, why windows fog up in winter, and why improperly insulated walls can suffer mold growth. HVAC designers use dew point calculations to size dehumidification systems, specify insulation requirements, and determine the risk of condensation on ductwork, chiller pipes, and other cold surfaces.
Farmers and growers monitor dew point closely because dew formation affects crop health. Persistent dew on plant surfaces promotes fungal diseases such as powdery mildew, downy mildew, and botrytis blight. When the dew point is above air temperature (which theoretically cannot happen — it would mean supersaturation), it signals that fog or rain is imminent. Irrigation timing, pesticide applications, and harvest decisions are all influenced by dew point forecasts.
In aviation, the temperature-dew point spread is one of the standard parameters reported in METAR weather observations. When this spread narrows to 2°C or less, pilots are alerted to the high probability of fog formation. Icing conditions also depend on dew point — supercooled droplets can form when the dew point is below 0°C but the air temperature is slightly above. Runway surface icing, carburetor icing, and airframe icing are all assessed partly through dew point analysis.
Many industrial processes are sensitive to moisture. Electronics manufacturing requires strict humidity control to prevent condensation on circuit boards and electrostatic discharge. Pharmaceutical manufacturing uses dew point monitoring to ensure product quality during blending and coating. Painting and coating operations depend on dew point to prevent blushing (moisture trapped in paint film) and adhesion failures. Compressed air systems monitor dew point to prevent moisture in pneumatic tools and pipelines.
Dew forms when surfaces cool to the dew point through radiative heat loss — typically on clear, calm nights when the sky acts as a heat sink. Surfaces such as grass, leaves, and car roofs lose heat by radiating infrared energy to the cold sky. When the surface temperature drops to the dew point, water vapor in the contact layer of air condenses onto the surface as liquid droplets.
When the dew point is below 0°C, the process produces frost instead of liquid dew. Frost forms directly from water vapor depositing as ice crystals (deposition), without first becoming liquid. The "frost point" is technically different from the dew point for subfreezing temperatures, but in everyday usage the terms are often used interchangeably.
No. The dew point can never exceed the air temperature. If you calculate a dew point higher than the air temperature, it means your inputs are physically inconsistent (for example, a relative humidity value greater than 100%). The maximum possible relative humidity is 100%, at which point the dew point equals the air temperature and the air is fully saturated.
Most people find dew points below 13°C (55°F) comfortable. The range of 10–13°C (50–55°F) is widely considered ideal — air that is fresh without being dry or clammy. Discomfort begins to increase noticeably above 16°C (60°F), and most people find dew points above 18°C (65°F) oppressive.
Relative humidity expresses moisture as a percentage of the air's maximum capacity at a given temperature. As temperature rises during the day, the air's capacity to hold moisture increases, so the same amount of water vapor represents a smaller percentage — relative humidity falls. The dew point, which measures the actual moisture content directly, stays nearly constant throughout the day unless new moisture is added or removed from the air mass.
Both dew point and wet bulb temperature measure atmospheric moisture, but they are different quantities. The wet bulb temperature is the lowest temperature a surface can reach through evaporative cooling, measured by wrapping a thermometer bulb in a wet cloth. It is always between the dry bulb (air) temperature and the dew point. Dew point is the temperature at which the air itself becomes saturated. At 100% relative humidity, all three — air temperature, wet bulb temperature, and dew point — are equal.
The August-Roche-Magnus formula with constants a = 17.27 and b = 237.7°C is accurate to within 0.1°C for most meteorological temperature ranges (approximately −40°C to +60°C). For extreme temperatures or scientific research requiring higher precision, more complex formulas (such as the Buck equation or Tetens equation) may be preferred. For everyday weather observation, agriculture, HVAC, and general science, the Magnus formula is more than sufficient.
The calculator accepts all three units for both air temperature and dew point. Meteorologists worldwide primarily use Celsius. In the United States, Fahrenheit is common in everyday weather reporting. Kelvin is used in scientific and thermodynamic calculations. The calculator converts internally to Celsius for the computation and then converts the output to whichever unit you selected, so any combination works.