Calculate dew point temperature from air temperature and relative humidity using the Magnus formula. Includes comfort scale, wet bulb, cloud base estimation.
The **Dew Point Calculator** determines the temperature at which air becomes saturated and water vapor begins to condense, using the widely-used Magnus formula. Dew point is one of the most practical humidity measurements — it directly tells you when condensation, fog, or frost will form.
Unlike relative humidity, which changes with temperature, dew point remains constant as air warms or cools (at constant pressure). This makes it the preferred metric for comfort assessment, weather forecasting, and building science. A dew point above 21°C feels oppressively humid regardless of the temperature, while below 10°C the air feels pleasantly dry.
Enter air temperature and relative humidity to instantly calculate dew point, dew point spread, wet bulb temperature, mixing ratio, and estimated cloud base altitude. The comfort scale visualization helps you understand what the numbers mean for everyday comfort and health. It also helps translate a humidity reading into the condensation and comfort questions people usually care about in practice. Check the example with realistic values before reporting.
Use this calculator when you need dew point as a practical moisture benchmark rather than a raw humidity percentage.
It is useful for forecasting condensation, judging comfort, checking fog risk, and understanding how warm or cool surfaces will behave in humid air. The supporting metrics help connect the dew point to the everyday weather, HVAC, and building questions that usually drive the calculation.
Dew Point (Magnus formula): Td = b × γ / (a − γ) Where: - γ = (a × T) / (b + T) + ln(RH / 100) - a = 17.27 - b = 237.7°C - T = air temperature (°C) - RH = relative humidity (%) Cloud base estimate: h ≈ (T − Td) / 2.5 × 304.8 meters
Result: 16.69°C dew point
At 25°C and 60% RH, γ = (17.27 × 25)/(237.7 + 25) + ln(0.60) = 1.133. Dew point = 237.7 × 1.133 / (17.27 − 1.133) = 16.69°C. This is in the "comfortable" range.
Dew point temperature is the temperature to which air must be cooled (at constant pressure and moisture content) to reach saturation. At the dew point, relative humidity is 100% and any further cooling causes condensation. This seemingly simple concept has profound implications across many fields.
The relationship between dew point and comfort is well-established. The U.S. National Weather Service uses dew point thresholds for heat advisories: below 13°C is comfortable, 13-18°C is slightly humid, 18-21°C is humid, and above 24°C is extremely uncomfortable. These thresholds are more reliable than heat index or relative humidity for assessing mugginess.
Condensation within wall assemblies is a major cause of building damage. When warm, humid interior air penetrates to a point in the wall where the temperature equals the dew point, moisture condenses — potentially causing mold, rot, and structural damage. Building scientists analyze dew point profiles through wall sections to locate condensation planes and specify vapor barriers accordingly.
Meteorologists rely heavily on dew point for several forecasts: - **Fog prediction:** When the temperature-dew point spread narrows to 2-3°C and winds are calm, radiation fog is likely. - **Thunderstorm potential:** High dew points (>18°C) indicate abundant atmospheric moisture, fueling convective storms. - **Frost forecasting:** When the dew point is below 0°C and clear skies allow radiative cooling, frost forms.
The dew point spread also estimates cumulus cloud base altitude — a crucial parameter for aviation weather planning and outdoor photography planning.
If air cools to the dew point, condensation forms. This is why grass gets dewy overnight — the surface cools below the dew point. It also determines when windows fog up or pipes sweat.
RH depends on temperature: 50% RH at 35°C feels far more oppressive than 50% RH at 15°C. Dew point gives a direct comparison — a dew point of 20°C always feels sticky, regardless of temperature.
The difference between air temperature and dew point. When the spread approaches zero, fog, clouds, or condensation are imminent. Pilots use spread to predict low-visibility conditions.
The Magnus formula is accurate to within 0.35°C for temperatures between -40°C and 50°C and RH from 1% to 100%. For extreme conditions, the more complex Arden Buck equation is preferred.
When the dew point is below 0°C, water vapor deposits directly as frost rather than liquid dew. The frost point is slightly different from the dew point when both are below freezing due to different saturation pressures over ice vs water.
No — the dew point can equal the air temperature (at 100% RH) but never exceed it under normal conditions. Use this as a practical reminder before finalizing the result.