Dew Point Calculator

What Is Dew Point?

The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid droplets. When the air temperature drops to the dew point, dew forms on surfaces, fog may develop, and clouds can form at higher altitudes. A dew point of 15 C (59 F) feels comfortable, while a dew point above 20 C (68 F) feels oppressively humid. Enter the current temperature and relative humidity in the calculator above to find the dew point instantly.

How Is Dew Point Calculated?

The Magnus formula approximates dew point from temperature and relative humidity: Td = (b times alpha) / (a - alpha), where alpha = (a times T) / (b + T) + ln(RH/100), with constants a = 17.27 and b = 237.7 C. For 25 C air at 60% humidity: alpha = (17.27 times 25)/(237.7 + 25) + ln(0.6) = 1.644 + (-0.511) = 1.133. Dew point = (237.7 times 1.133)/(17.27 - 1.133) = 269.3/16.14 = 16.7 C. The calculator handles this computation instantly for any temperature and humidity combination you enter.

Why Does Dew Point Matter More Than Humidity?

Relative humidity is misleading because it depends on temperature. Air at 30 C and 50% humidity contains far more moisture than air at 10 C and 50% humidity. Dew point measures the actual moisture content regardless of temperature. A dew point of 21 C always feels muggy whether the temperature is 25 C or 38 C. Meteorologists and HVAC engineers prefer dew point because it directly indicates moisture level. Weather forecasts that report dew point give you a more accurate sense of how the air will feel than relative humidity alone can provide.

Dew Point Comfort Scale

Below 10 C (50 F): dry, very comfortable. 10-15 C (50-59 F): comfortable. 16-18 C (60-64 F): slightly humid, noticeable. 19-21 C (66-70 F): humid, uncomfortable for some people. 22-24 C (72-75 F): very humid, oppressive. Above 24 C (75 F): extremely humid, dangerous for prolonged outdoor activity. The US Gulf Coast and Southeast regularly see dew points above 22 C in summer. Desert regions like Arizona have dew points below 5 C. Understanding this scale helps you prepare for outdoor activities, plan HVAC settings, and interpret weather forecasts more accurately than relative humidity percentages alone.

Dew Point in Building Science

In construction, the dew point determines where condensation occurs inside walls. When warm humid indoor air meets a cold surface inside the wall assembly, moisture condenses at the point where the temperature drops to the dew point. This condensation causes mold growth, wood rot, and insulation failure. Building scientists calculate the dew point profile through wall assemblies to ensure vapor barriers and insulation are positioned correctly. In cold climates, the vapor barrier goes on the warm side (interior). In hot humid climates, it goes on the exterior. Getting this wrong leads to moisture damage that can cost thousands in repairs.

Dew Point and Aviation

Pilots use dew point to predict fog, cloud formation, and icing conditions. When the temperature-dew point spread (the gap between air temperature and dew point) narrows to within 2-3 degrees, fog becomes likely. METARs (aviation weather reports) always include both temperature and dew point. A report showing temperature 15 C and dew point 14 C indicates near-saturated air with high fog probability. At altitude, the dew point determines where clouds form: rising air cools approximately 2 C per 1,000 feet, so a surface temperature of 20 C with dew point 10 C produces clouds at roughly 5,000 feet above ground level.

Dew Point vs Frost Point

When the dew point is below 0 C (32 F), moisture deposits as frost rather than liquid dew, and the temperature is technically called the frost point. The calculation is similar but uses slightly different constants because ice has a lower vapor pressure than liquid water. Gardeners and farmers monitor frost point to protect crops: when the air temperature approaches the frost point on clear calm nights, frost can damage or kill sensitive plants. Frost advisories are issued when the frost point and expected overnight low temperature converge. Covering plants or using smudge pots raises the local temperature above the frost point to prevent ice crystal formation on plant surfaces.

Frequently asked questions