Dew Point Calculator
Calculate the temperature at which air becomes saturated and dew or frost begins to form. Useful for weather analysis, HVAC planning, and assessing outdoor comfort levels.
About this calculator
The dew point is the temperature to which air must be cooled — at constant pressure and moisture content — for water vapor to condense into liquid. It is a direct measure of atmospheric moisture: a higher dew point means more water vapor in the air. This calculator uses an approximation based on the Magnus formula. Given air temperature T (°F) and relative humidity RH (%), the formula is: DP = T − ((100 − RH) / 5) × [ln(RH/100) + (17.27 × T) / (237.7 + T)] / 17.27. Note that the Magnus formula is strictly defined with temperature in Celsius; the formula here adapts it for Fahrenheit inputs. A dew point above 60 °F starts to feel humid to most people; above 70 °F it feels oppressively muggy. Meteorologists use dew point rather than relative humidity for comfort assessment because it does not change with air temperature.
How to use
Suppose the air temperature is 85 °F and relative humidity is 70%. Plug T = 85, RH = 70 into the formula: DP = 85 − ((100 − 70) / 5) × [ln(70/100) + (17.27 × 85) / (237.7 + 85)] / 17.27. Step by step: (100 − 70) / 5 = 6. ln(0.70) ≈ −0.3567. (17.27 × 85) / (237.7 + 85) = 1467.95 / 322.7 ≈ 4.549. Numerator: −0.3567 + 4.549 = 4.192. Divide by 17.27: 4.192 / 17.27 ≈ 0.2427. DP = 85 − 6 × 0.2427 ≈ 85 − 1.456 ≈ 73.5 °F — a very uncomfortable dew point.
Frequently asked questions
What is the difference between dew point and relative humidity?
Relative humidity expresses how much water vapor is in the air as a percentage of the maximum it could hold at that temperature. It changes as temperature changes, even if the actual moisture content stays constant — that is why a 90% humidity morning can drop to 40% by afternoon without any rain. Dew point, by contrast, is a fixed temperature that directly represents the absolute amount of moisture in the air and does not change unless moisture is added or removed. Meteorologists and comfort researchers prefer dew point because it is a more stable and physically meaningful measure of how humid the air truly is.
When does condensation or dew actually form on surfaces?
Dew forms when the surface temperature of an object drops to or below the dew point of the surrounding air, causing the air in contact with that surface to release its water vapor as liquid. This commonly happens at night when the ground and objects radiate heat away and cool down faster than the air. Frost forms instead of dew when the dew point is at or below 32 °F (0 °C). In buildings, condensation on windows and pipes occurs for the same reason — the surface temperature is below the indoor air's dew point, which is why proper insulation and ventilation are critical in humid climates.
How can I use dew point to assess outdoor comfort and heat risk?
Dew point is one of the best single-number comfort indicators available. Below 50 °F, the air feels dry and comfortable. Between 50–60 °F, most people feel comfortable but notice some moisture. From 60–65 °F the air starts feeling noticeably humid. Above 65 °F it becomes muggy and uncomfortable, and above 70 °F the air feels oppressive and physical exertion becomes more dangerous. Dew points above 75 °F — which occur in tropical regions and Gulf Coast heat waves — are associated with serious heat stress risk because the body's ability to cool through sweating is severely impaired.