How to use the dew point temperature calculator
The dew point is the temperature at which air, cooled without changing its moisture content, begins to form liquid water. When a pipe, a duct or a window pane sits below the dew point of the surrounding air, moisture collects on that surface. Enter a dry-bulb temperature and a relative humidity and the page returns the dew point, the dew-point depression, and both the saturation and the actual vapor pressure of that air.
Saturation pressure uses es = 6.112 x exp(17.67 x T / (T + 243.5)) in hectopascals, with T in degrees Celsius; the page converts your Fahrenheit input internally. The dew point is inverted through g = ln(RH/100) + 17.67T/(T+243.5) and Td = 243.5g / (17.67 - g). This coefficient pair is reported to stay within about 0.5% from -49 to 140 degrees F, so inputs outside that span are blocked with a message. Other published sets, such as 17.62 / 243.12, shift the last decimals. The sources are secondary, current as of October 2026, and the original documents are worth checking.
This calculator does not cover supply-air setpoints or a condensation verdict. Checking a surface against its dew point with a safety margin belongs to a separate tool. It is a reference aid and does not replace design work; review by a qualified engineer is required.
Frequently asked questions
Yes. At 100% the actual vapor pressure equals the saturation vapor pressure, so the inverted dew point matches the dry-bulb reading and the depression reads 0.00 degrees F.
It shows how much margin a surface still has before it reaches the dew point. Whether condensation actually forms depends on measured surface temperature and a chosen margin, which this page does not judge.
The saturation-pressure formula is published in SI units, so the page keeps hPa instead of converting. An arbitrary conversion would add error without adding information.