Dew Point Calculator

Dew point is the number that explains why a 75 degree afternoon can feel crisp one day and tropical the next. This dew point calculator converts air temperature and relative humidity into the temperature where condensation begins. It is useful for weather interpretation, HVAC troubleshooting, greenhouse management, storage rooms, camera gear, basements, and any situation where moisture becomes a problem only after a surface gets cold enough.

How the calculation works

The tool takes air temperature and relative humidity, converts Fahrenheit inputs to Celsius when needed, and applies the Magnus approximation described below. Unlike relative humidity, which rises and falls as air warms or cools, dew point stays closer to the actual amount of water vapor in the air. That is why forecasters often use dew point to describe mugginess: a 24 degrees Celsius day at 55 percent relative humidity has a much lower moisture load than a 32 degrees Celsius day at the same percentage.

The result appears in the selected unit and also lists the dew point in Celsius. If you need the inverse relationship, the relative humidity calculator works from temperature and dew point. In hot conditions, the heat index calculator uses humidity to estimate apparent temperature. For practical moisture loss from water surfaces, the evaporation rate calculator connects temperature, wind, humidity, and area.

Formula

The approximation uses constants $a = 17.27$ and $b = 237.7$. After converting the input temperature to degrees Celsius, it computes:

$\gamma = \frac{a \times T}{b + T} + \ln\left(\frac{RH}{100}\right)$

Then it solves for dew point:

$T_{\text{d}} = \frac{b \times \gamma}{a - \gamma}$

where:

• $T_{\text{d}}$ is dew point in degrees Celsius before any Fahrenheit conversion.
• $T$ is air temperature in degrees Celsius.
• $RH$ is relative humidity in percent.
• $\ln$ is the natural logarithm.
• $a$ and $b$ are the constants implemented by the calculator.

If Fahrenheit is selected, the displayed dew point is converted back with:

$T_{\text{d,F}} = T_{\text{d}} \times \frac{9}{5} + 32$

The form rounds the primary result to two decimals.

Example: calculating dew point

Use the default inputs: air temperature 24 degrees Celsius and relative humidity 55 percent. First compute the temperature part: 17.27 times 24 divided by 237.7 plus 24. The numerator is 414.48 and the denominator is 261.7, giving about 1.5846. Next add the natural logarithm of 55 divided by 100. The logarithm of 0.55 is about negative 0.5978, so $\gamma$ is about 0.9868.

Now multiply 237.7 by 0.9868, giving about 234.6. Divide by 17.27 minus 0.9868, or about 16.2832. The dew point is 14.39 degrees Celsius after rounding, which is the rounded result for this example. A pipe, window, roof deck, or leaf surface at roughly 14.39 degrees Celsius could begin collecting condensation if exposed to that air.

Interpreting dew point values

For outdoor comfort, dew point is often more intuitive than relative humidity. Dew points below about 10 degrees Celsius, or 50 degrees Fahrenheit, usually feel dry to many people. Around 16 degrees Celsius, or the low 60s Fahrenheit, humidity becomes noticeable. Around 21 degrees Celsius, or the low 70s Fahrenheit, many people describe the air as oppressive, especially with sun and weak wind. These are comfort rules of thumb, not official medical limits; heat illness risk requires air temperature, exertion, clothing, sun, hydration, and cooling access.

For buildings, compare dew point with surface temperature. If indoor air has a 15 degrees Celsius dew point and an uninsulated basement wall is 13 degrees Celsius, condensation is plausible even if the room relative humidity does not look alarming. That is why cold bridges, single-pane windows, metal ducts, and chilled water pipes often show moisture first. The EPA’s mold guidance focuses on controlling moisture sources and drying wet materials quickly because relative humidity alone cannot describe surface wetness.

Gardeners and growers can also use dew point to anticipate wet leaves, disease pressure, and greenhouse ventilation needs. A high evening dew point means air must cool only a little before saturation, so leaves may stay wet longer. Pair it with plant growth when planning crops and with daylight hours when seasonal drying time changes.

Limitations and mistakes to avoid

Do not enter a relative humidity from one room and a temperature from another. The formula assumes both observations describe the same air. Do not treat dew point as the same thing as wet-bulb temperature; wet-bulb includes evaporative cooling and ventilation effects. Also be careful at zero percent relative humidity. The form accepts zero, but the logarithm of zero is undefined, so the result can be non-useful. For extremely dry air, enter a small positive percentage instead.

The Magnus approximation is accurate enough for common weather and indoor conditions, but it is still an approximation. Industrial drying, cryogenic temperatures, compressed air systems, and precision psychrometrics may need a full thermodynamic calculation and calibrated instruments.

Sources

• National Weather Service Louisville, Relative Humidity — explains dew point and relative humidity in operational weather terms.
• National Weather Service, Dry bulb, wet bulb, and dew point temperature — definitions for common moisture variables.
• EPA, A Brief Guide to Mold, Moisture and Your Home — building moisture and mold-prevention guidance.