Dew Point Calculator

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Air temperature:

Choose the unit printed by the thermometer or station.

Relative humidity:

Accepted range is 1–100%; use a co-located calibrated sensor when the decision is sensitive.

Surface temperature:

The surface is at condensation risk when it reaches the modeled dew point.

Station pressure:

Standard-atmosphere default is 1013.25 hPa; use station pressure rather than sea-level-adjusted pressure when available.

Measure	Value	Meaning	Copy
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Condensation check

Compared surface

{{ values.surface_status }} {{ formatSignedDelta(values.surface_margin_c) }} from dew point.

Air saturation distance

{{ values.air_status }} {{ formatDelta(values.dew_point_depression_c) }} of cooling room.

Humidity threshold

{{ thresholdDisplay }} at the current air and surface temperatures.

This is a screening estimate. Sensor placement, local cold spots, ice surfaces, and calibration error can move the real condensation boundary.

Damp windows, sweating pipes, fogged lenses, and moisture on cold equipment all begin with the same comparison: the surface temperature falls to the dew point of the surrounding air. Dew point is the temperature at which that air reaches saturation if it cools without gaining or losing water vapor.

Relative humidity changes when temperature changes, even when the amount of moisture in the air stays constant. Dew point is therefore often the clearer measure of the air's moisture content. Warm air at 50% relative humidity can have a higher dew point than cold air at 90%, and may feel much muggier.

Dew point: The modeled saturation temperature for the current air temperature and relative humidity.
Dew-point depression: Air temperature minus dew point. A small spread means little cooling is needed to reach saturation.
Surface margin: Surface temperature minus dew point. A zero or negative margin indicates modeled condensation conditions.

The coldest relevant surface controls a condensation check. A room may look safe at the thermostat while a window edge, uninsulated duct, refrigeration line, thermal bridge, or metal enclosure is several degrees colder. Measuring the air and surface at different places or times can produce a reassuring number that does not describe the actual moisture boundary.

Dew point is a screening estimate, not proof that a surface will stay dry. Sensor accuracy, airflow, surface coatings, radiant cooling, pressure, and ice conditions can shift what happens locally. Use a margin rather than treating a tenth of a degree as a hard safety boundary.

How to Use This Tool:

Take the air and humidity readings together and compare them with the coldest surface that matters.

Enter Air temperature and choose the thermometer's unit.
Enter Relative humidity from a co-located reading. Values must be from 1% through 100%.
Enter Surface temperature for the cold spot you are checking, not a room-average temperature.
Open Advanced only when station pressure is known. Use actual station pressure rather than a sea-level-adjusted weather value.
Read the dew point and Surface status first, then use the moisture profile and RH curve for supporting context.

Interpreting Results:

A positive surface margin means the entered surface is warmer than the modeled dew point. The smaller the margin, the more easily measurement error or a brief temperature drop can reverse the result. A margin of 0°C or less is labeled Condensing now.

The air-status label describes cooling room, not the compared surface. A wide air spread can coexist with condensation on a much colder pipe or window. Check the surface margin whenever surface wetting is the decision.

Surface margin interpretation boundaries
Surface margin	Status	Practical meaning
≤ 0°C	Condensing now	The modeled dew point is at or above the surface temperature.
> 0°C and < 2°C	Very tight margin	A small error or temperature drop can remove the margin.
≥ 2°C and < 5°C	Narrow margin	Monitor the coldest location and changing conditions.
≥ 5°C	Clear margin	The entered surface is comfortably above the modeled dew point.

Technical Details:

The calculation uses the Alduchov–Eskridge Magnus approximation over water. Air temperature is converted to degrees Celsius, relative humidity is used as a fraction, and the result is converted back to the selected display unit.

Formula Core:

First, an intermediate moisture term is formed and then solved for dew-point temperature.

γ = \ln (\frac{RH}{100}) + \frac{a T}{b + T} T_{d} = \frac{b γ}{a - γ}

Here, T is air temperature in °C, RH is relative humidity in percent, T_d is dew point in °C, a is 17.625, and b is 243.04°C.

Saturation and actual vapor pressure provide the remaining moisture quantities.

e_{s} (T) = 6.112 \times \exp (\frac{17.625 T}{243.04 + T}) e = \frac{RH}{100} e_{s} (T)

Both pressures are in hectopascals. Station pressure does not change dew point or surface margin; it enters only the mixing-ratio and specific-humidity calculations.

Derived dew point and moisture quantities
Quantity	Definition	Unit
Surface margin	Surface temperature minus dew point	°C difference
Dew-point depression	Air temperature minus dew point	°C difference
Surface relative humidity	Actual vapor pressure divided by saturation pressure at the surface, multiplied by 100	%
Condensation threshold	Saturation pressure at the surface divided by saturation pressure in the air, multiplied by 100	% RH
Absolute humidity	216.7 times actual vapor pressure divided by absolute air temperature	g/m³
Mixing ratio	621.945 times vapor pressure divided by station pressure minus vapor pressure	g/kg dry air
Specific humidity	622 times vapor pressure divided by station pressure minus 0.378 times vapor pressure	g/kg moist air

Rule Core:

Dew-point depression is labeled Near saturation below 1°C, Small spread from 1°C to under 3°C, Moderate spread from 3°C to under 8°C, and Wide spread at 8°C or more. Comfort labels use dew point in °F: below 50 is Dry air, 50 to under 60 is Comfortable, 60 to under 65 is Slightly sticky, 65 to under 70 is Humid, 70 to under 75 is Oppressive, and 75 or more is Very oppressive.

Accepted air temperatures are −80°C to 80°C, surface temperatures are −100°C to 120°C, relative humidity is 1% to 100%, and station pressure is 300 to 1100 hPa. The displayed values are rounded for reading after the full-precision calculation.

Accuracy Notes:

The Magnus constants model saturation over liquid water. Ice or frost conditions may need an ice-specific formulation.
A single surface reading can miss colder edges, corners, joints, or locations with weak airflow.
The comfort labels are practical weather categories, not health or building-control limits.
At the exact boundary, model precision can be smaller than ordinary sensor error. Keep a working margin and verify sensitive decisions with calibrated instruments.

Worked Examples:

Indoor wall check

Air at 25°C and 60% RH has a modeled dew point of about 16.7°C. A 20°C wall has a margin of about 3.3°C, so the result is Narrow margin. Checking the colder window edge may change the decision.

Cool surface in humid air

At the same 25°C air temperature, raising RH to 75% moves the dew point to about 20.3°C. A 20°C surface then has a margin near −0.3°C and is labeled Condensing now.

References:

Improved Magnus Form Approximation of Saturation Vapor Pressure, O. A. Alduchov and R. E. Eskridge, Journal of Applied Meteorology, 1996.
Dew Point vs. Humidity, NOAA National Weather Service.