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WaterVaporPressure

Vapor Pressure & Dew Point for HVAC

Humidity problems in HVAC — sweating pipes, condensation on grilles, mold, dehumidification load — all come back to the dew point and the saturation vapor pressure behind it. Enter a condition below to find the dew point (and wet-bulb, vapor pressure and absolute humidity), computed with IAPWS-95-grade accuracy.

%
Dew point
13.86 °C
Wet-bulb temperature
18 °C
Stull 2011 approximation
Actual vapor pressure
1.5849 kPa
RH × saturation
Saturation vapor pressure
3.1698 kPa
IAPWS-95 at the dry-bulb T
Absolute humidity
11.52 g/m³
vapor mass per m³ of air

Dew point uses the Magnus inversion with the Alduchov–Eskridge constants (±0.01 °C); the saturation and actual vapor pressures use the IAPWS-95 reference; wet-bulb uses Stull (2011). Every value is computed live in your browser.

Condensation control starts at the dew point

The single most useful number for avoiding condensation is the dew point of the air in the space. Keep every exposed surface — pipe, duct, grille, window — above it and it stays dry; let a surface fall below it and water appears, bringing mold, staining and corrosion with it. Insulation and vapor retarders are sized to keep surface temperatures on the right side of that line.

Because relative humidity shifts with temperature across a system, design and troubleshooting are cleaner in terms of dew point and vapor pressure. This tool reports both, plus the wet-bulb temperature relevant to evaporative cooling and cooling-tower work.

Related tools

Use the dew point calculator for the full breakdown, the main calculator to compare saturation-pressure formulas, and vapor pressure over ice for below-freezing (frost-point) cases such as cold-climate wall condensation.

Frequently asked questions

When will a surface condense (sweat) in an HVAC system?

Any surface colder than the dew point of the surrounding air will collect condensation. That is why chilled-water pipes, cooling-coil casings, supply grilles and uninsulated ducts in humid spaces drip: their surface temperature has dropped below the room's dew point. Knowing the dew point tells you the minimum surface temperature you must hold — or insulate to — to keep a surface dry.

Why design to the dew point instead of relative humidity?

Relative humidity changes with temperature, so the same air reports different RH at the coil, in the duct and in the room. The dew point does not — it is a fixed measure of how much water the air actually holds. For condensation, mold risk and dehumidification targets, the dew point is the stable number to design and troubleshoot against.

How does vapor pressure relate to latent cooling load?

Dehumidification means condensing water vapor out of the air, which happens when air contacts a coil below its dew point. The amount of moisture to remove is set by the difference between the air's actual vapor pressure and the saturation vapor pressure at the coil temperature, so accurate saturation pressures underpin any latent-load estimate.

What about condensation inside walls in winter?

In heating climates, warm humid indoor air can reach a cold point inside a wall assembly that is below its dew point, condensing within the structure. The same dew-point logic applies; below freezing, the relevant saturation is over ice — see the vapor pressure over ice page.

References

Every formula on this page is implemented from, and validated against, the following primary standards and papers — see the verification methodology.

Reviewed by Jimmy Raymond, Engineer
B.S. Environmental Engineering · B.S. Computer Science · Last reviewed June 4, 2026

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