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Dew Point Calculator

Find the dew point from any two known conditions — then check whether a specific surface will actually condense. Uses the ASHRAE Hyland-Wexler formulation with the Buck enhancement factor, correct over ice as well as water.

Built and verified by Kyle Lorinos, PE — HVAC design engineer. See the method and validation →

Conditions

Check a surface
A window pane, an uninsulated duct, a cold-water pipe, a slab, a basement wall. Leave blank to skip.
SSPC / ASTM D3276 / ISO 8502-4 requirement for surface prep, application and cure.
Advanced — elevation
Affects the humidity ratio and the enhancement factor. Dew point itself barely moves with elevation.
Dew point
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Method, and what makes this different

The saturation curve

Saturation pressure comes from the ASHRAE Hyland-Wexler correlation, using the separate formulation over ice below 0 °C and over liquid water above it. Calculators built on the Antoine equation apply the liquid-water curve everywhere, which drifts exactly where condensation questions matter most — near and below freezing.

The enhancement factor

Moist air is not an ideal mixture. The effective saturation pressure of water vapor in air sits roughly 0.4 % above that over a pure water surface, so we apply the Buck enhancement factor with separate forms over water and ice. Omitting it understates humidity ratio by 0.3–0.6 % — the single most common discrepancy between free calculators and commercial reference software.

Dew point versus frost point

Below freezing, water vapor deposits directly as frost rather than dew. The two are not the same temperature for the same air. Where the result falls below freezing this calculator reports it as a frost point and says so, rather than silently labelling it dew point.

The surface check

Dew point on its own rarely answers the question people actually have, which is whether a particular surface will sweat. Enter the surface temperature and you get the margin, in degrees, along with what that margin means.

The coating rule

For protective coatings, SSPC, ASTM D3276 and ISO 8502-4 all require the substrate to be at least 5 °F (3 °C) above the dew point during surface preparation, application and cure. Meeting dew point exactly is not sufficient; the spread is the specification. Tick the box and the verdict applies that threshold instead of the bare condensation point.

Validated against ASHRAE tabulated values and independently against commercial psychrometric software across 12 conditions from −18 to 54 °C (0–130 °F) and 10–100 % RH, including sub-freezing and exactly saturated states — 60 property comparisons, all within tolerance.