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Psychrometrics

Dew Point Calculator

Computes the dew-point temperature of moist air from its dry-bulb temperature, relative humidity and pressure, by inverting the Hyland–Wexler saturation pressure at the actual water-vapour partial pressure.

The dew point is the temperature at which moist air, cooled at constant pressure, first becomes saturated and water begins to condense. This calculator finds it by inverting the Hyland–Wexler saturation pressure at the actual water-vapour partial pressure, and also reports the humidity ratio, wet-bulb temperature, enthalpy and specific volume. Enthalpy is per kilogram of dry air, on the dry-air/liquid-water 0 °C datum.

TypeInteractive engineering calculator

Calculator

Air state
°C

Validated range 0–50 °C

%

1–100 %

kPa

80–110 kPa (101.325 = sea level)

Dew-point temperature13.9 °C
Humidity ratio9.92 g/kg da
Wet-bulb temperature17.88 °C
Specific enthalpy50.43 kJ/kg da
Specific volume0.8581 m³/kg da
Audit trail
  • Saturation pressure p_ws = 3.1692 kPa (Hyland–Wexler 1983, over water)
  • Enhancement factor f = 1.00434 (Buck 1981)
  • Partial pressure p_w = f·(RH/100)·p_ws = 1.5915 kPa
  • Humidity ratio W = 0.621945·p_w/(P − p_w) = 9.9246 g/kg dry air
  • Dew point t_dp = 13.868 °C (inverse of f·p_ws = p_w, iterative)
  • Wet-bulb t_wb = 17.881 °C (psychrometric balance, iterative)
  • Enthalpy h = 1.006·t + W·(2501 + 1.86·t) = 50.433 kJ/kg dry air
  • Specific volume v = 287.042·T·(1 + 1.607858·W)/P = 0.8581 m³/kg dry air
Copyable summary

At the standard 25 °C dry-bulb / 50 % RH state (an exact node on the humid-air dataset), moist air at sea level has a humidity ratio of 9.9 g/kg dry air, a wet-bulb temperature of 17.9 °C, a dew point of 13.9 °C and a specific enthalpy of 50.4 kJ/kg dry air.

Sea-level over-water psychrometrics, computed live; cross-checked against the humid-air dataset. Properties and calculations only — not equipment selection or building-services design guidance.

Related: Wet-bulb temperature · Dew point · Humidity ratio & enthalpy · Humid-air properties · Water & steam

Formulas

Saturation pressure (Hyland–Wexler 1983)
ln(p_ws) = C8/T + C9 + C10·T + C11·T² + C12·T³ + C13·ln(T)
Water-vapour partial pressure (with Buck 1981 enhancement factor f)
p_w = f · (RH/100) · p_ws(t)
Dew point (solved iteratively for t_dp)
f(t_dp) · p_ws(t_dp) = p_w
Humidity ratio
W = 0.621945 · p_w / (P − p_w)

Diagram

t_dbW100% RHstatet_dpt_db

Worked example

Air at 25 °C dry-bulb, 50 % relative humidity, 101.325 kPa. Find the dew point.

  1. 01Saturation pressure (Hyland–Wexler): p_ws(25 °C) = 3.169 kPa
  2. 02Enhancement factor (Buck 1981): f = 1.0043
  3. 03Partial pressure: p_w = 0.50 × 1.0043 × 3.169 = 1.591 kPa
  4. 04Find t_dp such that f(t_dp)·p_ws(t_dp) = 1.591 kPa (iteration converges to t_dp ≈ 13.9 °C)
  5. 05Check: p_ws(13.9 °C) ≈ 1.585 kPa, so the partial pressure equals saturation there
Result

The dew point is about 13.9 °C (humidity ratio 9.92 g/kg dry air, wet-bulb 17.9 °C, enthalpy 50.4 kJ/kg dry air).

FAQ

How is the dew point found?
By inverting the saturation-pressure curve: the dew point is the temperature at which the saturation pressure equals the air’s actual water-vapour partial pressure. The calculator solves this iteratively on the monotonic Hyland–Wexler curve.
Can I use a pressure other than sea level?
Yes — the pressure input is adjustable over 80–110 kPa. Outside that range the calculator refuses rather than extrapolating.
What about a dew point below freezing?
This version uses the over-water saturation correlation, so a computed dew point below 0 °C is an over-water value, not a frost point, and is treated as indicative. The validated dry-bulb input range is 0–50 °C.
Is the dew point the same as the wet-bulb temperature?
No. The dew point is always at or below the wet-bulb temperature, which is at or below the dry-bulb temperature; the three coincide only at 100 % relative humidity.

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