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🌡️ Dew Point Calculator

Dew Point Calculator

Inputs

%

Results

Dew Point Temperature-°C
Absolute Humidity- g/m³
Wet Bulb Temperature- °C

Calculator Description

The dew point is the temperature at which water vapor in air becomes saturated and condensation occurs. This calculator takes dry-bulb temperature and relative humidity as inputs and computes dew point, wet-bulb temperature, and absolute humidity.

What this calculator finds

Using the Magnus formula, it calculates the dew point temperature from dry-bulb temperature and relative humidity. Additionally, wet-bulb temperature, saturation vapor pressure, actual vapor pressure, and absolute humidity (mixing ratio) are provided.

Why it matters

  • Condensation prevention: temperatures below dew point cause surface condensation, leading to corrosion and mold
  • Indoor comfort: human comfort depends heavily on dew point (recommended: 10–15°C)
  • HVAC design: dew point is a key parameter for cooling load calculation and dehumidifier sizing

Formula

Magnus Formula (Dew Point)

The Magnus formula is a widely used empirical formula for dew point calculation, providing high accuracy in the range of −40°C to 50°C.

Td=b α(T,RH)a−α(T,RH)T_d = \dfrac{b\,\alpha(T, RH)}{a - \alpha(T, RH)}
α(T,RH)=aTb+T+ln⁡ ⁣(RH100)\alpha(T, RH) = \dfrac{a T}{b + T} + \ln\!\left(\dfrac{RH}{100}\right)
  • Td — Dew point temperature [°C]
  • T — Dry-bulb temperature [°C]
  • RH — Relative humidity [%]
  • a, b — Constants (a = 17.27, b = 237.7 °C)
  • ln — Natural logarithm

Saturation vapor pressure formula

es(T)=6.1078 exp⁡ ⁣(17.27 T237.7+T)e_s(T) = 6.1078\,\exp\!\left(\dfrac{17.27\,T}{237.7 + T}\right)

Unit is hPa (mbar); actual vapor pressure e = e_s × RH/100.

Wet-bulb temperature approximation (Stull)

Tw≈T arctan⁡ ⁣(0.151977RH+8.313659)+arctan⁡(T+RH)−arctan⁡(RH−1.676331)T_w \approx T\,\arctan\!\left(0.151977\sqrt{RH + 8.313659}\right) + \arctan(T + RH) - \arctan(RH - 1.676331)
+  0.00391838 RH3/2arctan⁡(0.023101 RH)−4.686035+\; 0.00391838\,RH^{3/2}\arctan(0.023101\,RH) - 4.686035

Useful Tips

Practical tips

  • Dew point below 15°C is comfortable; above 20°C causes significant discomfort
  • Keeping surface temperature above the dew point is key to preventing condensation
  • In HVAC design, cooling coil outlet temperature must be set below the target dew point

Limitations & cautions

  • The Magnus formula is for pure air-water vapor mixtures; errors occur with high-concentration pollutants
  • Pressure correction is needed when atmospheric pressure deviates significantly from standard (1013.25 hPa)
  • Wet-bulb approximation is within ±0.3°C; use psychrometric charts for precise measurements