Wet Bulb Temperature Calculator

Overview

How to Use This Calculator

1. Select input combination — choose from Dry-Bulb + Relative Humidity, Dry-Bulb + Dew Point Temperature, or Dry-Bulb + Humidity Ratio.

2. Enter dry-bulb temperature — in °C or °F.

3. Enter the second input based on your selected combination — relative humidity (%), dew point temperature (°C/°F), or humidity ratio (g/kg or gr/lb).

4. Click "Calculate" — get wet-bulb temperature, wet-bulb depression (ΔT), relative humidity, humidity ratio, dew point, vapor pressure, and enthalpy.

Use the entering wet-bulb for cooling tower / evaporative cooler sizing and coil total-load selection; size to the ASHRAE 0.4%/1% design wet-bulb for the location.

Inputs & Outputs

Formula

Metric:   P_sat(T) = 0.61078 × exp(17.625 × T / (243.04 + T))   [kPa]
Imperial: P_sat(T) = 0.08855 × exp(17.625 × Tc / (243.04 + Tc)) [psi]
          where Tc = (T_°F − 32) / 1.8

Metric:
  W_sat_wb = 621.945 × P_sat(T_wb) / (P_atm − P_sat(T_wb))   [g/kg]
  W = W_sat_wb − 0.000799 × (T_db − T_wb) × (1000 + W_sat_wb) [g/kg]

Imperial:
  W_sat_wb = 4350 × P_sat(T_wb) / (P_atm − P_sat(T_wb))       [gr/lb]
  W = W_sat_wb − 0.000437 × (T_db − T_wb) × 7000              [gr/lb]

f(T_wb) = W_sat(T_wb) − A × (T_db − T_wb) × K − W_actual = 0

T_wb[n+1] = T_wb[n] − f(T_wb[n]) / f'(T_wb[n])

Metric:   W = 621.945 × P_v / (P_atm − P_v)   [g/kg]
Imperial: W = 4350 × P_v / (P_atm − P_v)       [gr/lb]

Metric:   P_v = P_atm × W / (621.945 + W)   [kPa]
Imperial: P_v = P_atm × W / (4350 + W)       [psi]

Metric:
  α = ln(P_v / 0.61078)
  T_dp = 243.04 × α / (17.625 − α)   [°C]

Imperial:
  P_v_kPa = P_v_psi × 6.8948
  α = ln(P_v_kPa / 0.61078)
  T_dp(°C) = 243.04 × α / (17.625 − α)
  T_dp(°F) = 32 + 1.8 × T_dp(°C)

Imperial: h = 0.240 × T_db + (W / 7000) × (1061 + 0.444 × T_db)   [BTU/lb]
Metric:   h = 1.006 × T_db + (W / 1000) × (2501 + 1.86 × T_db)    [kJ/kg]

Key Facts

• Wet-bulb temperature always lies between dry-bulb and dew point temperatures: T_dp ≤ T_wb ≤ T_db. At 100% RH, all three temperatures are equal. As humidity decreases, the spread between them increases.
• Cooling towers cannot cool water below the entering wet-bulb temperature. The approach (leaving water temperature minus wet-bulb) is the primary performance metric — typical design approaches range from 5–10°F (3–6°C) for HVAC towers.
• The ASHRAE design wet-bulb temperature for a location is the 0.4% or 1% annual exceedance value from ASHRAE Climatic Design Conditions. Equipment sized to the 0.4% design wet-bulb will meet load for 99.6% of annual hours.
• Wet-bulb depression (T_db − T_wb) directly indicates evaporative cooling potential. A depression of 20°F (11°C) or more indicates conditions favorable for evaporative cooling. A depression below 5°F (3°C) means evaporative cooling is ineffective.
• The Sprung psychrometric constant used in this calculator (A = 0.000799 °C⁻¹ metric, A = 0.000437 °F⁻¹ imperial) applies to mechanically ventilated (aspirated) wet-bulb thermometers with air velocity ≥ 3 m/s (600 fpm).
• A wet-bulb temperature of 35°C (95°F) is considered the theoretical upper limit of human survivability — above this temperature, the human body cannot cool itself through perspiration regardless of shade, wind, or hydration.

Applications

• Cooling tower sizing and performance evaluation — entering wet-bulb temperature determines the theoretical minimum cold water temperature and tower approach
• Evaporative cooler (swamp cooler) design — wet-bulb temperature sets the lowest achievable supply air temperature; wet-bulb depression indicates cooling potential
• HVAC cooling coil entering-air conditions — wet-bulb temperature determines total enthalpy and latent load on the coil
• Outdoor heat stress assessment — wet-bulb temperature is the dominant component of WBGT (Wet Bulb Globe Temperature) used for occupational and athletic heat safety
• Psychrometric analysis and field measurement — convert sling psychrometer readings (dry-bulb + wet-bulb) to full psychrometric state
• ASHRAE climate data interpretation — design wet-bulb temperatures from ASHRAE Climatic Design Conditions are used for equipment sizing in every HVAC project

Standards & References

• ASHRAE Handbook — Fundamentals (2021), Ch. 1 Psychrometrics (Sprung wet-bulb equation) — Primary reference for wet-bulb temperature formulations, the Sprung psychrometric equation, and the relationship between wet-bulb and other psychrometric properties.
• ASHRAE Standard 55-2020 — Thermal Environmental Conditions for Human Occupancy — References wet-bulb temperature indirectly through humidity limits and the psychrometric comfort envelope.
• ASHRAE Climatic Design Conditions — Provides design wet-bulb temperatures (0.4%, 1%, 2% annual exceedance) for equipment sizing at locations worldwide.
• Magnus Formula (Alduchov-Eskridge, 1996) — Used for saturation pressure calculation. Constants 17.625 and 243.04 are optimized for the temperature range −40°C to +60°C.
• Sprung Psychrometric Equation — Constants A = 0.000437 °F⁻¹ (Imperial) and A = 0.000799 °C⁻¹ (Metric) apply to mechanically ventilated wet-bulb sensors with air velocity ≥ 3 m/s.
• CTI Standard STD-201 — Thermal Performance Testing of Cooling Towers — Defines wet-bulb measurement requirements for cooling tower acceptance testing.

Common Mistakes to Avoid

• Confusing wet-bulb with dew point temperature. Both are lower than dry-bulb, but they measure different things. Dew point depends only on moisture content and is the condensation threshold. Wet-bulb depends on both moisture content and dry-bulb temperature and represents the evaporative cooling limit. They are equal only at 100% RH.
• Using dry-bulb temperature instead of wet-bulb for cooling tower sizing. Cooling tower capacity is governed by the entering wet-bulb temperature, not the dry-bulb. Using dry-bulb leads to oversized towers in humid climates and undersized towers in dry climates.
• Assuming wet-bulb temperature is constant when dry-bulb changes. Unlike dew point, wet-bulb temperature changes with dry-bulb temperature even at constant moisture content. Heating air at constant humidity ratio raises the wet-bulb temperature (though less than the dry-bulb rise).
• Applying the wrong Sprung constant for the sensor type. The psychrometric constant A = 0.000799 °C⁻¹ (0.000437 °F⁻¹) applies to mechanically aspirated wet-bulb sensors. Naturally ventilated sensors use a larger constant (approximately 0.0012 °C⁻¹), producing a lower computed humidity ratio.
• Neglecting altitude correction for wet-bulb calculations. At higher altitudes, lower atmospheric pressure increases evaporation rate, lowering the wet-bulb temperature for the same moisture content. The standard formulas assume sea-level pressure (101.325 kPa / 14.696 psi).
• Using wet-bulb temperature for condensation risk assessment. Condensation occurs when a surface falls below the dew point, not the wet-bulb temperature. Use the Dew Point Temperature Calculator for condensation analysis.

Frequently Asked Questions