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Wet bulb calculator

Thermodynamic wet-bulb temperature from dry bulb and relative humidity, at any altitude.

Free, right in your browser. Every result shows its work.

°F
%
ft

Sets the pressure from the standard atmosphere. 0 = sea level.

Result

Wet bulb
67.18°F
Dew point
54.43°F
Relative humidity
30%
Humidity ratio
0.009002lb/lb (63.01 gr/lb)
Enthalpy
31.51Btu/lb dry air
Specific volume
14.06ft³/lb dry air
Vapor pressure
0.2097psia
Pressure
14.696psia
  1. 1.Pressure at 0 ft: p = 14.696 psia
  2. 2.Saturation pressure at 90 °F (Hyland–Wexler): p_ws = 0.6989 psia
  3. 3.p_w = RH × p_ws = 0.3 × 0.6989 = 0.2097 psia
  4. 4.W = 0.621945 × p_w / (p − p_w) = 0.009002 lb/lb
  5. 5.RH = p_w / p_ws = 30%
  6. 6.Dew point: the temperature where p_ws(t) = p_w → 54.43 °F
  7. 7.Wet bulb: solve the ASHRAE wet-bulb relation for this W → 67.18 °F
  8. 8.h = 0.240(90) + 0.009002 × [1061 + 0.444(90)] = 31.51 Btu/lb dry air
  9. 9.v = 0.370486 × (90 + 459.67)(1 + 1.607858W) / p = 14.06 ft³/lb dry air

How it's tested on the exam

Shows up on: PE Mechanical (HVAC & Refrigeration) · PE Mechanical (Thermal & Fluid Systems)

  • Wet bulb sets the limit for evaporative cooling. A cooling tower's leaving water can approach the entering air's wet bulb but never reach it; an evaporative cooler's leaving air likewise.
  • Wet bulb and dew point are different. At anything below 100% RH the order is always dew point < wet bulb < dry bulb.
  • Enthalpy tracks wet bulb. Lines of constant wet bulb and constant enthalpy are nearly parallel on the chart — useful for a fast check, not exact.

Worked example

Outdoor air is 90 °F and 30% RH at sea level. What is the lowest temperature a direct evaporative cooler could approach?

  1. 1.Find W from 90 °F and 30% RH: 0.00900 lb/lb
  2. 2.Solve the ASHRAE wet-bulb relation for that W: 67.2 °F

About 67.2 °F — the entering wet bulb is the limit.

Where it is in the Handbook

  • Psychrometrics — Psychrometric Properties — PE Mechanical Reference Handbook

Section names as printed in the current NCEES PE Reference Handbooks (Mechanical 2.1, Civil 2.2); numbering can shift between versions. On exam day you'll use your own approved calculator — here's which models NCEES allows.