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Relative humidity calculator

Relative humidity from dry bulb plus dew point or wet bulb.

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

°F
°F
ft

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

Result

Relative humidity
50.53%
Dew point
60°F
Wet bulb
66.82°F
Humidity ratio
0.01104lb/lb (77.29 gr/lb)
Enthalpy
31.31Btu/lb dry air
Specific volume
13.85ft³/lb dry air
Vapor pressure
0.2563psia
Pressure
14.696psia
  1. 1.Pressure at 0 ft: p = 14.696 psia
  2. 2.Saturation pressure at 80 °F (Hyland–Wexler): p_ws = 0.5074 psia
  3. 3.p_w = p_ws at the dew point 60 °F = 0.2563 psia
  4. 4.W = 0.621945 × p_w / (p − p_w) = 0.01104 lb/lb
  5. 5.RH = p_w / p_ws = 50.53%
  6. 6.Dew point: the temperature where p_ws(t) = p_w → 60 °F
  7. 7.Wet bulb: solve the ASHRAE wet-bulb relation for this W → 66.82 °F
  8. 8.h = 0.240(80) + 0.01104 × [1061 + 0.444(80)] = 31.31 Btu/lb dry air
  9. 9.v = 0.370486 × (80 + 459.67)(1 + 1.607858W) / p = 13.85 ft³/lb dry air

How it's tested on the exam

Shows up on: PE Mechanical (HVAC & Refrigeration)

  • Relative humidity changes with temperature even when moisture doesn't. Heating air at constant W lowers its RH; that's why winter indoor air feels dry.
  • RH is a ratio of vapor pressures (p_w / p_ws at the same dry bulb), not of humidity ratios. The two differ slightly; the difference matters near saturation.
  • Mixing airstreams: mix by mass and enthalpy/humidity ratio, then recompute RH. Averaging the two RH values is wrong.

Worked example

Air at 80 °F has a dew point of 60 °F. What is its relative humidity?

  1. 1.p_w = p_ws(60 °F); p_ws = p_ws(80 °F)
  2. 2.RH = p_w / p_ws = 50.5%

About 50.5%.

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.