CastorPrep

Dew point calculator

Dew point from dry bulb and relative humidity — the surface temperature where moisture starts to condense.

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

°F
%
ft

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

Result

Dew point
55.12°F
Wet bulb
62.55°F
Relative humidity
50%
Humidity ratio
0.009236lb/lb (64.65 gr/lb)
Enthalpy
28.11Btu/lb dry air
Specific volume
13.68ft³/lb dry air
Vapor pressure
0.215psia
Pressure
14.696psia
  1. 1.Pressure at 0 ft: p = 14.696 psia
  2. 2.Saturation pressure at 75 °F (Hyland–Wexler): p_ws = 0.4301 psia
  3. 3.p_w = RH × p_ws = 0.5 × 0.4301 = 0.215 psia
  4. 4.W = 0.621945 × p_w / (p − p_w) = 0.009236 lb/lb
  5. 5.RH = p_w / p_ws = 50%
  6. 6.Dew point: the temperature where p_ws(t) = p_w → 55.12 °F
  7. 7.Wet bulb: solve the ASHRAE wet-bulb relation for this W → 62.55 °F
  8. 8.h = 0.240(75) + 0.009236 × [1061 + 0.444(75)] = 28.11 Btu/lb dry air
  9. 9.v = 0.370486 × (75 + 459.67)(1 + 1.607858W) / p = 13.68 ft³/lb dry air

How it's tested on the exam

Shows up on: PE Mechanical (HVAC & Refrigeration)

  • Dew point is a property of the air, not the surface. Any surface colder than the air's dew point — a supply duct, a cold-water pipe, a window — will sweat.
  • Sensible cooling doesn't move the dew point. Cooling air without removing moisture keeps W, and therefore the dew point, constant until the air reaches it.
  • Below freezing it's a frost point. Under 32 °F the saturation pressure is taken over ice, which is what this calculator does.

Worked example

A room is held at 75 °F and 50% relative humidity. Will a chilled-water pipe at 52 °F sweat?

  1. 1.p_w = 0.50 × p_ws(75 °F)
  2. 2.Dew point is the temperature where p_ws(t) = p_w: 55.1 °F
  3. 3.52 °F is below 55.1 °F

Yes — the pipe surface is below the 55.1 °F dew point, so it condenses. Insulate it with a vapor retarder.

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.