Dew Point & Relative Humidity (Dry/Wet Bulb)
Calculate relative humidity and dew point from dry bulb and wet bulb temperature readings. Essential for psychrometric analysis and HVAC commissioning.
Results
What is it?
Relative humidity (RH) expresses how much moisture air holds relative to its maximum capacity at that temperature. The dew point is the temperature at which air must be cooled (at constant pressure) for condensation to begin. These two values are the foundation of psychrometric analysis � used to design HVAC systems, predict condensation on cold surfaces, and ensure occupant comfort.
How to use
Use a sling psychrometer or digital hygrometer with wet/dry bulb readings. Enter both temperatures. The calculator applies the Magnus formula for saturation vapor pressure and the Sprung formula for actual vapor pressure. RH and dew point are derived from the vapor pressure ratio.
Example scenario
Outdoor summer conditions: 75�F dry bulb, 62�F wet bulb. Calculator gives RH � 53%, dew point � 54�F. The chilled water coil leaving air temperature of 55�F is just above the dew point, so the coil will dehumidify slightly. ASHRAE 55 comfort zone for 75�F is RH 30�60%, so this is within comfort range.
Pro tip
A dew point above 60�F (15.6�C) feels "muggy" to most people. Above 70�F dew point is extremely uncomfortable. For IAQ and mold prevention, keep indoor dew point below 59�F (15�C). In HVAC commissioning, measure wet and dry bulb at coil entering and leaving air to calculate sensible heat ratio and verify latent cooling capacity.