What formula computes the dew point?

The tool uses the Magnus approximation. It finds the saturation vapor pressure at the dry-bulb temperature, multiplies by relative humidity to get the actual vapor pressure, then inverts the Magnus equation to find the temperature at which that vapor pressure would be saturated, which is the dew point.

How is wet-bulb temperature found?

There is no closed-form solution, so the tool iterates: it guesses a wet-bulb temperature, computes the humidity ratio implied by the psychrometric wet-bulb equation, and compares it to the actual humidity ratio, adjusting until they match. This is the same balance a sling psychrometer measures physically.

What is the humidity ratio in grains per pound?

Humidity ratio is the mass of water vapor per pound of dry air. HVAC convention reports it in grains, where 7,000 grains equal one pound. It is the quantity you balance when sizing dehumidification, because two air states with the same dry-bulb can hold very different amounts of moisture.

Why does altitude change the numbers?

Humidity ratio and enthalpy depend on barometric pressure, which falls with altitude. At higher elevations the same vapor pressure corresponds to a higher humidity ratio. Entering your altitude applies the standard pressure-versus-elevation correction so grains and enthalpy are accurate for your site.

What is enthalpy used for?

Enthalpy in BTU per pound of dry air is the total heat content, sensible plus latent. Comparing the enthalpy of two air streams (for example, return versus mixed air) gives the total cooling or heating energy across a coil, which is the basis of economizer and energy-recovery decisions.

Psychrometric Dew Point & Wet Bulb Calculator

The psychrometric dew point and wet bulb calculator derives the full moist-air state from just dry-bulb temperature and relative humidity. It gives technicians the dew point, wet-bulb, humidity ratio, and enthalpy needed for duct design, dehumidifier sizing, and refrigerant charge verification, without a paper psychrometric chart.

How it works

The calculation chains standard moist-air relations:

Saturation vapor pressure at the dry-bulb temperature comes from the Magnus formula.
Actual vapor pressure is that saturation pressure times relative humidity.
Dew point is found by inverting the Magnus formula on the actual vapor pressure.
Humidity ratio is W = 0.62198 x Pv / (P - Pv), where P is barometric pressure.
Enthalpy is h = 0.240 x T + W x (1061 + 0.444 x T) in BTU per pound of dry air.
Wet-bulb temperature is solved iteratively so its implied humidity ratio matches W.

Altitude adjusts the barometric pressure P using the standard atmosphere relation, which shifts humidity ratio and enthalpy accordingly.

Example and tips

At 75 F dry-bulb and 50 percent relative humidity at sea level, the dew point is about 55 F, the wet-bulb about 62.5 F, the humidity ratio roughly 65 grains per pound, and the enthalpy near 28 BTU per pound. If the dew point of supply air rises above a surface temperature, condensation forms there, which is why you check dew point before insulating ductwork or chilled-water lines. For dehumidifier sizing, work in grains per pound, not relative humidity, because grains are what the equipment actually removes.