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.

What is the Psychrometric Dew Point & Wet Bulb Calculator?

Compute dew point, wet-bulb temperature, humidity ratio in grains per pound, and enthalpy in BTU per pound from dry-bulb temperature and relative humidity using the Magnus formula and iterative psychrometric relations, with optional altitude pressure correction. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Psychrometric Dew Point & Wet Bulb Calculator

Name: Psychrometric Dew Point & Wet Bulb Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Worked example

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.

Now raise the relative humidity to 70 percent at the same dry-bulb. The dew point climbs to around 63 °F, the wet-bulb to about 66 °F, and the humidity ratio to roughly 92 grains per pound. The enthalpy rises to about 32 BTU per pound — a 14 percent increase in total heat load from that humidity jump alone, which matters significantly when sizing a cooling coil.

When each output matters most

Dew point is the condensation boundary. If supply air in a duct has a dew point of 55 °F and an uninsulated fitting runs at 50 °F, condensation forms. Use the dew point to verify that insulation keeps surfaces above the dew point, and to evaluate comfort: occupants start noticing stickiness around 60 °F dew point and become uncomfortable above 65 °F.

Humidity ratio in grains per pound is what dehumidifiers and cooling coils actually remove. Two rooms at the same relative humidity but different temperatures carry different grain counts — grains tell you the real moisture load. For dehumidifier sizing, multiply the air mass flow rate by the grain difference between entering and leaving conditions.

Enthalpy in BTU per pound of dry air captures both sensible heat (temperature) and latent heat (moisture). Comparing enthalpy across an economizer or energy-recovery ventilator gives the total energy transferred without needing separate sensible and latent calculations. The enthalpy difference times the mass flow rate of dry air is the total coil load.

Wet-bulb temperature is what a sling psychrometer measures. It is also the lower bound for evaporative cooling — no direct evaporative cooler can cool air below the entering wet-bulb temperature.

Altitude correction

At 5,000 ft elevation, atmospheric pressure falls to roughly 24.9 in Hg compared to the sea-level 29.92. Because humidity ratio depends on the ratio of vapor pressure to total pressure, the same 50 percent RH at 75 °F carries more grains per pound at altitude than at sea level — a detail that is easy to overlook and that leads to undersized dehumidification if the correction is omitted.