Psychrometric Calculator - Humidity Ratio, Dew Point, Enthalpy

A psychrometric calculator turns two air measurements - dry bulb temperature and relative humidity - plus a station pressure into the full set of ASHRAE moist air properties: humidity ratio, specific volume, specific enthalpy, dew point, wet bulb, absolute humidity, and degree of saturation.

• • HVAC cooling coil selection: Compute the inlet and outlet specific enthalpy of an evaporator coil so a sensible heat ratio matches the manufacturer's catalog data.
• • Indoor air quality and thermal comfort: Check whether an occupied room sits inside the ASHRAE 55 comfort polygon by comparing humidity ratio and operative temperature.
• • Industrial drying and curing: Estimate how much water a drying oven can pull from a product by tracking humidity ratio before and after the process.
• • Greenhouse humidity control: Set a target vapor pressure deficit by pairing dry bulb and relative humidity readings from a greenhouse sensor.

Psychrometrics is the engineering science of moist air. The psychrometric calculator replaces a printed chart with a numeric answer that matches the ASHRAE Handbook of Fundamentals. Cooling coils transfer both sensible and latent heat, and storage rooms must stay below a critical relative humidity. The calculator returns every dependent property from the same two inputs so each downstream check pulls from one consistent set of values.

When you need the mass per unit volume of the same moist air sample, the air density calculator uses the same ideal gas extension with humidity to give density in kg/m^3, g/L, and lb/ft^3.

The calculator converts dry bulb temperature and relative humidity to partial vapor pressure with the Tetens equation, then applies the ASHRAE humidity ratio, specific volume, and enthalpy formulas. Dew point comes from the inverse Magnus form, absolute humidity from the ideal gas law applied to the vapor, and wet bulb from the Stull 2011 closed-form approximation.

• Tdb: Dry bulb temperature in degrees Celsius, converted to Kelvin for the gas-law terms.
• RH: Relative humidity as a fraction from 0 (dry air) to 1 (saturated air).
• P: Absolute atmospheric pressure in pascals at the air sample location.
• Ps: Saturation vapor pressure at Tdb from Tetens in kilopascals; Pv equals RH times Ps.
• W: Humidity ratio in kilograms of water vapor per kilogram of dry air.
• h: Specific enthalpy in kJ/kg of dry air, the ASHRAE sensible plus latent sum.

The Tetens equation sets saturation vapor pressure at the dry bulb temperature. Multiplying by relative humidity gives the actual partial pressure, and the humidity ratio follows from the ideal gas mixing law. Specific volume and enthalpy are weighted by the humidity ratio. Dew point uses the same Magnus constants in reverse, and the wet bulb follows from the closed-form Stull 2011 expression evaluated below.

According to Stull, 2011, J. Appl. Meteor. Climatol. 50: 226 (Wet-Bulb Temperature from Relative Humidity and Air Temperature), the closed-form approximation Twb = T*atan(0.151977*sqrt(RH+8.313659)) + atan(T+RH) - atan(RH-1.676331) + 0.00391838*RH^1.5*atan(0.023101*RH) - 4.686035 returns wet bulb from dry bulb and relative humidity with an absolute error under about 0.4 degrees C between 5 and 99 percent RH, the exact expression this calculator evaluates for the Twb output.

When you want to step back and solve for pressure, volume, or moles instead of a property of moist air, the ideal gas calculator takes the same P, V, n, T inputs and returns whichever gas-law quantity the equation leaves open.

Four building blocks cover every number the psychrometric calculator returns.

For a humidity-focused companion that starts from the same Tetens equation and reports the difference between saturation and actual vapor pressure, the vapor pressure deficit calculator is the natural next step.

Five steps turn a temperature and humidity reading into a defensible moist air state.

1. 1 Enter the dry bulb temperature: Type the air temperature in degrees Celsius. Leave the default of 25 degrees C for a typical indoor sample.
2. 2 Enter the relative humidity: Type the relative humidity as a percentage from 0 (dry air) to 100 (saturated). The default of 50 percent matches a comfortable office.
3. 3 Enter the station pressure: Type the absolute atmospheric pressure at the sample location. The default of 101.325 kPa matches ISA standard sea level. Use the pressure unit selector for hPa, mbar, atm, mmHg, or psi.
4. 4 Read the moist air properties: The result panel returns humidity ratio, specific volume, specific enthalpy, dew point, absolute humidity, degree of saturation, wet bulb, and the two vapor pressures used in the calculation.
5. 5 Check the dew point for condensation risk: Compare the dew point to the surface temperatures you control. Any surface cooler than the dew point will condense moisture out of the air.

A weather station reports 30 degrees C, 60 percent RH, and 101.325 kPa. The calculator returns a humidity ratio of 0.01316 kg/kg, an enthalpy of 63.67 kJ/kg dry air, a dew point of 21.40 degrees C, and a wet bulb of 23.40 degrees C. Chilled water coils below 21 degrees C will form condensate.

When you only need the mass of water vapor per cubic meter of air and do not need humidity ratio or enthalpy, the absolute humidity calculator takes dry bulb, relative humidity, and pressure and returns that single number.

A dedicated psychrometric calculator removes the lookup work that printed psychrometric charts and hand-solved humidity formulas still require.

• • Full ASHRAE state in one calculation: Returns nine moist air properties at once: humidity ratio, specific volume, specific enthalpy, dew point, absolute humidity, degree of saturation, wet bulb, and both vapor pressures.
• • Works at any altitude: Accepts absolute pressure as an input, so the same calculator handles sea level, mountain stations, and pressurized cabins.
• • Printable chart replacement: Produces the same humidity ratio and enthalpy values a printed ASHRAE chart gives, so engineers skip manual interpolation.
• • Dew point and wet bulb on demand: Reports dew point and wet bulb from closed-form expressions so you do not have to solve the iterative energy balance by hand.

When the same dry bulb and relative humidity reading also has to predict the condensation level for fog or cloud formation, the cloud base calculator turns the dew point and a lapse rate into a cloud base height.

Three inputs drive the answer, and three limits tell you when the answer starts to drift from real measurements.

• • The Tetens equation fits saturation vapor pressure over liquid water between 0 and 50 degrees C. Outside that range the saturation term drifts and the dew point loses precision near the freezing point.
• • The Stull 2011 wet bulb formula is a closed-form approximation with about a 0.4 degree C error band between 5 and 99 percent RH. Iterate the energy balance for high-precision wet bulb work.
• • The calculator treats the gas as an ideal mixture of dry air and water vapor. Pressures above 10 atmospheres or below 0.1 atmosphere need non-ideal gas corrections the formula does not capture.

According to Engineering Toolbox - Moist Air Psychrometric Table (29.92 inHg), the ASHRAE Handbook of Fundamentals reference values for saturation pressure, water vapor weight, specific volume, and total enthalpy at standard atmospheric pressure (29.92 inHg, 101.325 kPa) are tabulated against dry bulb temperature from minus 40 to 200 degrees F, giving engineers a direct cross-check against the calculator's outputs at sea level without reaching for a printed chart.

According to Alduchov and Eskridge, 1996, J. Appl. Meteor. 35: 601 (Improved Magnus Form Approximation of Saturation Vapor Pressure), the August-Roche-Magnus (Magnus-Tetens) form Ps = 6.1094*exp(17.625*T/(T+243.04)) hPa with T in degrees Celsius matches ASHRAE-grade saturation pressure over liquid water to within about 0.4 percent between 0 and 50 degrees C, providing the regression-based benchmark for the simplified Tetens constants (0.61078, 17.27, 237.3) used in this calculator's Ps output.

When you also need the dry bulb temperature at altitude to feed back into the humidity calculation, the altitude temperature calculator gives the ICAO lapse-rate temperature at any elevation above sea level.