Psychrometric Calculator

A psychrometric calculator computes the thermodynamic properties of moist air from a small set of known measurements. Enter the dry-bulb temperature plus any one of relative humidity, wet-bulb temperature, dew point, or humidity ratio, and this tool returns the full state: relative humidity, humidity ratio, enthalpy, specific volume, wet-bulb and dew-point temperatures, saturation and vapor pressures, air density, and absolute humidity — all from the Tetens and ASHRAE psychrometer equations.

Wet-bulb temperature cannot be solved in closed form, so this calculator iterates the ASHRAE psychrometer (adiabatic-saturation) equation: W = ((2501 − 2.43·Twb)·Ws(Twb) − 1.006·(Tdb − Twb)) / (2501 + 1.89·Tdb − 4.19·Twb). Bisection finds the Twb that reproduces your known humidity ratio. At 25 °C and 50 % RH the result is 17.9 °C, matching published psychrometric tables within 0.1 °C.

Dew point is the temperature at which the current vapor pressure becomes the saturation pressure. From vapor pressure Pv = RH · Psat(Tdb), dew point is the inverse of the Tetens equation: Tdp = 237.3 · ln(Pv/0.6108) / (17.27 − ln(Pv/0.6108)), with Pv in kPa. For 25 °C air at 50 % RH, Pv is 1.584 kPa and the dew point is 13.9 °C.

Humidity ratio (ω) is the mass of water vapor per mass of dry air, usually shown in g/kg or grains per pound. It is calculated as ω = 0.62198 · Pv / (P − Pv), where Pv is vapor pressure and P is total atmospheric pressure. Unlike relative humidity, it does not change when air is heated or cooled without adding or removing moisture — at 25 °C, 50 % RH and sea level it equals 9.88 g/kg.

Relative humidity is a percentage: how full the air is relative to saturation at its current temperature (RH = Pv / Psat). Humidity ratio is an absolute mass ratio (kg water per kg dry air). Warm the air and RH falls while humidity ratio stays constant; that is why humidity ratio, not RH, is used for coil loads, ventilation, and drying calculations.

Higher altitude means lower atmospheric pressure, and humidity ratio ω = 0.62198 · Pv / (P − Pv) grows as P shrinks. At the same 25 °C and 50 % RH, air at 1500 m (P ≈ 84.5 kPa) holds about 11.9 g/kg versus 9.9 g/kg at sea level, and its specific volume is larger. Use the altitude presets or enter station pressure directly for correct results.

Enthalpy is the total heat content per kilogram of dry air: h = 1.006·T + ω·(2501 + 1.86·T) kJ/kg. The first term is sensible heat of the dry air; the second is latent heat carried by the vapor (about 2501 kJ/kg at 0 °C). At 25 °C and 50 % RH, h = 50.3 kJ/kg — roughly half sensible, half latent. Enthalpy differences drive every heating, cooling, and dehumidification load calculation.

Specific volume v is the volume occupied by one kilogram of dry air plus its moisture: v = 0.287 · T_K · (1 + 1.6078·ω) / P, in m³/kg with P in kPa. Fan and duct sizing needs it to convert mass flow to volume flow (CFM). At 25 °C, 50 % RH, sea level, v = 0.858 m³/kg.

The chart plots dry-bulb temperature on the horizontal axis against humidity ratio on the vertical axis, with curved relative-humidity lines and the bold saturation curve on the left. Your state point is marked with crosshairs; the dashed horizontal line to the saturation curve reads the dew point, and the highlighted curve through the point is its relative humidity. Click anywhere on the chart to load that state into the calculator.

No. The wet bulb is cooled by evaporation, so it is always at or below the dry bulb. The two are equal only for saturated air (100 % RH) — and the dew point joins them there too. If you enter a wet-bulb value above the dry-bulb temperature, the calculator flags it as invalid.

Wet-bulb temperature sets the physical limit for evaporative cooling: no evaporative cooler or cooling tower can cool below the entering wet bulb. It also determines coil condensation, dehumidification capacity, and heat-stress indices. Designers size equipment against local design wet-bulb values, not dry-bulb alone.

This tool solves the properties of a single air state point (one dry-bulb temperature plus one moisture property). A mixed-air calculator blends two different airstreams — for example outdoor air plus return air — into one mixture. For mixing two streams, use our mixed-air calculator; for the dry-bulb-only weighted average of several streams, use our mixed-air-temperature calculator.

Saturation pressure uses the Tetens equation, which matches ASHRAE reference tables within 0.35 % from −40 to 50 °C (typically under 0.1 % near room temperature). The wet-bulb iteration converges to 0.001 °C. The sanity check — 25 °C, 50 % RH, 101.325 kPa → Psat 3.168 kPa, ω 9.877 g/kg, h 50.31 kJ/kg, dew point 13.86 °C — reproduces published table values.

Both. SI mode uses °C, kPa, g/kg, kJ/kg, and m³/kg; IP mode uses °F, inHg, grains per pound, Btu/lb, and ft³/lb. The internal math always runs in SI (Tetens + ASHRAE equations) and converts only at input and output, so both unit systems give identical physical answers.