☁️ Psychrometric Calculator

Calculate moist air properties including dew point, absolute humidity, and enthalpy from dry bulb temperature and relative humidity.

Inputs

Dry Bulb Temp (T_db)

Relative Humidity (RH)

Calculations are based on Standard Atmospheric Pressure (101.325 kPa / 14.696 psi) at sea level.

Results

Dew Point (T_dp)

0.00°F

Humidity Ratio (W)

0.00grains/lb

Specific Enthalpy (h)

0.00Btu/lb

Air State Overview

Calculator Description

The psychrometric chart describes the thermodynamic state of moist air — a mixture of air and water vapor. From just the dry-bulb temperature and relative humidity you can obtain every property needed for HVAC design: humidity ratio, enthalpy, dew point and more.

What this calculator finds

From dry-bulb temperature T, relative humidity RH and atmospheric pressure P_atm it computes saturation vapor pressure P_ws, actual vapor pressure P_w, humidity ratio W and specific enthalpy h. It is the basis of cooling/heating load and humidification/dehumidification design.

Why it matters

Estimating cooling and humidification loads for air handlers
Checking dew point to assess condensation risk
Tracking state changes through mixing, heating, cooling and humidification processes

Formula

Magnus-Tetens Formula (Vapor Pressure)

A practical empirical relation for the saturation vapor pressure P_ws at temperature T (°C). The actual vapor pressure P_w follows from the relative humidity.

P_{ws} = 0.61078\,\exp\!\left(\dfrac{17.27\,T}{T + 237.3}\right)

P_w = P_{ws}\left(\dfrac{RH}{100}\right)

Humidity Ratio & Enthalpy

Humidity ratio W is the mass of water vapor per kg of dry air; specific enthalpy h is the total heat (sensible + latent) per kg of dry air.

W = \dfrac{0.621945\,P_w}{P_{atm} - P_w}

h = 1.006\,T + W(2501 + 1.86\,T)

T — Dry bulb temperature [°C (°F)]
P_ws — Saturation vapor pressure [kPa]
P_w — Actual vapor pressure [kPa]
W — Humidity ratio (kg water vapor / kg dry air)
h — Specific enthalpy (kJ per kg dry air)
P_atm — Atmospheric pressure (typically 101.325 kPa)

How the formula works

Higher temperature raises saturation vapor pressure exponentially, so the same RH carries more moisture.
Humidity ratio W is proportional to actual vapor pressure P_w and inversely related to (P_atm − P_w).
In enthalpy, 2501 is water's latent heat of vaporization (at 0°C); the W term (latent heat) usually dominates.

Worked example

At 25°C, RH 60%, P_atm 101.325 kPa: P_ws = 0.61078·exp(17.27×25/(25+237.3)) ≈ 3.17 kPa, so P_w = 3.17×0.60 ≈ 1.90 kPa. W = 0.621945×1.90/(101.325−1.90) ≈ 0.0119 kg/kg. h = 1.006×25 + 0.0119×(2501+1.86×25) ≈ 25.2 + 30.1 ≈ 55.3 kJ/kg.

Useful Tips

Practical tips

At high altitude lower P_atm slightly raises humidity ratio for the same conditions — correct P_atm.
When a cooling coil drops below dew point, dehumidification (condensation) occurs and h falls together with W.
Air mixing can be found as the mass-weighted average of humidity ratio and enthalpy.

Limitations & cautions

The Magnus-Tetens relation is accurate roughly 0–50°C; error grows at extreme temperatures.
This model uses the ideal-gas approximation and is unsuitable for high-pressure (enclosed) states.
Air with salts/impurities or plasma states is not considered.