# What is the psychrometric state?

Psychrometric calculator: humidity ratio, relative humidity, dew point, wet bulb, enthalpy, specific volume and density of moist air from the dry bulb and the relative humidity, wet bulb or dew point.

- Page: https://www.acalculator.org/physics/psychrometric-calculator
- JSON spec: https://www.acalculator.org/physics/psychrometric-calculator.json
- Version: 4d1a8aaa69a3

## Default answer

Example with the default inputs (Dry-bulb temperature 76.9999999999999 °F, Humidity from Relative humidity, Relative humidity 50%, Air pressure 14.6959487755135 psi): The humidity ratio is 9.88 g/kg, with a dew point of 13.9 °C, a wet bulb of 17.9 °C and an enthalpy of 50.32 kJ/kg.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| t | Dry-bulb temperature | The air temperature from an ordinary thermometer. |
| by | Humidity from | Which humidity value you know: relative humidity, wet-bulb temperature or dew point. |
| rh | Relative humidity | The relative humidity, from 0.1% to 100%. |
| twb | Wet-bulb temperature | The wet-bulb temperature; it cannot be above the dry bulb. |
| tdp | Dew point | The dew point; it cannot be above the dry bulb. |
| p | Air pressure | The actual air pressure; 101.325 kPa (14.696 psi) at sea level. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| w | Humidity ratio (g/kg) | Grams of water vapour per kilogram of dry air: 1,000 × 0.62198 p_w ÷ (p − p_w). |
| grains | Humidity ratio (gr/lb) | Grains of water vapour per pound of dry air (7,000 grains to the pound). |
| rh | Relative humidity (%) | p_w ÷ p_sat at the dry bulb. |
| dewPoint | Dew point | The temperature at which the air becomes saturated when cooled (a frost point below 0 °C). |
| wetBulb | Wet bulb | The adiabatic saturation temperature. |
| enthalpy | Enthalpy (kJ/kg dry air) | 1.006 t + W (2501 + 1.86 t), from 0 °C dry air and liquid water. |
| volume | Specific volume (m³/kg dry air) | R_d × T ÷ (p − p_w): the volume holding 1 kg of dry air. |
| volumeFt | Specific volume (ft³/lb dry air) | The specific volume in cubic feet per pound of dry air. |
| density | Density (kg/m³) | (1 + W) ÷ v: the mass of moist air per cubic metre. |
| vapour | Vapour pressure (kPa) | The partial pressure of water vapour p_w. |
| saturationP | Saturation pressure (kPa) | The Buck saturation vapour pressure at the dry bulb. |
| saturationDeg | Degree of saturation (%) | W ÷ W_s, with W_s the humidity ratio of saturated air at the dry bulb. |

## Method

p_sat(t) by Buck (1996); p_w = RH × p_sat(t), p_sat(dew point), or from the wet bulb t*: W = ((2501 − 2.326 t*) W_s* − 1.006 (t − t*)) ÷ (2501 + 1.86 t − 4.186 t*); W = 0.62198 p_w ÷ (p − p_w); h = 1.006 t + W (2501 + 1.86 t); v = 287.058 T ÷ (p − p_w).

## Assumptions

- Moist air is a mix of ideal gases; enthalpy is zero for dry air and liquid water at 0 °C.
- Below 0 °C the saturation pressure is over ice (the dew point is a frost point); the wet-bulb balance uses 4.186 t* for the added water at any temperature.

## Worked examples

1. t = 298.15, by = rh, rh = 50, p = 101,325 gives w = 9.87943, enthalpy = 50.317847, twb = 291.039404. Source: Wikipedia, Arden Buck equation (Buck 1996: 6.1121 exp((18.678 − T/234.5)(T/(257.14 + T))) hPa over water, 6.1115 exp((23.036 − T/333.7)(T/(279.82 + T))) hPa over ice), https://en.wikipedia.org/wiki/Arden_Buck_equation (retrieved 2026-10-05); Engineering ToolBox, Humidity Ratio of Air (x = 0.62198 p_w ÷ (p_a − p_w)), https://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html (retrieved 2026-10-05); Engineering ToolBox, Enthalpy of Moist Air (h = 1.006 t + x (1.86 t + 2501) kJ/kg), https://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html (retrieved 2026-10-05).
2. t = 293.15, by = dew, tdp = 283.15, p = 101,325 gives rh = 52.509908, tdp = 283.15, w = 7.629633. Source: Wikipedia, Arden Buck equation (Buck 1996: 6.1121 exp((18.678 − T/234.5)(T/(257.14 + T))) hPa over water, 6.1115 exp((23.036 − T/333.7)(T/(279.82 + T))) hPa over ice), https://en.wikipedia.org/wiki/Arden_Buck_equation (retrieved 2026-10-05); Engineering ToolBox, Humidity Ratio of Air (x = 0.62198 p_w ÷ (p_a − p_w)), https://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html (retrieved 2026-10-05).
3. t = 303.15, by = wet, twb = 303.15, p = 101,325 gives rh = 100, tdp = 303.15. Source: Wikipedia, Arden Buck equation (Buck 1996: 6.1121 exp((18.678 − T/234.5)(T/(257.14 + T))) hPa over water, 6.1115 exp((23.036 − T/333.7)(T/(279.82 + T))) hPa over ice), https://en.wikipedia.org/wiki/Arden_Buck_equation (retrieved 2026-10-05).

## FAQ

### What is a psychrometric calculator?

It does the work of a psychrometric chart: from two properties of moist air (here the dry bulb and one humidity value) and the pressure, it finds all the others, such as humidity ratio, enthalpy and dew point.

### How do you calculate the humidity ratio?

W = 0.62198 × p_w ÷ (p − p_w), with p_w the vapour pressure and p the air pressure. At 25 °C and 50% RH at sea level, p_w = 1,584 Pa and W = 0.00988 kg/kg, or 9.88 g of water per kg of dry air.

### How do you calculate the enthalpy of moist air?

h = 1.006 × t + W × (2501 + 1.86 × t) kJ per kg of dry air, with t in °C. At 25 °C and W = 0.00988 that is 25.15 + 25.17 = 50.32 kJ/kg.

### What is the difference between wet bulb and dew point?

The dew point is the temperature at which the air becomes saturated when cooled with no water added. The wet bulb is the temperature reached by evaporating water into it until it is saturated. The dew point is never above the wet bulb, and both equal the dry bulb at 100% RH.

### How does air pressure change the results?

At a lower pressure (higher altitude) the same vapour pressure is a larger share of the air, so the humidity ratio, the enthalpy and the specific volume are higher at the same temperature and RH.

### What is the specific volume?

The volume of moist air that holds 1 kg of dry air: v = 287.058 × T ÷ (p − p_w). It is about 0.86 m³/kg (13.7 ft³/lb) at 25 °C and 50% RH.

## Sources

- Wikipedia, Arden Buck equation: Buck (1996) saturation vapour pressure over water and over ice, retrieved 2026-10-05. https://en.wikipedia.org/wiki/Arden_Buck_equation
- Engineering ToolBox, Humidity Ratio of Air: x = 0.62198 p_w ÷ (p_a − p_w), retrieved 2026-10-05. https://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html
- Engineering ToolBox, Enthalpy of Moist Air: h = 1.006 t + x (1.86 t + 2501) kJ/kg, retrieved 2026-10-05. https://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html
- Wikipedia, Density of air: the gas constant of dry air R_d = 287.058 J/(kg·K), retrieved 2026-10-05. https://en.wikipedia.org/wiki/Density_of_air