# Mixed Air Calculator (Psychrometric Mixing)

URL: https://speedycalculator.com/calculators/physics/mixed-air/
Last updated: 2026-10-30

> The mixed air calculator finds the temperature, humidity ratio, relative humidity and enthalpy when outdoor air and return air are mixed in an air-handling unit.

## Inputs

- Air pressure (kPa, hPa, psi)
- Outdoor airflow (m³/h, CFM, L/s)
- Outdoor temperature (°C, °F, K)
- Outdoor RH (%)
- Return airflow (m³/h, CFM, L/s)
- Return air temperature (°C, °F, K)
- Return air RH (%)

## Results

- Mixed humidity ratio
- Mixed enthalpy
- Mixed air temperature
- Mixed relative humidity
- Outdoor air fraction

## Formula

```
W = 0.622·p_v/(p − p_v),  h = 1.006T + W(2501 + 1.86T)  · mixing by mass:  W_mix = Σṁ·W/Σṁ,  h_mix = Σṁ·h/Σṁ
```

## Explanation

HVAC systems blend fresh outdoor air with recirculated return air. The mixture's temperature and humidity – found by mass-weighted averaging of humidity ratio and enthalpy – set the load on the coils.

### How to use the Mixed Air Calculator

- **Air pressure**
- **Outdoor airflow**
- **Outdoor temperature**
- **Outdoor RH (%)**
- **Return airflow**
- **Return air temperature**
- **Return air RH (%)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the air pressure and the airflow, temperature and relative humidity of the outdoor and return air streams.

### Tips

- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.

### Common mistakes to avoid

- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).

### Inputs explained

FieldWhat to enterAir pressureUnits: kPa, hPa, psiOutdoor airflowUnits: m³/h, CFM, L/sOutdoor temperatureUnits: °C, °F, KOutdoor RH (%)A numberReturn airflowUnits: m³/h, CFM, L/sReturn air temperatureUnits: °C, °F, KReturn air RH (%)A number

### What each result means

- **Mixed humidity ratio** = ([m1]×[W1] + [m2]×[W2])÷([m1] + [m2])×1000
- **Mixed enthalpy** = ([m1]×(1.006×[Outdoor temperature] + [W1]×(2501 + 1.86×[Outdoor temperature])) + [m2]×(1.006×[Return air temperature] + [W2]×(2501 + 1.86×[Return air temperature])))÷([m1] + [m2])
- **Mixed air temperature** = ([Mixed enthalpy] - 2501×[Mixed humidity ratio]÷1000)÷(1.006 + 1.86×[Mixed humidity ratio]÷1000)
- **Mixed relative humidity** = ([Mixed humidity ratio]÷1000×[Air pressure]÷(0.622 + [Mixed humidity ratio]÷1000))÷(611.2×exp(17.62×[Mixed air temperature]÷(243.12 + [Mixed air temperature])))×100
- **Outdoor air fraction** = [m1]÷([m1] + [m2])×100

### Example values

How the main result changes when **Air pressure** changes and everything else stays at the example values above:

Air pressure (kPa)Mixed air temperature (°C)50.6625.966375.9925.9644101.3325.9634126.6625.9628151.9925.9624202.6525.9619

## Worked example

Inputs — Air pressure: 101.325 kPa; Outdoor airflow: 500 m³/h; Outdoor temperature: 32 °C; Outdoor RH (%): 60; Return airflow: 1,500 m³/h; Return air temperature: 24 °C; Return air RH (%): 50.

Results — Mixed humidity ratio = 11.3847 g/kg; Mixed enthalpy = 55.1421 kJ/kg; **Mixed air temperature = 25.9634 °C**; Mixed relative humidity = 54.4294 %; Outdoor air fraction = 24.2526 %.

## Key terms

- **Return air**: Air drawn back from the conditioned space.

## FAQ

**Why mix by mass and not volume?**
Air density differs with temperature and humidity.

**Is the mixed temperature the simple average?**
Close to the flow-weighted average, but not exactly.

**What is the humidity ratio?**
Mass of water vapour per mass of dry air.

**Can mixing cause condensation?**
Yes, if the mixed state falls above saturation.

**What formula does the Mixed Air Calculator use?**
W = 0.622·p_v/(p − p_v),  h = 1.006T + W(2501 + 1.86T)  · mixing by mass:  W_mix = Σṁ·W/Σṁ,  h_mix = Σṁ·h/Σṁ.


## Sources

- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)
- [ASHRAE Handbook – Fundamentals (psychrometrics)](https://www.ashrae.org/technical-resources/ashrae-handbook)