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Air Mixing Calculator

Find the temperature, humidity, dew point and enthalpy of air from two streams mixed together — such as return air and outdoor ventilation air.

Method reviewed October 25, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Mixed-air dry-bulb
78.9°F
Mixed-air relative humidity
48.4%

Mixed air: 78.9 °F at 48 % RH (dew point 57.8 °F).

Mixed-air dew point
57.8°F
Mixed-air wet-bulb
65.3°F
Humidity ratio
71.3gr/lb
Enthalpy
30.09BTU/lb
Stream B share of the mix (by mass)
19.3%
Show the working
  1. Dry-air mass of each stream ∝ CFM ÷ specific volume
    stream B is 19.3 % of the dry-air mass
  2. Mixed humidity ratio and enthalpy are mass-weighted averages
    W = 71.3 gr/lb, h = 30.09 BTU/lb
  3. Mixed dry-bulb from enthalpy and humidity ratio
    78.89 °F

How mixing is calculated

When two air streams mix without heating or cooling, the mass of dry air, the moisture and the energy are all conserved. The calculator converts each airflow to a mass of dry air using its specific volume, then takes the mass-weighted average of the humidity ratio and enthalpy. The mixed temperature, relative humidity, dew point and wet-bulb follow from those two properties.

Moist-air properties use the ASHRAE Handbook—Fundamentals equations at the standard-atmosphere pressure for your elevation, and every release is checked against PsychroLib, an independent implementation.

Mass, not volume

A cubic foot of warm air weighs less than a cubic foot of cool air, so averaging by CFM slightly overweights the warmer stream. For ordinary return and outdoor air the difference is small — a fraction of a degree — but it grows as the temperatures move further apart. The result also shows the outdoor stream’s share by mass.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Stream A airflow (e.g. return air)
1,600 CFM
Stream A dry-bulb
75 °F
Stream A relative humidity
50 %
Stream B airflow (e.g. outdoor air)
400 CFM
Stream B dry-bulb
95 °F
Stream B relative humidity
40 %
Site elevation
0 ft

Result

Mixed-air dry-bulb
78.9 °F
Mixed-air relative humidity
48.4 %
Mixed-air dew point
57.8 °F
Mixed-air wet-bulb
65.3 °F
Humidity ratio
71.3 gr/lb
Enthalpy
30.09 BTU/lb

Mixed air: 78.9 °F at 48 % RH (dew point 57.8 °F).

Assumptions

  • Adiabatic mixing: dry-air mass, moisture and enthalpy are conserved. Properties use the ASHRAE 2017 equations (as in PsychroLib) at the pressure for your elevation.

Limitations

  • If the mixed state would be supersaturated (fog), the result is not physical — check the inputs.

Frequently asked questions

What is mixed air temperature used for?

It is the temperature entering the coil when outdoor air is mixed with return air, so it sets the coil load and is used to check economizers and ventilation dampers.

Can I estimate the outdoor-air fraction from temperatures?

Yes, roughly: outdoor fraction ≈ (return − mixed) ÷ (return − outdoor) by temperature. It is unreliable when the return and outdoor temperatures are close together.

Why can mixing two clear air streams make fog?

Mixing very cold air with warm, humid air can produce a mix above 100 % relative humidity, because saturation is curved on a psychrometric chart. The calculator stops and tells you if your inputs would do that, because a single temperature and humidity cannot describe fog.

Sources

  1. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  2. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.

Spotted an error? Report a problem with this calculator — see our corrections policy.