Formula & worked example
Tmix=(Qoa·Toa+Qra·Tra)/(Qoa+Qra); OA%=Qoa/(Qoa+Qra)·100; OA%target=(Ttarget−Tra)/(Toa−Tra)·100.
1,000 CFM outdoor air at 35°C mixed with 4,000 CFM return air at 24°C gives 26.2°C mixed air and 20% outside air.
HVAC engineering calculator · Updated Sep 17, 2026
Mixed Air Temperature Calculator — HVAC Mixing & Outside Air %
Mix outside and return air by airflow, calculate mixed-air temperature and outside-air fraction, or reverse-solve the outside-air percentage needed for a target mixed-air temperature.
Inputs
Live result
Mixed air temperature 26.2 °C (79.16 °F)
Total airflow 5,000 CFM
Outside-air fraction 20%
Return-air fraction 80%
Outside air needed for 25 °C target: 9.0909091%
Target is reachable by mixing these two streams
Assumptions & limits
- Temperature-only mixing assumes similar air heat capacity and density.
- For humid-air mixing and condensation risk, use psychrometric enthalpy and humidity-ratio analysis.
Frequently asked questions
What does this Mixed Air Temperature Calculator calculate?
Mix outside and return air by airflow, calculate mixed-air temperature and outside-air fraction, or reverse-solve the outside-air percentage needed for a target mixed-air temperature.
Why are the assumptions shown?
HVAC results depend on air properties, measured conditions and design conventions. MAXScanner exposes the governing assumptions so the result can be checked instead of hiding them behind a single number.
Can this replace final HVAC design?
No. Use it for transparent sizing and cross-checking, then verify final selections against measured conditions, equipment data and applicable design standards.