Mixed Air Calculator (HVAC Temperature)
Calculate mixed air temperature from outdoor and return air using weighted average formulas. Includes economizer threshold check.
All calculations happen in your browser. No data is sent to any server.
Calculated based on ASHRAE Standard weighted average:
MAT = (OA_CFM × OA_T + RA_CFM × RA_T) ÷ (OA_CFM + RA_CFM) — equivalent to
MAT = (OAF × OA_T) + ((1 − OAF) × RA_T).
Results assume dry air and do not account for humidity or latent heat.
How to use this calculator
Three simple steps to calculate your mixed air temperature
Choose your method
Select either "Outdoor Air Fraction" or "Airflow (CFM)" mode based on your available data.
Enter your values
Fill in outdoor temperature, return temperature, and either fraction or airflow CFM.
Calculate & interpret
Click "Calculate" to get mixed air temperature and read the operational guidance.
How the mixed air calculation works
Mixed air temperature (MAT) is the temperature of air entering the HVAC cooling or heating coil after outdoor and return air blend. The calculation uses a weighted average based on each airstream's proportion of total flow [citation:5].
Where %OA is outdoor air fraction, %RA = 1 − %OA, OAT is outdoor temperature, and RAT is return temperature from conditioned space [citation:7]. The calculator also accepts CFM airflow values directly, using the formula MAT = (OAcfm × OAT + RAcfm × RAT) / (OAcfm + RAcfm). This method follows ASHRAE Standard weighted average principles for dry-bulb temperature mixing.
Key factors that affect mixed air temperature
Outdoor air fraction
Higher %OA pulls the MAT closer to outdoor temperature. In cooling season, reducing OA lowers coil load and saves energy.
Outdoor vs return temperature difference
A wide gap between OAT and RAT makes small fraction changes cause large MAT swings — critical during heat waves or freezing weather.
Humidity (latent load)
Dry-bulb calculation ignores moisture. When outdoor air is humid, actual cooling demand exceeds what temperature alone suggests [citation:7].
When to use mixed air temperature calculations
Cooling coil sizing
Designers need entering coil temperature to select capacity. For example, 95°F outdoor + 75°F return at 20% OA gives 79°F MAT — a moderate load compared to 100% OA at 95°F.
Economizer control setup
When outdoor air is cooler than return air (e.g., 55°F OA vs 72°F RA), increasing OA fraction reduces mechanical cooling. MAT confirms whether economizer operation actually lowers coil load [citation:7].
Freeze protection for coils
In heating season, MAT below 40°F risks coil freezing. Operators increase return air fraction or add preheat when MAT drops below safe threshold.
Accuracy and limitations
This calculator assumes dry air and uses only dry-bulb temperature — the standard method for sensible heat calculations in HVAC design [citation:5]. Results are accurate for determining coil entering temperature when the coil operates in dry (sensible-only) conditions.
What the tool does not account for: Humidity (latent heat), wet-bulb temperature, or condensation effects. When outdoor air is humid, the mixed air condition includes moisture that increases total cooling load beyond what dry-bulb temperature predicts. For humid climates, use enthalpy-based methods alongside this calculator [citation:7].
Professional HVAC engineers reference ASHRAE Handbook — Fundamentals for detailed psychrometric analysis. The calculation formula is sourced from industry-standard HVAC design references.
Frequently asked questions
The mixed air temperature entering HVAC coils, based on outdoor temperature, return temperature, and either outdoor air fraction or CFM airflow values.
MAT = (%OA × OAT) + (%RA × RAT). For CFM mode: MAT = (OAcfm × OAT + RAcfm × RAT) / (OAcfm + RAcfm). Both are weighted averages from the law of conservation of energy [citation:5].
The outdoor air fraction. A 10% increase in OA draws MAT 10% closer to outdoor temperature. When OAT and RAT differ by 20°F or more, small fraction changes significantly affect results.
The dry-bulb calculation is mathematically exact for sensible heat. However, actual mixed air downstream may reach saturation if humid air mixes with cold outdoor air, causing condensation [citation:2]. In humid conditions, the true coil load is higher than dry-bulb suggests.
Compare MAT to your coil design range. For cooling, MAT above 80°F indicates high load — consider reducing OA fraction or adding capacity. For heating, MAT below 40°F suggests freeze protection needed. Use the result to adjust economizer or ventilation settings.
No. All calculations run in your browser. No inputs leave your device — no server, no analytics, no storage.
For coil selection, system design, or troubleshooting existing equipment. This calculator provides preliminary entering temperature. Licensed mechanical engineers perform full load calculations considering humidity, occupancy, and climate data.