Mixed Air Temperature Calculator
Enter the temperature and flow rate of each air stream to find the resulting mixed air temperature. The calculator handles two-stream or three-stream mixing and six solve modes: find the mixed temperature, any individual stream temperature, or the required flow rate. Switch between Celsius and Fahrenheit and between CFM, m3/s, and L/s. Results update instantly as you type.
Formula
Worked example
A rooftop AHU draws 2,000 CFM of 95°F outdoor air and 8,000 CFM of 75°F return air. Mixed air temperature = (95 x 2,000 + 75 x 8,000) / (2,000 + 8,000) = (190,000 + 600,000) / 10,000 = 79°F. The outdoor-air fraction is 2,000 / 10,000 = 20%.
What is mixed air temperature and why it matters
In any central air-handling unit (AHU), outdoor air and return air from the occupied zones are blended in a mixing plenum before being conditioned by coils and distributed through supply ducts. The resulting temperature of that blend is called the mixed air temperature (MAT). MAT governs how much cooling or heating the downstream coil must provide to reach the desired supply-air setpoint, and it determines whether a free-cooling economizer can be used instead of mechanical refrigeration. Underestimating MAT on a hot day leads to undersized cooling coils; overestimating it wastes energy by overcooling on mild days.
The mixed air temperature formula
The formula is a flow-weighted average of the two (or more) stream temperatures: T_mix = (T1 x Q1 + T2 x Q2) / (Q1 + Q2). Each stream temperature is multiplied by its volumetric flow rate, the products are summed, and the result is divided by the total flow. The formula assumes (1) adiabatic mixing - no heat exchange with surroundings, (2) uniform density across both streams - valid for standard HVAC at near-atmospheric pressure, and (3) well-mixed conditions - no stratification. For high-precision psychrometric work, mass flow rates based on dry-air density replace volumetric flows, but the volumetric approximation is accurate to within 1% for typical HVAC temperature and humidity ranges.
Outdoor-air fraction and ASHRAE 62.1 minimums
The outdoor-air fraction (OAF) is the percentage of supply air that comes from outdoors: OAF = Q_outdoor / (Q_outdoor + Q_return). ASHRAE Standard 62.1 sets minimum OAF requirements for each occupancy type to maintain acceptable indoor air quality. Offices typically require 15-20% OA; classrooms 20-35%; hospital rooms 30-50%; laboratories may require 100% OA when fume hoods are present. An economizer cycle raises the OAF to as high as 100% whenever outdoor conditions are mild enough to provide free cooling, reducing compressor energy. At 100% OA (full economizer), T_mix equals the outdoor temperature.
Reverse-solve modes and design applications
Beyond finding the mixed temperature, engineers often need to work backward. Given a target mixed temperature (such as a coil entering-air setpoint of 55°F for a DX system), how much outdoor air can the economizer admit before the mixed air is too warm? Or, given fixed flow rates, what outdoor temperature triggers switchover to mechanical cooling? This calculator solves all six variants: forward mixing (two and three streams), find Q1, find Q2, find T1, and find T2. The three-stream mode covers AHUs with separate outside-air, return-air, and exhaust-bypass or transfer-air connections. All modes use the same underlying energy balance and produce the same outdoor-air fraction breakdown.
Typical HVAC outdoor-air fractions by occupancy
| Building type | Typical OA fraction | Notes |
|---|---|---|
| Office (densely occupied) | 15-20% | Minimum ventilation per person plus per area |
| Classroom | 20-35% | Higher fresh air demand per ft2 of floor area |
| Conference room | 25-40% | Transient high-density occupancy drives peak demand |
| Retail / lobby | 10-15% | Large volume dilutes occupancy-based loads |
| Hospital patient room | 30-50% | Infection-control guidelines mandate high OA |
| Laboratory (fume hoods) | 90-100% | Hazardous exhaust often prohibits recirculation |
| Restaurant kitchen | 80-100% | Makeup air replaces exhaust hood volume |
| Natatorium (pool) | 50-70% | High humidity requires large OA for dehumidification |
ASHRAE 62.1 guidance on minimum outdoor-air fractions for common building types.
Frequently asked questions
What is the mixed air temperature formula?
T_mix = (T1 x Q1 + T2 x Q2) / (Q1 + Q2). Each stream temperature is multiplied by its volumetric flow rate; the products are summed and divided by total flow. For three streams, add a third T3 x Q3 term in the numerator and Q3 in the denominator.
Does it matter whether I use CFM, m3/s, or L/s?
No, as long as you use the same unit for both streams. The formula is a ratio, so the units cancel. The calculator lets you switch between CFM, m3/s, and L/s for convenience.
Should I use volumetric or mass flow rates?
For standard HVAC design, volumetric flow rates give results accurate to within about 1%. Strictly correct psychrometric analysis uses mass flow rates of dry air (kg/s or lb/min), because air density varies slightly with temperature and humidity. At temperatures below 120°F (50°C) and relative humidities below 90%, the volumetric approximation is adequate for coil sizing and economizer control.
What is an outdoor-air fraction and what is a typical value?
The outdoor-air fraction is the ratio of outdoor air flow to total supply air flow: OAF = Q_OA / Q_total. ASHRAE 62.1 minimums range from about 10% for large retail spaces to 100% for laboratories with fume hoods. Most office and commercial systems run 15-30% OA at design conditions, rising to 100% during economizer operation.
What is an economizer, and how does mixed air temperature relate to it?
An air-side economizer raises the outdoor-air damper to provide free cooling whenever outdoor air is cool and dry enough. Controllers compare the outdoor dry-bulb temperature (or enthalpy) to a setpoint - typically 55-65°F (13-18°C) dry bulb. When outdoor air is below the setpoint, 100% outdoor air is admitted, setting T_mix equal to the outdoor temperature and eliminating or reducing compressor load. The mixed air calculator shows directly what T_mix will be at any outdoor-air fraction, making it straightforward to find the crossover point.
How do I find the outdoor-air flow rate needed to hit a target mixed temperature?
Use the "Solve for stream 1 flow rate (Q1)" mode. Enter the outdoor temperature (T1), the return-air temperature (T2), the return-air flow rate (Q2), and the desired mixed temperature. The calculator rearranges the formula as Q1 = Q2 x (T2 - T_mix) / (T_mix - T1) and solves directly.
Does altitude affect the calculation?
Altitude reduces air density, so a given volumetric flow rate carries less mass - and therefore less sensible heat - than at sea level. The volumetric mixed-air formula still gives the correct mixed temperature (the ratio of flow rates is unchanged), but you should derate coil and fan capacities for altitude when sizing equipment. At 5,000 ft (1,500 m), air density is roughly 17% lower than at sea level.
Can I use this for humidity or enthalpy mixing?
The same flow-weighted formula applies to humidity ratio (g/kg or grains/lb): W_mix = (W1 x Q1 + W2 x Q2) / (Q1 + Q2). Enthalpy mixing follows the same pattern with mass flow rates. This calculator focuses on dry-bulb temperature; for full psychrometric analysis including dew point, enthalpy, and relative humidity of the mixed stream, use a dedicated psychrometric tool.