Mixed Air Calculator — HVAC Psychrometrics
Determine the thermodynamic properties of the mixed-air point in an HVAC air-handling unit. Enter the outdoor air percentage, dry-bulb temperature and relative humidity for both the outdoor and return air streams, and get the mixed air temperature, humidity ratio and specific enthalpy at sea-level pressure.
%
°C
%
°C
%
Flow-weighted average of outdoor and return air dry-bulb temperatures
25%
outdoor airOutdoor air
25%
Return air
75%
- 1
Outdoor air fraction
25 ÷ 100 = 0.25 - 2
Return air fraction
1 − 0.25 = 0.75 - 3
Mixed dry-bulb temperature
0.25 × 35 + 0.75 × 24 = 26.75Flow-weighted average of outdoor and return air temperatures.
How does this calculator work?
Mixed air properties = weighted average of OA and RA fractions. T_ma = f·T_oa + (1−f)·T_ra; W_ma = f·W_oa + (1−f)·W_ra; h = 1.006T + W(2501 + 1.86T). For 25% OA at 35°C/60% RH mixed with 75% RA at 24°C/50% RH: T_ma ≈ 26.75°C. Calculated at sea-level pressure.
Formula
How this is calculated
In a central air-handling unit (AHU), outdoor air (OA) and recirculated return air (RA) are blended at a mixing box before conditioning. The thermodynamic properties of the resulting mixed air (MA) are computed by mass-flow-weighted averages. Because air density varies only slightly with temperature and humidity in normal HVAC conditions, volume-flow percentages are used as an approximation for mass-flow fractions.
Dry-bulb temperature and humidity ratio both mix linearly: T_ma = f · T_oa + (1−f) · T_ra, where f is the outdoor air fraction (0–1). The specific enthalpy of moist air is calculated from the standard ASHRAE psychrometric relation h = 1.006T + W(2501 + 1.86T) kJ/kg dry air, where W is the humidity ratio in kg/kg derived from relative humidity using the Magnus approximation for saturation vapour pressure at 101.325 kPa.
The mixed-air point is critical in HVAC design because it determines the cooling or heating load imposed on the conditioning coil. Increasing the outdoor air fraction raises the mixed-air enthalpy in summer (more cooling required) and lowers it in winter (more heating required). Economiser control modulates OA to minimise this load.
Frequently asked questions
The mixed-air point (MA) is the state of air immediately after the outdoor air (OA) and return air (RA) streams are blended in the mixing box of an air-handling unit, before any heating or cooling coil. Its temperature and enthalpy determine the load on the conditioning equipment.
Humidity ratio (W, in kg water per kg dry air) is a conserved quantity during mixing — unlike relative humidity, which is temperature-dependent and does not mix linearly. Tracking W lets you correctly compute the enthalpy (total heat content) of the mixed air and predict condensation or dehumidification requirements.
No — this calculator assumes standard atmospheric pressure of 101.325 kPa (sea level). At high altitude, actual atmospheric pressure is lower, which affects saturation vapour pressure, humidity ratio, and enthalpy. For high-altitude HVAC design, enter site-specific barometric pressure into a full psychrometric chart tool.
Also known as
TG we-Calculate Editorial Team. (2026). Mixed Air Calculator — HVAC Psychrometrics [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mixed-air-calculator
TG we-Calculate Editorial Team. "Mixed Air Calculator — HVAC Psychrometrics." TG we-Calculate. 2026. https://we-calculate.com/calculator/mixed-air-calculator.
TG we-Calculate Editorial Team, "Mixed Air Calculator — HVAC Psychrometrics," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mixed-air-calculator
@misc{wecalculate_mixed_air_calculator, title = {Mixed Air Calculator — HVAC Psychrometrics}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mixed-air-calculator}}, year = {2026}, note = {TG we-Calculate} }
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