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Mixed Air Calculator — Full Psychrometric Mixer for HVAC

Mix two airstreams — outside air with return air, or any two flows — and get the complete mixed-air psychrometric state: dry-bulb temperature, relative humidity, humidity ratio, enthalpy, dew point, wet bulb, specific volume and mass flow. Exact mass-weighted mixing (not naive CFM averaging), AHU/economizer mode, reverse solver, fog warning and coil-load estimate. 100% in your browser — no sign-up, no data leaves your device.

Temperature: Airflow: Pressure:

Barometric pressure (sea level = 101.325 kPa)

Altitude preset

Stream 1 — e.g. outside air

Dry-bulb temperature Relative humidity % Airflow (CFM)

Stream 2 — e.g. return air

Dry-bulb temperature Relative humidity % Airflow (CFM)

AHU mixing-box view: this ahu mixed air calculator mode sets the total supply airflow and the outside-air damper position; outside air and return air conditions define the rest. This is the economizer calculation — watch the mixed-air temperature and coil load move as you change the OA%.

Total supply airflow (CFM)

Outside air % of total

Resulting flows

—

Outside air

Dry-bulb temperature Relative humidity %

Return air

Dry-bulb temperature Relative humidity %

Reverse solver: stream 1 is fixed. Tell it the second stream's conditions and the mixed-air temperature you want, and it finds the required stream-2 airflow.

Stream 1 dry-bulb

Stream 1 RH %

Stream 1 airflow (CFM)

Stream 2 dry-bulb

Stream 2 RH %

Target mixed-air dry-bulb

Coil-load estimate (optional)

How much cooling (or heating) does it take to bring the mixed air to your supply setpoint? Positive = cooling load.

Supply setpoint dry-bulb

Supply setpoint RH %

—

Worked example: 1,000 CFM at 35°C/60% RH mixed with 1,000 CFM at 20°C/40% RH

Two equal CFM flows do not give the naive average of 27.5°C, because the warmer stream is less dense. Converting each stream to dry-air mass flow first (m = V / v):

Stream	Tdb	RH	ω (humidity ratio)	h (enthalpy)	v (spec. volume)	m (mass flow)
Stream 1	35.0°C	60%	21.42 g/kg	90.18 kJ/kg	0.9029 m³/kg	31.36 kg/min
Stream 2	20.0°C	40%	5.80 g/kg	34.84 kJ/kg	0.8381 m³/kg	33.79 kg/min
Mixed air	27.33°C	58.4%	13.32 g/kg	61.47 kJ/kg	0.8724 m³/kg	65.15 kg/min

Mixed: dew point 18.46°C, wet bulb 21.9°C, vapour pressure 2.13 kPa. Note the mixed RH is 58.4% — not the 50% a naive average would suggest. Press the "Worked example" preset above to reproduce this live.

Mixed air formulas (exact psychrometric mixing)

These are the governing equations of mixed air psychrometrics. Mixing is adiabatic: dry-air mass and water vapour are conserved, and the energy balance has no heat or work terms.

m₁ + m₂ = m₃ (dry-air mass balance)

m₁·ω₁ + m₂·ω₂ = m₃·ω₃ (vapour mass balance)

m₁·h₁ + m₂·h₂ = m₃·h₃ (energy balance — adiabatic)

Each stream's properties come from its dry-bulb temperature T and relative humidity φ at pressure P:

Saturation pressure (Tetens): p_vs = 0.6108 · e^{17.27·T / (T + 237.3)} kPa

Humidity ratio: ω = 0.621945 · p_v / (P − p_v), p_v = φ · p_vs

Enthalpy: h = 1.006·T + ω·(2501 + 1.86·T) kJ/kg dry air

Specific volume: v = 0.287042·(T + 273.15)·(1 + 1.607858·ω) / P m³/kg

Crucially, mass flows weight the balances: m = V / v, where each stream uses its own specific volume. The mixed dry-bulb temperature is then recovered from h₃ and ω₃, the dew point by exact inversion of the Tetens equation, and the wet bulb from the ASHRAE psychrometric relation e = p_vs(T_wb) − P·0.00066·(T − T_wb).

The common shortcut T_mix = (%OA × T_OA) + (%RA × T_RA) skips the mass conversion and the moisture terms — fine for a quick sanity check, but use the exact solution above (which this tool computes) whenever humidity, coil load or condensation risk matters.

HVAC mixed air explained: reference and related calculations

HVAC mixed air explained in one paragraph: outside air and return air meet in the AHU mixing box, and the blend that leaves it — defined by dry-air mass balances on temperature, moisture and energy — is what the coil must condition. This mixed air psychrometric calculator solves those balances exactly, so it also works as a mixed air properties calculator (ten properties per stream), a mixed air temperature hvac calculator, an ahu mixing box calculator, a mixed air plenum calculator, and an adiabatic mixing of air calculator.

How to calculate mixed air temperature: convert each stream's airflow to dry-air mass flow (m = V / v), then weight the enthalpies — never average by CFM alone. That is also how to calculate mixed air psychrometrics beyond temperature: mass-weight the humidity ratio ω the same way, then derive RH, dew point and wet bulb from the mixed T and ω. A full mixed air temperature calculation example with numbers is worked above; the equivalent mixed air psychrometric chart example is: plot both stream states, draw the straight psychrometric mixing line between them (this tool is a psychrometric mixing line calculator in equation form), and mark the mixture at the mass-flow ratio along it.

Quantity this two airstream mixing calculator reports	What it means
Mixed air dry bulb temperature	The headline result: mass-weighted blend of the two stream temperatures. A mixed air temperature sensor calculation in a real AHU averages several duct sensors to approximate it.
Mixed air relative humidity	Derived from mixed T and ω — never averaged directly.
Humidity ratio ω — this mixed air humidity calculator output, i.e. a mixed air humidity ratio calculator	kg of moisture per kg of dry air, mass-weighted from both streams.
Specific enthalpy h (the mixed air enthalpy calculator result; mixed air specific enthalpy)	kJ per kg of dry air; the quantity the energy balance actually mixes.
Dew point (a mixed air dew point calculator)	Exact Tetens inversion from the mixed humidity ratio.
Mixed air wet bulb temperature	From the ASHRAE psychrometric relation, solved iteratively.
Mixed air specific volume	m³ per kg of dry air — converts the mixed mass flow back to CFM.
Mixed air mass flow rate	m₁ + m₂ in kg/min; the only correct weighting basis.

CFM-based mixing: this mixed air calculator cfm input mode also accepts m³/min. To get mixed air temperature with CFM inputs right, every CFM value is divided by its stream's specific volume before weighting — that conversion is what a proper cfm mixing calculator temperature tool must do, and it is why equal CFMs of different-density air do not average evenly.

AHU and economizer use: the AHU tab is an outside air percentage calculator hvac damper-analysis tool — set any OA% of the classic return air outside air mixture and read the result. Economizer mixed air temperature drops toward the outside condition as OA% rises, which is the free-cooling effect. Wondering how much outside air to get mixed air temperature at your target? The Reverse tab is a mixed air reverse calculator that solves the required second-stream airflow numerically.

Condensation: can mixed air cause condensation? Yes — fog in mixing box hvac systems is a classic winter failure mode, when the straight mixing line dips into the saturated region. As a mixed air condensation calculator, this tool raises a fog warning whenever the mixed dry bulb falls below either parent stream's dew point.

Load: the coil section is a mixed air cooling load calculator, reporting the mixed air to supply air load as total, sensible and latent kW (plus BTU/hr and tons). Covering mixing two air streams psychrometrics end to end, this approach replaces chart lookups for everyday design checks — though, as always, field-verify critical designs.

Scope boundary: which mixed-air tool do you need?

This page is the full psychrometric mixer — a complete mixed air psychrometrics calculator: temperature plus humidity ratio, relative humidity, enthalpy, dew point, wet bulb, specific volume and mass flow for both streams and the mixture — with AHU/economizer analysis, a reverse solver and condensation warnings.

If you only need the dry-bulb temperature of the blend and nothing else, the lighter companion tool is faster: mixed air temperature calculator.

Frequently asked questions

What is mixed air in an HVAC system?

Mixed air is the airstream produced when two airflows with different conditions are combined — in HVAC this is almost always outside (fresh) air blended with return (recirculated) air in the AHU mixing box, before the cooling or heating coil. Its temperature, humidity and enthalpy sit between the two parent streams, weighted by their dry-air mass flows, and it is the actual load the coil has to condition.

What is the mixed air temperature formula?

The exact adiabatic-mixing relation is the mass-weighted enthalpy balance m1·h1 + m2·h2 = m3·h3 together with the humidity balance m1·W1 + m2·W2 = m3·W3, from which the mixed dry-bulb temperature is recovered. The familiar shortcut T_mix = (%OA × T_OA) + (%RA × T_RA) is only an approximation: it ignores density differences between the streams and the latent (moisture) content. This calculator solves the exact mass-weighted equations, so use it when humidity matters.

Why can't I just average the two temperatures by CFM?

Because CFM is a volume flow and the mixing laws are mass balances. Cold air is denser than warm air, so 1,000 CFM of cold outside air carries more dry-air mass than 1,000 CFM of warm return air and pulls the mixture temperature toward itself. Each stream must first be converted to mass flow (m = V / v, using its own specific volume) before weighting. The error from naive CFM averaging is typically a few tenths of a degree, but it grows in winter when the streams differ a lot in density.

Is air mixing an adiabatic process?

Yes, for engineering purposes mixing is treated as adiabatic: heat loss from the mixing box to the surroundings is negligible and no shaft work crosses the boundary, so the energy balance reduces to m1·h1 + m2·h2 = m3·h3 with m1 + m2 = m3 for the dry-air mass. On a psychrometric chart the mixed state always falls on the straight line joining the two parent states, at a position set by the mass-flow ratio.

How do you calculate the humidity of mixed air?

Mix the humidity ratio (mass-weighted: W_mix = (m1·W1 + m2·W2) / (m1 + m2)), never the relative humidity. Relative humidity is strongly non-linear in temperature, so averaging 60% and 40% RH does not give 50% — you must compute W_mix and T_mix first, then derive RH from them. In the worked example below, 35°C/60% mixed with 20°C/40% gives about 58.4% RH, not 50%.

How do you find the dew point of the mixed air?

From the mixed humidity ratio W_mix: the dew point is the temperature whose saturation pressure equals the mixture's actual vapour pressure Pv = W_mix·P / (0.621945 + W_mix). This calculator inverts the Tetens saturation-pressure equation numerically to get it exactly. If the mixed dry-bulb temperature is below the dew point of either parent stream, moisture will condense or fog will form in the mixing box — the calculator flags this automatically.

Can mixing air cause condensation or fog in the AHU?

Yes. When cold outside air meets warm humid return air in winter, the mixture can become saturated even though neither stream was: the straight mixing line on the psychrometric chart can cross into the fog region. The warning sign is mixed dry-bulb temperature below the dew point of the return (or outside) air stream. Expect wet mixing-box surfaces, dripping dampers and possible coil freeze-up; the usual fixes are preheating the outside air, raising the minimum outside-air temperature, or lowering the outside-air fraction.

What is an AHU mixing box and economizer?

The mixing box is the AHU section where the outside-air and return-air dampers meet before the coil. An airside economizer modulates those dampers: when outside air is cooler (and dry enough) than return air, it opens the outside-air damper toward 100% for free cooling and the mixed-air temperature drops toward the outside condition. Use this outside air return air mix calculator (AHU mode above) with your outside-air percentage to see the mixed state and the resulting coil load for any damper position.

How much outside air do I need for a target mixed-air temperature?

Use the Reverse mode: enter the fixed stream (e.g. your return air flow), the second stream's conditions (e.g. outside air), and the mixed-air dry-bulb temperature you want. The calculator solves the mass and energy balances numerically and returns the required second-stream flow in CFM (or m³/min) plus the full psychrometric state of the resulting mixture. The target must lie strictly between the two stream temperatures.

Does altitude or barometric pressure change the result?

Yes for moisture quantities, barely for temperature. At lower pressure the same relative humidity holds more moisture per kilogram of dry air (W rises), so the humidity ratio, dew point and enthalpy of the mixture shift; specific volume also grows, changing the CFM-to-mass conversion. Set your site pressure (or altitude) in the calculator — at 2,000 m the worked example below mixes to about 17.1 g/kg instead of 13.3 g/kg at sea level.

What is the difference between this and a mixed-air-temperature calculator?

A mixed-air-temperature calculator solves only the dry-bulb temperature of the blend — a single number. This tool is the full psychrometric version: it additionally computes humidity ratio, relative humidity, enthalpy, dew point, wet bulb, specific volume, vapour pressure and mass flow for both parent streams and the mixture, plus fog warnings, AHU/economizer analysis and coil-load estimates. If you only need the temperature, our companion mixed-air temperature calculator is the lighter tool.

How accurate is this calculator compared to a psychrometric chart?

It matches published worked examples to within chart-reading error: against the OmniCalculator mixed-air worked example (32°C/60% outside air with 14°C saturated return air), this engine reproduces the mixed state as 18.93°C dry bulb, 88.8% RH, 12.15 g/kg humidity ratio, 49.86 kJ/kg enthalpy and 17.03°C dew point — all within 0.1 of the published values. Saturation pressure uses the Tetens equation, enthalpy the standard 1.006·T + W·(2501 + 1.86·T) relation, and the dew point is found by exact numerical inversion.

What units can I use?

Temperatures in °C or °F, airflows in CFM or m³/min, pressure in kPa or inHg (with sea-level and altitude presets). Results are shown in the selected temperature unit with SI psychrometric units (kJ/kg, g/kg, m³/kg, kPa) and the equivalent imperial flow. All psychrometric properties are computed in SI internally and converted only for display.

Can I use this for real engineering design?

This is an educational and estimating tool, not certified engineering data. The psychrometric math is exact for ideal adiabatic mixing at the pressure you enter, but real mixing boxes have imperfect mixing, stratification, damper leakage and heat gains that shift the result. Always verify critical designs against measured data and applicable standards such as ASHRAE 62.1 for ventilation and your local energy code.

Free tool by NRTechStudio

Mixed Air Calculator — Full Psychrometric Mixer for HVAC

Stream 1 — e.g. outside air

Stream 2 — e.g. return air

Outside air

Return air

Coil-load estimate (optional)

Worked example: 1,000 CFM at 35°C/60% RH mixed with 1,000 CFM at 20°C/40% RH

Mixed air formulas (exact psychrometric mixing)

HVAC mixed air explained: reference and related calculations

Scope boundary: which mixed-air tool do you need?

Frequently asked questions

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