What is the difference between mixing ratio and specific humidity?

Mixing ratio is vapor mass per dry air mass (W). Specific humidity is vapor mass per total moist air mass (q = W/(1+W)). They are nearly identical at low humidity levels.

Why use mixing ratio instead of RH?

RH changes with temperature even if no moisture is added or removed. Mixing ratio is conserved during adiabatic processes, making it more useful for tracking actual moisture.

What is absolute humidity?

The mass of water vapor per unit volume of air (g/m³). Unlike mixing ratio, it depends on both temperature and pressure.

How is the dew point related?

The dew point is the temperature at which the current water-vapor content exactly reaches saturation. Cooling below that temperature causes condensation to begin.

Why is moist air lighter than dry air?

Water vapor has a lower molecular mass than the nitrogen and oxygen it displaces, so humid air can be less dense than dry air at the same temperature and pressure. That density difference is why humid air can slightly reduce lift and change ventilation behavior.

Does pressure affect mixing ratio?

Yes. At lower atmospheric pressure, the same relative humidity corresponds to a slightly different vapor-pressure balance, which shifts the resulting mixing ratio.

Mixing Ratio of Air Calculator

Calculate humidity mixing ratio, specific humidity, absolute humidity, dew point, enthalpy, and moist air density from temperature, RH, and pressure.

Temperature

Temp Unit

Relative Humidity (%)

Atmospheric Pressure

Pressure Unit

Mixing Ratio (W)

9.877 g/kg

Mass of water vapor per mass of dry air: W = 0.622 × Pv / (P − Pv)

Saturation Mixing Ratio (Ws)

20.073 g/kg

Maximum moisture at this temperature and pressure

Specific Humidity

9.780 g/kg

q = W / (1 + W) — vapor mass per total air mass

Absolute Humidity

11.51 g/m³

Mass of water vapor per unit volume of air

Volume Mixing Ratio

15,631 ppm

1.563% by volume

Dew Point

13.9 °C

Temperature where condensation begins

Enthalpy

50.31 kJ/kg

Total heat content (sensible + latent)

Moist Air Density

1,147.169 kg/m³

Moist air is lighter than dry air at the same T, P

Saturation Level

Dry (0%)Current: 49.2%Saturated (100%)

Temp (°C)	Ws (g/kg)	W at 50% RH	AH (g/m³)
-10	1.76	0.88	1.18
0	3.77	1.89	2.42
10	7.63	3.82	4.70
20	14.69	7.35	8.64
25	20.07	10.04	11.51
30	27.18	13.59	15.16
35	36.54	18.27	19.77
40	48.83	24.41	25.52
45	64.97	32.48	32.63
50	86.23	43.11	41.36

Planning notes, formulas, and examples

About the Mixing Ratio of Air Calculator

The mixing ratio (or humidity ratio) is the mass of water vapor per unit mass of dry air, typically expressed in grams per kilogram. It is a fundamental quantity in atmospheric science, HVAC engineering, and meteorology because — unlike relative humidity — it doesn't change with temperature alone. Two air masses with the same mixing ratio contain the same amount of moisture regardless of temperature.

Understanding the mixing ratio is essential for weather forecasting (predicting cloud formation, precipitation), HVAC design (sizing dehumidifiers, selecting coils), and industrial processes where moisture control matters. It connects directly to dew point, enthalpy, and air density, forming the backbone of psychrometric calculations.

This calculator computes the mixing ratio along with specific humidity, absolute humidity, volume mixing ratio (ppm), dew point, enthalpy, and moist air density. Multiple temperature and pressure units are supported, with altitude-aware presets for different climates. A saturation bar shows how close the air is to condensation, and a temperature sweep table reveals how moisture capacity changes across the temperature range.

When This Page Helps

Use this calculator when you need the actual moisture content of air rather than a temperature-dependent humidity percentage, especially for HVAC sizing, psychrometrics, and weather analysis. It is a better fit when you care about conserved moisture content instead of a number that shifts with temperature. That makes it easier to compare indoor and outdoor air on the same moisture basis.

How to Use the Inputs

Enter the air temperature and select the unit (°C, °F, or K).
Enter the relative humidity as a percentage.
Enter the atmospheric pressure and select the unit (Pa, atm, hPa, or inHg).
Use preset buttons for common climate conditions.
Review mixing ratio, specific humidity, absolute humidity, dew point, enthalpy, and density.
Check the saturation bar to see how close the air is to dew point.
Use the temperature sweep table to compare moisture capacity at different temperatures.

Formula used

Saturation Pressure: Ps = 610.78 × exp(17.27T / (T + 237.3)) Pa
Mixing Ratio: W = 0.622 × Pv / (P − Pv) kg/kg
Specific Humidity: q = W / (1 + W)
Absolute Humidity: ρv = Pv / (Rv × T)
Volume Mixing Ratio: χ = Pv / P
Enthalpy: h = 1.006T + W(2501 + 1.86T) kJ/kg
Dew Point: Td = 237.3 × α / (17.27 − α)

Example Calculation

Result: W = 9.88 g/kg, Ws = 19.77 g/kg, Td = 13.9°C

At 25°C and 50% RH, each kilogram of dry air holds 9.88 g of water vapor. The air could hold up to 19.77 g/kg at saturation. Cooling below 13.9°C will cause condensation.

Tips & Best Practices

Check that all inputs use the same scale and assumptions before trusting the result.
Compare the answer with the worked example or a rough estimate to catch entry mistakes.

Why Mixing Ratio Matters

Relative humidity is useful for comfort discussions, but it changes as air warms or cools even when no moisture is added or removed. Mixing ratio tracks the actual mass of water vapor carried by the air, which is why meteorologists and HVAC engineers use it for process and weather calculations.

Practical HVAC Reading

When comparing indoor and outdoor air streams, look at dew point and enthalpy alongside mixing ratio. That combination tells you whether latent load or sensible load is driving equipment selection. It also helps explain why two spaces can share the same temperature but feel very different.

Common Interpretation Errors

Do not mix up absolute humidity, specific humidity, and mixing ratio. They are related, but they are not interchangeable in design calculations. Pressure assumptions also matter at altitude, so use the local atmospheric pressure when you are not working near sea level.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

Mixing ratio is vapor mass per dry air mass (W). Specific humidity is vapor mass per total moist air mass (q = W/(1+W)). They are nearly identical at low humidity levels.