Humidity Ratio Calculator

Calculate humidity ratio and related psychrometric properties of moist air using ASHRAE Fundamentals 2017 equations.

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How to Calculate Humidity Ratio

What Is Humidity Ratio?

Humidity ratio (W) is the mass of water vapor per unit mass of dry air, expressed in lb_w/lb_da. It is the fundamental property used in psychrometric calculations for HVAC design, dehumidification sizing, and comfort analysis.

ASHRAE Psychrometric Equations

Saturation Pressure: P_ws = f(T) (ASHRAE eq 5/6, Hyland-Wexter formulation)

Partial Pressure: P_w = RH/100 × P_ws

Humidity Ratio: W = 0.62198 × P_w / (P - P_w) (ASHRAE eq 22)

Enthalpy: h = 0.240 × T_db + W × (1061 + 0.444 × T_db) (ASHRAE eq 32)

Where P = barometric pressure in psia, T_db = dry bulb temperature in °F.

Worked Example

Scenario: Standard air at 70°F dry bulb, 50% RH, sea level (29.92 inHg).

Saturation pressure at 70°F: P_ws ≈ 0.3633 psia
Partial pressure: P_w = 0.50 × 0.3633 = 0.1817 psia
Total pressure: P = 29.92 × 0.491154 = 14.693 psia
Humidity ratio: W = 0.62198 × 0.1817 / (14.693 - 0.1817) ≈ 0.0078 lb/lb
Enthalpy: h = 0.240 × 70 + 0.0078 × (1061 + 0.444 × 70) ≈ 28.1 BTU/lb

Practical Tips

Standard air conditions (70°F, 50% RH) yield W ≈ 0.0078 lb/lb — a common baseline for HVAC calculations.
Altitude reduces barometric pressure, which increases humidity ratio at the same temperature and RH.
Converting to grains (× 7000) is common in industrial dehumidification specifications.
The dew point temperature indicates when condensation will occur on surfaces.

Code References

ASHRAE Fundamentals 2017 Chapter 1

Frequently Asked Questions

What is the difference between humidity ratio and relative humidity?

Relative humidity (RH) is the ratio of actual vapor pressure to saturation vapor pressure at a given temperature — it varies with temperature. Humidity ratio (W) is the actual mass of water vapor per mass of dry air — it remains constant as air temperature changes (unless moisture is added or removed). HVAC calculations use humidity ratio because it is an absolute measure.

Why does altitude affect humidity ratio?

At higher altitudes, barometric pressure is lower. Since humidity ratio W = 0.62198 × P_w / (P - P_w), lower total pressure P means the denominator is smaller, resulting in a higher humidity ratio for the same temperature and relative humidity. This is why mountain air can feel surprisingly moist even at moderate RH levels.

What is dew point temperature?

Dew point is the temperature at which air becomes saturated (100% RH) if cooled at constant pressure. When air temperature drops below the dew point, condensation occurs. In HVAC, dew point is critical for preventing condensation on ductwork, windows, and building surfaces.

What are grains per pound?

Grains per pound (gr/lb) is an alternative unit for humidity ratio. One grain equals 1/7000 of a pound. To convert: gr/lb = W (lb/lb) × 7000. This unit is commonly used in industrial dehumidification specifications and by HVAC equipment manufacturers.