Humidity ratio is the mass of water vapor contained in a unit mass of dry air, and it is the right-hand vertical axis on every standard psychrometric chart. That single fact resolves a persistent confusion: humidity ratio and relative humidity (RH) are not the same property, do not behave the same way, and are not interchangeable in engineering calculations.
This article defines humidity ratio precisely, distinguishes it from RH and specific humidity, explains how to locate and read it on a psychrometric chart, and walks through the standard formula with a complete numerical example.
Key Takeaways
- Humidity ratio (W) is defined as the mass of water vapor per unit mass of dry air, expressed in grains per pound (gr/lb) in imperial units or grams per kilogram of dry air (g/kg) in SI units, as specified in the ASHRAE Handbook of Fundamentals, Chapter 1.
- Humidity ratio does not change when air temperature changes; relative humidity does, even when no moisture is added or removed from the air.
- Specific humidity and humidity ratio are numerically close but not interchangeable: specific humidity uses total moist air mass as its denominator, while humidity ratio uses dry air mass alone.
- The standard psychrometric formula for humidity ratio is W = 0.622 × Pv / (P − Pv), where Pv is the partial pressure of water vapor and P is total barometric pressure.
- At elevations above 5,000 feet, lower barometric pressure raises the humidity ratio output for the same vapor pressure, requiring altitude-corrected psychrometric data.
- Humidity ratio, not relative humidity, is the correct design variable for sizing humidification systems and calculating moisture load in heating, ventilation, and air conditioning (HVAC) applications.
What Is Humidity Ratio?
Humidity ratio (symbol W, sometimes written as ω) is the ratio of the mass of water vapor to the mass of dry air in a given air sample. In imperial units, it is expressed in grains of moisture per pound of dry air (gr/lb) or pounds of water per pound of dry air (lb/lb). In SI units, it is expressed as grams per kilogram of dry air (g/kg). “Dry air” refers to the total air mixture minus the water vapor it contains, which is the denominator the definition depends on. This definition is the one used in the ASHRAE Handbook of Fundamentals, Chapter 1.
Why the Units Matter
One pound of dry air at typical indoor conditions contains roughly 50 to 80 grains of moisture, equivalent to approximately 7 to 11 g/kg. These numbers help calibrate the scale of the variable before a reader encounters it on a chart axis. A value of 0 gr/lb represents perfectly dry air with no moisture content, a condition that does not exist in real facilities. A value above 100 gr/lb indicates very humid air approaching saturation at warm temperatures.
Humidity Ratio vs. Relative Humidity: The Key Difference
Relative humidity is a percentage expressing how much water vapor is in the air relative to the maximum it could hold at that temperature. That maximum changes with temperature, which means RH changes whenever temperature changes, even if no moisture is added or removed. Humidity ratio does not behave this way. Because it measures actual moisture mass rather than a percentage of capacity, it remains constant when temperature changes and the moisture content of the air stays the same.
A concrete example illustrates this. Air at 70°F and 40% RH has a humidity ratio of approximately 43 gr/lb. If that same air is heated to 90°F without adding any moisture, the humidity ratio stays at 43 gr/lb while the RH drops to roughly 20%. The moisture content has not changed. Only the air’s capacity to hold moisture has increased, pulling the percentage down. This is why RH alone is an unreliable indicator of actual moisture content when temperature varies.
The psychrometric chart plots both variables because they answer different questions. For more on how relative humidity is defined and measured, see our guide to relative humidity. For a side-by-side comparison of absolute and relative measures, see our comparison of absolute humidity vs relative humidity.
- Humidity ratio: Measures actual moisture mass per unit mass of dry air; does not change with temperature; the correct input for HVAC load calculations.
- Relative humidity: Measures moisture as a percentage of saturation capacity; rises and falls with temperature; useful for comfort assessment and equipment specifications.
Why HVAC Engineers Use Humidity Ratio Instead of RH
Calculating how much water a humidification system must add to a facility requires knowing the actual mass of moisture in the air, not its percentage of saturation. Engineers calculate the moisture addition rate as: mass flow rate of air multiplied by the difference between the target humidity ratio (W2) and the current humidity ratio (W1). Humidification equipment is rated in pounds of moisture per hour or kilograms per hour, and those ratings derive directly from humidity ratio differences across the system, not from RH differences.
Humidity Ratio vs. Specific Humidity: A Precision Distinction
Specific humidity is defined as the mass of water vapor per unit mass of moist air, meaning the denominator includes the water vapor itself. Humidity ratio uses dry air mass alone as its denominator. In practice, the two values are numerically very close: specific humidity is always slightly smaller than humidity ratio for the same air sample, because the moist air denominator is larger than the dry air denominator.
Despite the small numerical gap, the terms are not interchangeable in precise engineering and thermodynamic calculations. The ASHRAE Handbook of Fundamentals uses humidity ratio (W) as the primary variable for psychrometric chart construction and HVAC system design. Standards such as ISO 7933 (analytical determination and interpretation of heat stress) also require precise psychrometric definitions where conflating these terms can introduce calculation error.
The practical rule is straightforward: when reading values from a psychrometric chart or performing system sizing calculations, use humidity ratio as defined by ASHRAE.
When the Difference Actually Affects Calculations
In facilities with high air volume flow rates, a small per-unit-mass error compounds quickly. If a facility engineer uses specific humidity values where the calculation requires humidity ratio, the resulting moisture load estimate will be slightly low. At air flow rates typical of large manufacturing or data center spaces, that margin accumulates into a meaningful discrepancy in equipment sizing over an annual operating cycle.
How to Read Humidity Ratio on a Psychrometric Chart
On a standard North American psychrometric chart following ASHRAE Fundamentals conventions at sea-level barometric pressure, humidity ratio appears on the right-hand vertical axis. The horizontal gridlines running across the chart from left to right are lines of constant humidity ratio. Any point on one of those gridlines represents air with the same moisture mass content, regardless of its dry bulb temperature.
To locate humidity ratio for a known air state, follow this sequence:
- Identify the air state point on the chart using dry bulb temperature on the horizontal axis and one other known variable, such as wet bulb temperature, relative humidity line, or dew point temperature.
- From the state point, draw a horizontal line to the right-hand vertical axis.
- Read the humidity ratio value where that horizontal line meets the axis.
- Note the units printed on the axis: gr/lb on imperial charts, g/kg on SI charts.
- If the state point falls between printed gridlines, interpolate proportionally between the two nearest values.
A worked example: air at 75°F dry bulb and 60% RH falls at approximately 65 gr/lb (approximately 9.3 g/kg) on a standard sea-level psychrometric chart. Most engineering software and online psychrometric calculators will confirm this reading, but the physical chart and the formula below are the reference standards.
The enthalpy calculator returns humidity ratio alongside enthalpy from the same temperature and RH inputs. For context on how dew point temperature relates to this reading, see our comparison of dew point vs humidity.
What the Saturation Curve Tells You About Humidity Ratio Limits
The curved left boundary of the psychrometric chart is the saturation curve, representing 100% RH. Humidity ratio values along this curve are the maximum possible moisture content for each dry bulb temperature. At 75°F, saturated air holds approximately 131 gr/lb. At 50°F, saturated air holds approximately 53 gr/lb. These limits explain why cooling air without removing moisture causes condensation: the humidity ratio stays constant while the saturation limit drops until they meet at the dew point temperature.
How to Calculate Humidity Ratio: Formula and Worked Example
The standard psychrometric formula for humidity ratio is:
W = 0.622 × Pv / (P − Pv)
W is humidity ratio in pounds of water per pound of dry air (or kg/kg in SI). Pv is the partial pressure of water vapor in the air sample. P is total atmospheric (barometric) pressure. The constant 0.622 is the ratio of the molecular weight of water (18.015 g/mol) to the molecular weight of dry air (28.966 g/mol). It does not change.
The following worked example uses realistic industrial conditions at sea level.
Given conditions: Dry bulb temperature = 75°F, RH = 60%, barometric pressure P = 14.696 psia (sea-level standard).
- Find the saturation pressure of water at 75°F. From ASHRAE steam tables, Psat at 75°F is approximately 0.4298 psia.
- Calculate the partial vapor pressure: Pv = RH × Psat = 0.60 × 0.4298 = 0.2579 psia.
- Apply the formula: W = 0.622 × 0.2579 / (14.696 − 0.2579).
- Compute the denominator: 14.696 − 0.2579 = 14.438.
- Compute the numerator: 0.622 × 0.2579 = 0.1604.
- Divide: W = 0.1604 / 14.438 = 0.01111 lb/lb.
- Convert to gr/lb: 0.01111 × 7000 = 77.8 gr/lb.
- Convert to g/kg: 0.01111 × 1000 = 11.1 g/kg.
The formula result of 77.8 gr/lb differs slightly from the 65 gr/lb chart-reading estimate given in the previous section. That difference reflects rounding in the chart-reading step: the formula is authoritative, and the psychrometric chart is a visual approximation. Enthalpy and other moist air properties calculated from the same state point follow the same principle.
Humidity Ratio at Saturation: Maximum Moisture in Air
When Pv equals the full saturation pressure at a given dry bulb temperature, the formula yields the maximum possible humidity ratio for that temperature. At 75°F, saturated air (100% RH) produces W approximately equal to 0.0187 lb/lb, or approximately 131 gr/lb. At 50°F, the saturated value is approximately 0.0077 lb/lb, or approximately 54 gr/lb. Saturated air at lower temperatures holds significantly less moisture by mass. This is the physical basis of the dehumidification process by cooling: reducing temperature lowers the saturation limit until it intersects the existing humidity ratio, causing condensation.
How Altitude Affects Humidity Ratio Readings
Standard North American psychrometric charts are produced at a barometric pressure of 29.921 inHg, equivalent to approximately 14.696 psia at sea level. At 5,000 feet elevation, barometric pressure drops to approximately 12.23 psia. Because total pressure P appears in the denominator of the humidity ratio formula, a lower P produces a higher W for the same partial vapor pressure.
This means that at altitude, air at 60% RH and 75°F contains a higher humidity ratio than air at the same RH and temperature at sea level, even though the relative humidity percentage reads identically. Engineers designing HVAC or humidity control systems for high-altitude facilities must use altitude-corrected psychrometric charts or apply the formula directly with local barometric pressure. The ASHRAE Handbook of Fundamentals provides altitude-corrected psychrometric data in Chapter 1.
Using Corrected Barometric Pressure in the Formula
Re-running the same 75°F / 60% RH example at Denver (elevation approximately 5,280 feet, P approximately 12.15 psia) demonstrates the difference. Pv remains 0.2579 psia (same temperature and RH). W = 0.622 × 0.2579 / (12.15 − 0.2579) = 0.1604 / 11.892 = 0.01349 lb/lb, or approximately 94.4 gr/lb. That is meaningfully higher than the sea-level result of 77.8 gr/lb, for identical dry bulb temperature and relative humidity. Systems sized using sea-level charts for Denver installations will be undersized unless corrected barometric pressure is applied.
How Humidity Ratio Is Used in HVAC and Industrial Humidification System Design
Humidity ratio is the input variable for three core HVAC and industrial humidification engineering tasks. Each task requires the absolute moisture mass value that humidity ratio provides, not the temperature-dependent percentage that RH represents.
- Moisture load calculation: To determine how much water a humidification system must add, engineers calculate: moisture addition rate = air mass flow rate × (W2 − W1), where W1 is the current state and W2 is the target state.
- Equipment sizing: HVAC humidification systems and industrial humidifier systems are rated in pounds of moisture per hour or kilograms per hour, derived from the humidity ratio difference across the system.
- System verification: After installation, humidity ratio readings at supply and return air states confirm whether the system delivers the specified moisture load, providing a temperature-independent check that RH alone cannot.
Why Relative Humidity Alone Is Insufficient for System Sizing
An engineer who sizes a humidification system on RH targets without accounting for the required humidity ratio change risks specifying equipment that meets the RH reading at one temperature but cannot maintain it when air temperature varies. Sensible heat gains and losses in a facility shift dry bulb temperature continuously, which moves the RH reading without changing the actual moisture content. Humidity ratio is temperature-independent and is therefore the correct basis for sizing.
How Smart Fog Maintains Precise Humidity Ratio Control in Industrial Facilities
Maintaining a stable humidity ratio set point requires that all water introduced into the airstream reaches the vapor phase. Water that does not fully evaporate does not register as a humidity ratio change. It lands on surfaces instead, reducing actual vapor-phase moisture output below the specified level and introducing surface wetting that can damage equipment, products, or infrastructure.
A humidification system that produces an equal-sized droplet grid, where each droplet carries a slight electrostatic charge to prevent re-aggregation, ensures that droplets self-evaporate before reaching any surface under proper system design. Every unit of water introduced into the airstream converts to vapor. The result is humidity ratio control traceable to a set point, with plus or minus 1 to 2% RH precision and no surface wetting under proper system design.
Note: direct exposure to the fog stream will wet surfaces; the non-wetting specification applies under proper system design. For a full explanation of the underlying technology, see the Smart Fog technology overview.
No Moving Parts, No Constant Calibration
Mechanical wear in humidification systems causes output variability. As components degrade, moisture delivery drifts from the set point, and the humidity ratio at the supply air state no longer matches the design value. Systems with no moving parts in the humidification process do not experience this drift. Smart Fog systems are engineered to this standard, with maintenance intervals extending to every two years. For facilities where humidity ratio must be held within a specified range for product quality, indoor air quality compliance, or equipment protection, that stability is a direct operational benefit.
Full System Design, Not a Component Kit
A humidification component requires the facility to integrate it into an existing airflow configuration and verify that the delivered moisture load matches the psychrometric calculation. Smart Fog delivers a complete engineered system designed to the facility’s specific airflow rates, target humidity ratio, and space geometry. The delivered moisture load is calculated and verified against the same psychrometric principles this article has covered. That engineering completeness reduces the gap between specified and actual humidity ratio performance at commissioning.
Final Thoughts
Humidity ratio is the foundational psychrometric variable for moisture load analysis and humidification system sizing. It measures actual moisture mass per unit mass of dry air, remains constant when temperature changes, and is the quantity plotted on the right-hand vertical axis of every standard psychrometric chart. Relative humidity measures a percentage of saturation capacity and shifts with temperature, making it an incomplete basis for engineering calculations.
The formula W = 0.622 × Pv / (P − Pv) produces humidity ratio from known vapor pressure and barometric pressure data. At altitude, total pressure decreases and the same formula produces a higher output for identical RH conditions, which is why standard sea-level psychrometric charts require correction for high-elevation facilities.
For facilities that need humidity ratio held to a specified engineering standard, speak with a Smart Fog engineer to discuss a precision humidification system designed for the facility’s airflow and conditions.
FAQ
What is humidity ratio on a psychrometric chart?
Humidity ratio on a psychrometric chart is the mass of water vapor per unit mass of dry air, plotted on the right-hand vertical axis of the chart. On standard North American charts following ASHRAE conventions at sea-level pressure, it is expressed in grains per pound of dry air (gr/lb) or grams per kilogram of dry air (g/kg). To read it, locate the air state point and draw a horizontal line to the right-hand axis.
What is the difference between humidity ratio and relative humidity?
Humidity ratio is an absolute measure of moisture mass per unit mass of dry air; it does not change when temperature changes. Relative humidity is a percentage expressing how much water vapor is present relative to the maximum possible at a given temperature; it rises and falls with temperature even when no moisture is added or removed. For HVAC load calculations and system sizing, humidity ratio is the correct variable.
How do you calculate humidity ratio from relative humidity?
Use the formula W = 0.622 × Pv / (P − Pv). First, find the saturation pressure (Psat) at the air’s dry bulb temperature from ASHRAE steam tables. Multiply Psat by the decimal form of relative humidity to get the partial vapor pressure (Pv). Then apply the formula using local barometric pressure as P. At 75°F, 60% RH, and sea-level pressure (14.696 psia), the result is approximately 0.01111 lb/lb, or 77.8 gr/lb.
What units are used for humidity ratio?
Humidity ratio is expressed in grains of moisture per pound of dry air (gr/lb) in imperial units, or grams of water per kilogram of dry air (g/kg) in SI units. It may also be expressed as pounds of water per pound of dry air (lb/lb), which is the direct output of the psychrometric formula before unit conversion. There are 7,000 grains per pound, so multiply lb/lb by 7,000 to convert.
What is humidity ratio in saturated air, and how is it calculated?
In saturated air, the partial vapor pressure equals the full saturation pressure at that temperature, so Pv = Psat. Applying the formula W = 0.622 × Psat / (P − Psat) yields the maximum humidity ratio for that dry bulb temperature. At 75°F and sea-level pressure, saturated air holds approximately 131 gr/lb. At 50°F, the value drops to approximately 54 gr/lb, which is why cooling air causes condensation when its humidity ratio exceeds the new saturation limit.
How does altitude affect humidity ratio readings on a psychrometric chart?
Standard psychrometric charts are calibrated for sea-level barometric pressure (14.696 psia). At higher elevations, total pressure decreases, which reduces the denominator (P − Pv) in the humidity ratio formula and produces a higher W for the same vapor pressure. At Denver (approximately 5,280 feet, P approximately 12.15 psia), air at 75°F and 60% RH has a humidity ratio of approximately 94.4 gr/lb rather than the sea-level value of 77.8 gr/lb. Facilities at altitude should use altitude-corrected psychrometric data or apply the formula directly with local barometric pressure.
What is the difference between humidity ratio and specific humidity?
Humidity ratio (W) is the mass of water vapor divided by the mass of dry air. Specific humidity is the mass of water vapor divided by the mass of moist air, which includes the vapor. Specific humidity is always slightly smaller than humidity ratio for the same air sample. The ASHRAE Handbook of Fundamentals uses humidity ratio as the primary variable for psychrometric chart construction, and engineering calculations should use it accordingly to avoid small but cumulative sizing errors.
How does humidity ratio affect the sizing of HVAC humidification systems?
Humidification equipment is rated in pounds of moisture per hour or kilograms per hour. That rating is calculated from the humidity ratio difference between the current air state (W1) and the target air state (W2), multiplied by the air mass flow rate through the system. Sizing on relative humidity targets instead of humidity ratio differences risks specifying equipment that cannot maintain conditions when air temperature varies, because RH shifts with temperature while humidity ratio does not.
