Humidity Ratio Calculator – Core Psychrometric Quantities in One Tool
Humidity ratio is one of the most important concepts in psychrometrics and HVAC engineering. It connects temperature, moisture and energy in moist air and appears in load calculations, coil sizing, dehumidification, drying and comfort analysis. This Humidity Ratio Calculator from MyTimeCalculator brings together humidity ratio (ω), specific humidity, mixing ratio and a set of moist air properties in one interactive page.
Instead of manually reading psychrometric charts or re-deriving formulas, you can enter air temperature, relative humidity and pressure. The calculator estimates saturation and actual vapor pressure and then derives humidity ratio, specific humidity, dew point, absolute humidity, enthalpy and approximate density using widely accepted equations.
How the Humidity Ratio Calculator is Organized
The tool is divided into four focused tabs:
- Humidity Ratio: Compute ω from temperature, RH and pressure.
- Specific Humidity: Convert humidity ratio into specific humidity.
- Mixing Ratio: Highlight the mixing ratio interpretation (kg/kg and g/kg).
- Moist Air Properties: Summarize dew point, vapor pressures, humidity ratio, specific humidity, absolute humidity, enthalpy and density.
All tabs share a consistent unit system so you can work in either Celsius or Fahrenheit while keeping pressure in hPa (hectopascals), which is standard in meteorology and psychrometrics.
Tab 1 – Humidity Ratio (ω)
Humidity ratio (ω) is defined as the mass of water vapor per unit mass of dry air. In HVAC and psychrometric practice, it is typically expressed in kg/kg or g/kg (grams of water per kilogram of dry air). In this tab you provide:
- Air temperature (°C or °F)
- Relative humidity (%)
- Atmospheric pressure (hPa)
- Desired decimal precision
The calculator converts the temperature internally to °C (if needed), computes saturation vapor pressure at that temperature, multiplies by RH to obtain actual vapor pressure and then applies the standard relation:
ω = 0.62198 × Pv / (P − Pv)
where Pv is the actual water vapor partial pressure and P is the total atmospheric pressure. Results are shown both in kg/kg and g/kg, along with saturation and actual vapor pressure.
Tab 2 – Specific Humidity (q)
Specific humidity is the mass of water vapor per unit mass of moist air. It is related to humidity ratio by a simple conversion:
q = ω / (1 + ω)
Because ω is usually much smaller than one, the numeric values of humidity ratio and specific humidity are close, but they are not identical. In this tab the calculator:
- Computes humidity ratio ω as in Tab 1.
- Converts ω to specific humidity q.
- Reports q as both a dimensionless fraction and a percentage.
- Shows the associated actual vapor pressure.
Tab 3 – Mixing Ratio Interpretation
Many atmospheric science texts refer to the mixing ratio of water vapor, which is conceptually the same as humidity ratio: mass of water vapor per unit mass of dry air. This tab emphasizes that connection and presents:
- Mixing ratio in kg/kg.
- Mixing ratio in g/kg.
- Saturation and actual vapor pressure at the given state.
For typical HVAC and near-surface atmospheric conditions, humidity ratio and mixing ratio can be used interchangeably when the same units are chosen.
Tab 4 – Moist Air Properties at a Given State Point
The moist air properties tab gives a compact psychrometric summary for a single state defined by dry-bulb temperature, relative humidity and pressure. Using standard psychrometric approximations, the calculator computes:
- Dew point temperature: The temperature at which condensation begins if the air is cooled at constant pressure.
- Humidity ratio (ω): kg of water per kg of dry air, plus g/kg.
- Specific humidity (q): Mass fraction of water vapor in the moist air.
- Absolute humidity: g/m³ of water vapor in the air volume.
- Saturation and actual vapor pressure: hPa values based on temperature and RH.
- Moist air enthalpy: Approximate kJ/kg of dry air using a common engineering formula.
- Moist air density: Approximate kg/m³ considering both dry air and water vapor.
These properties are useful for system sizing, coil loads, dehumidification planning and comparing indoor versus outdoor air conditions during ventilation or economizer operation.
Tips for Using the Humidity Ratio Calculator
- Use the default pressure (1013.25 hPa) for sea-level or near sea-level conditions.
- Adjust pressure downward for higher altitudes or upward for pressurized spaces.
- Start with the Moist Air Properties tab to get a quick overview of a state point.
- Use the Humidity Ratio and Specific Humidity tabs when you need more focused values for engineering formulas.
The calculator is designed for education and planning. For detailed design work, always cross-check with official psychrometric charts, standards and professional software.
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Humidity Ratio & Psychrometrics FAQs
Frequently Asked Questions About Humidity Ratio & Moist Air
Quick answers to common questions in HVAC, building science and meteorology.
Relative humidity depends strongly on temperature and is not directly conserved when air is heated or cooled. Humidity ratio is based on mass, so it is more convenient for energy balance, coil calculations and system sizing. Heating air at constant moisture content changes RH but leaves humidity ratio unchanged.
Dehumidification removes moisture from the air, so humidity ratio decreases. On a psychrometric chart, the state point moves left toward the dry-air axis. Cooling coils that condense moisture typically reduce both temperature and humidity ratio at the same time.
Humidity ratio is based on partial pressures of water vapor and dry air. At higher elevations, total atmospheric pressure is lower, so the same vapor pressure corresponds to a different humidity ratio. Including pressure improves accuracy, especially for non-sea-level locations or pressurized systems.
Yes. As long as you have temperature, relative humidity and an approximate pressure, you can analyze both indoor and outdoor air. Comparing humidity ratio and enthalpy between indoor and outdoor conditions is a common way to assess ventilation or economizer performance.
It covers many of the same quantities and is very convenient for quick computations, but it does not show the full graphical relationships between variables. For design work and system visualization, psychrometric charts remain a valuable complement to numeric calculators like this one.