What this psychrometric chart calculator does
A psychrometric chart is one of the most useful tools in HVAC, indoor air quality, and industrial drying. It connects temperature, moisture, and energy of air in one framework. This calculator gives you the same core values you would normally read from a printed chart—without the ruler, pencil, and interpolation.
Enter three inputs (dry-bulb temperature, relative humidity, and atmospheric pressure), and the calculator returns a complete moist-air snapshot including humidity ratio, dew point, wet-bulb temperature, enthalpy, and specific volume.
Outputs you can use immediately
- Humidity ratio (w): Mass of water vapor per mass of dry air (kg/kg dry air).
- Dew point (Tdp): Temperature where condensation starts if air cools at constant moisture content.
- Wet-bulb temperature (Twb): Useful for evaporative cooling and field diagnostics.
- Moist air enthalpy (h): Total thermal energy of the air-vapor mixture (kJ/kg dry air).
- Specific volume (v): Air volume per unit dry-air mass (m³/kg dry air), useful for airflow conversions.
- Vapor pressure, VPD, and absolute humidity: Helpful for humidity control and greenhouse process work.
Why psychrometrics matters in real projects
1) HVAC sizing and coil load estimates
Cooling systems remove both sensible heat (temperature drop) and latent heat (moisture removal). Psychrometric calculations let you estimate both parts and avoid undersized or oversized equipment.
2) Ventilation and comfort control
Thermal comfort depends heavily on humidity. Air at 24°C and 50% RH can feel comfortable, while similar temperature at very high RH often feels stuffy. This calculator helps identify those differences quickly.
3) Drying and dehumidification processes
In drying rooms, pharmaceutical spaces, food plants, and archives, moisture control is often more important than temperature. Humidity ratio and dew point are key values for process consistency and product quality.
Key relationships behind the calculator
The calculator applies standard moist-air relationships:
- Saturation vapor pressure from temperature.
- Actual vapor pressure = RH × saturation pressure.
- Humidity ratio from vapor pressure and barometric pressure.
- Enthalpy from dry-bulb temperature and humidity ratio.
- Specific volume from ideal gas behavior with moisture correction.
These are the same fundamentals used in psychrometric charts and many engineering handbooks.
How to use this calculator correctly
- Choose temperature unit (°C or °F).
- Input dry-bulb temperature.
- Input relative humidity between 0 and 100%.
- Set local atmospheric pressure (or leave default for sea level).
- Click Calculate Properties.
If you’re working at altitude, always adjust pressure. Lower pressure changes humidity ratio and volume noticeably.
Practical interpretation tips
- High dew point: Air feels muggy; condensation risk increases on cold surfaces.
- Low humidity ratio: Air is dry; can increase static and discomfort in winter.
- High enthalpy: More total cooling energy required.
- Large VPD: Strong drying potential (useful in agriculture and dehydration work).
Limitations and assumptions
This calculator is designed for engineering estimates and practical planning:
- Wet-bulb temperature uses a widely accepted approximation (excellent for typical ambient conditions).
- Accuracy is strongest in common HVAC ranges and normal atmospheric pressure bands.
- For critical compliance work, validate with ASHRAE-grade software or laboratory instrumentation.