Calculator Input
Use dry-bulb temperature, relative humidity, and barometric pressure to estimate the moisture content of moist air.
Example Data Table
These sample cases show how humidity ratio changes as air temperature, pressure, and relative humidity change.
| Temperature (°C) | Pressure (kPa) | Relative Humidity (%) | Humidity Ratio (g/kg) | Specific Humidity | Dew Point (°C) |
|---|---|---|---|---|---|
| 20.00 | 101.325 | 40.00 | 5.7947 | 0.00576 | 6.00 |
| 25.00 | 101.325 | 50.00 | 9.8768 | 0.00978 | 13.86 |
| 30.00 | 100.000 | 60.00 | 16.2478 | 0.01599 | 21.38 |
| 35.00 | 99.000 | 70.00 | 25.7507 | 0.02510 | 28.70 |
Formula Used
1. Saturation vapor pressure
Pws = 0.61078 × exp((17.2694 × T) / (T + 237.29)) for temperatures at or above 0°C.
2. Actual vapor pressure
Pv = (RH / 100) × Pws
3. Humidity ratio
W = 0.62198 × Pv / (P − Pv)
Where: W is humidity ratio in kg water per kg dry air, Pv is actual vapor pressure, Pws is saturation vapor pressure, P is barometric pressure, and RH is relative humidity in percent.
This calculator uses a standard psychrometric relation and a common vapor-pressure approximation. It is suitable for education, HVAC estimates, air-handling checks, and general engineering review.
How to Use This Calculator
- Enter the air temperature.
- Select the correct temperature unit.
- Enter relative humidity from 0 to 100.
- Enter barometric pressure and choose its unit.
- Select the number of decimal places you want.
- Press the calculate button.
- Read the result cards above the form.
- Review the Plotly graph for the humidity trend.
- Download the result as CSV or PDF.
FAQs
1. What is humidity ratio?
Humidity ratio is the mass of water vapor mixed with each unit mass of dry air. It is commonly expressed as kg/kg dry air or g/kg dry air.
2. Is humidity ratio the same as relative humidity?
No. Relative humidity compares actual vapor pressure with saturation vapor pressure. Humidity ratio measures the actual amount of moisture carried by the air.
3. Why does pressure matter in this calculation?
Pressure changes the relationship between vapor pressure and dry-air mass. The same relative humidity can produce a different humidity ratio at a different barometric pressure.
4. Which temperature should I enter?
Enter dry-bulb air temperature. That is the standard air temperature measured by a normal thermometer before any wet-bulb or dew-point adjustment.
5. Can I use Fahrenheit or Kelvin?
Yes. The calculator accepts Celsius, Fahrenheit, and Kelvin. It converts the selected unit internally before evaluating the psychrometric equations.
6. What output units are included?
You get humidity ratio in kg/kg and g/kg, specific humidity, vapor pressures, dew point, degree of saturation, and grains per pound.
7. Are the results exact for every condition?
No. The results are strong engineering estimates based on standard formulas. Extreme conditions and specialized research work may require higher-order property models.
8. Where is this calculator useful?
It is useful in HVAC design, drying analysis, indoor air studies, weather education, psychrometric review, ventilation checks, and laboratory air-handling work.