Calculator Inputs
Example Data Table
| Scenario | Dry Bulb | RH | Pressure | Dew Point | Humidity Ratio | Absolute Humidity | Enthalpy |
|---|---|---|---|---|---|---|---|
| Balanced office | 24.0 °C | 50% | 101.3 kPa | 12.9 °C | 9.3 g/kg | 10.9 g/m³ | 47.8 kJ/kg |
| Humid classroom | 28.0 °C | 65% | 100.0 kPa | 20.8 °C | 15.6 g/kg | 17.5 g/m³ | 68.1 kJ/kg |
| Dry heated room | 20.0 °C | 35% | 95.0 kPa | 4.1 °C | 5.3 g/kg | 6.0 g/m³ | 33.5 kJ/kg |
Example values are rounded for readability and may vary slightly with equation choice.
Formula Used
1. Saturation vapor pressure: The calculator uses the Buck equation to estimate saturated water vapor pressure at the dry bulb temperature.
2. Actual vapor pressure: Actual vapor pressure = relative humidity × saturation vapor pressure / 100.
3. Dew point: Dew point is obtained by inverting the vapor pressure relation with a Magnus-style logarithmic expression.
4. Humidity ratio: W = 0.62198 × Pv / (P − Pv), where Pv is actual vapor pressure and P is station pressure.
5. Absolute humidity: Absolute humidity = 2167 × Pv / (T + 273.15), giving grams of water per cubic meter.
6. Enthalpy: h = 1.006T + W(2501 + 1.86T), in kJ per kilogram of dry air.
7. Moisture change: Room moisture adjustment = (target absolute humidity − current absolute humidity) × room volume.
How to Use This Calculator
- Enter the indoor dry bulb temperature and choose Celsius or Fahrenheit.
- Type the measured relative humidity from your hygrometer or sensor.
- Select how pressure should be handled: sea level, station pressure, or altitude.
- Enter room volume to estimate how much moisture must be added or removed.
- Enter the coldest indoor surface temperature to check condensation risk.
- Set a target humidity level for comfort or equipment protection.
- Press the calculate button to show the result summary above the form.
- Review the chart and export the results as CSV or PDF.
FAQs
1. What indoor humidity range is usually recommended?
Most buildings perform well between 30% and 60% relative humidity. Many homes and offices feel best around 40% to 50%, depending on temperature, ventilation, and occupancy.
2. Why does pressure matter in a humidity calculation?
Pressure affects humidity ratio, specific volume, and enthalpy. At higher elevations, the same temperature and relative humidity can produce different air properties than at sea level.
3. What is the difference between relative and absolute humidity?
Relative humidity compares current moisture with the maximum possible moisture at that temperature. Absolute humidity reports the actual water vapor mass in each cubic meter of air.
4. Why is dew point important indoors?
Dew point shows the temperature where condensation begins. If a wall, window, or duct surface drops below the dew point, water can form and increase mold or material damage risk.
5. Can this calculator help size a humidifier or dehumidifier?
It helps estimate the moisture addition or removal needed for the room air volume. Final equipment sizing should also consider infiltration, ventilation, occupancy, and moisture generation.
6. Why is the wet bulb temperature lower than dry bulb?
Wet bulb reflects evaporative cooling potential. Drier air allows more evaporation, so wet bulb drops further below dry bulb. When air approaches saturation, the two temperatures move closer together.
7. Does high humidity always mean condensation will occur?
No. Condensation depends on both moisture level and surface temperature. Warm humid air may stay stable until it touches a colder surface below the calculated dew point.
8. Is this equivalent to a full psychrometric chart?
It is a practical single-point psychrometric calculator. It covers common engineering outputs, but a full chart still helps visualize broader air-conditioning and process-air changes.