Water Vapor Pressure & Humidity
Water vapor pressure is the partial pressure of water vapor in equilibrium with liquid water. The Buck equation P = 0.61121 × e^((18.678 - T/234.5) × T/(257.14+T)) is highly accurate for 0–100°C. Essential for humidity, dew point, and meteorological calculations.
Why This Chemistry Calculation Matters
Why: Water vapor pressure underpins humidity calculations, dew point determination, HVAC design, and weather forecasting. Critical for food preservation and environmental science.
How: Enter temperature; the calculator applies Buck (most accurate), Antoine, Magnus, and Tetens formulas. Buck typically has error <0.01% for 0–100°C.
- ●Buck formula is most accurate for typical atmospheric temperatures.
- ●Relative humidity RH = (actual P / saturation P) × 100%.
- ●Dew point is the temperature at which air becomes saturated.
Water Vapor Pressure Examples
❄️ Water at 0°C
Vapor pressure at freezing point
🌡️ Water at 25°C
Room temperature vapor pressure
💧 Water at 50°C
Moderate temperature
🔥 Water at 80°C
Near boiling point
💨 Water at 100°C
Boiling point at 1 atm
🌊 Water at 20°C
Standard room temperature
☀️ Water at 30°C
Warm day temperature
🌡️ Water at 40°C
Hot day temperature
🔥 Water at 60°C
Hot water temperature
💨 Water at 90°C
Very near boiling
❄️ Water at 10°C
Cool temperature
🧊 Water at 5°C
Near freezing
🌤️ Water at 15°C
Mild temperature
☀️ Water at 35°C
Body temperature range
🔥 Water at 75°C
Hot water
Calculate Water Vapor Pressure
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
Buck formula error typically <0.01% for 0–100°C.
— NIST
At 25°C: P_sat ≈ 3.17 kPa (23.8 mmHg).
— NIST
At 100°C: P_sat = 101.325 kPa (boiling at 1 atm).
— IUPAC
RH = 100% at dew point; condensation begins.
— WMO
What is Water Vapor Pressure?
Water vapor pressure is the partial pressure of water vapor in the atmosphere. It represents the pressure exerted by water molecules in the gas phase when in equilibrium with liquid water at a given temperature. This is crucial for understanding humidity, cloud formation, and weather patterns.
Water Vapor Pressure Table
| Temperature (°C) | Temperature (°F) | Vapor Pressure (kPa) | Vapor Pressure (mmHg) | Vapor Pressure (atm) |
|---|---|---|---|---|
| 0 | 32 | 0.6112 | 4.5845 | 0.0060 |
| 5 | 41 | 0.8724 | 6.5439 | 0.0086 |
| 10 | 50 | 1.2279 | 9.2097 | 0.0121 |
| 15 | 59 | 1.7052 | 12.7899 | 0.0168 |
| 20 | 68 | 2.3383 | 17.5390 | 0.0231 |
| 25 | 77 | 3.1685 | 23.7660 | 0.0313 |
| 30 | 86 | 4.2451 | 31.8411 | 0.0419 |
| 35 | 95 | 5.6268 | 42.2042 | 0.0555 |
| 40 | 104 | 7.3824 | 55.3724 | 0.0729 |
| 50 | 122 | 12.3494 | 92.6284 | 0.1219 |
| 60 | 140 | 19.9451 | 149.6013 | 0.1968 |
| 70 | 158 | 31.2010 | 234.0272 | 0.3079 |
| 80 | 176 | 47.4103 | 355.6072 | 0.4679 |
| 90 | 194 | 70.1521 | 526.1858 | 0.6923 |
| 100 | 212 | 101.3078 | 759.8729 | 0.9998 |
Key Concepts
Relative Humidity
RH = (Actual vapor pressure / Saturation vapor pressure) × 100%. When RH = 100%, condensation occurs.
Dew Point
The temperature at which air becomes saturated with water vapor and condensation begins to form.
Saturation
When vapor pressure equals saturation vapor pressure, the air is at 100% relative humidity.
How Do Water Vapor Pressure Formulas Work?
Multiple empirical and theoretical formulas exist to calculate water vapor pressure. Each has different accuracy ranges and applications. The Buck formula is generally most accurate for typical temperatures (0-100°C).
🔬 Antoine Equation
log₁₀(P) = A - B/(C + T)
0-100°C: A=8.07131, B=1730.63, C=233.426
100-374°C: A=8.14019, B=1810.94, C=244.485
Most accurate for high temperatures (>100°C)
📊 Buck Formula (Arden Buck)
P = 0.61121 × e^((18.678 - T/234.5) × T/(257.14+T))
Most accurate for 0-100°C range
Error typically <0.01% in typical conditions
🌡️ Magnus Formula
P = 0.61094 × e^(17.625×T/(T+243.04))
Good accuracy for 0-50°C
Commonly used in meteorology
💧 Tetens Formula
P = 0.61078 × e^(17.27×T/(T+237.3))
Similar to Magnus, good for 0-50°C
Used in agricultural and environmental applications
When to Use Water Vapor Pressure Calculations
Water vapor pressure calculations are essential in meteorology, HVAC design, food preservation, and many other fields.
Meteorology
Calculate humidity, dew point, and predict weather patterns including cloud formation and precipitation.
- Weather forecasting
- Humidity calculations
- Dew point determination
HVAC Systems
Design heating, ventilation, and air conditioning systems with proper humidity control.
- Humidity control
- Condensation prevention
- Comfort calculations
Food Preservation
Optimize drying processes, predict spoilage, and design food storage systems.
- Freeze-drying
- Dehydration
- Shelf life prediction
Water Vapor Pressure Formulas Comparison
Accuracy Comparison (0-100°C)
Buck Formula: Most accurate, error <0.01%
Antoine Equation: Very accurate, especially >100°C
Tetens Formula: Good for 0-50°C, error ~0.05%
Magnus Formula: Good for 0-50°C, error ~0.06%
Simple Formula: Less accurate, error ~1-7%
Practical Water Vapor Pressure Examples
Example: Room Temperature
Given:
- Temperature: 25°C
- Calculate vapor pressure
Solution (Buck):
P = 0.61121 × e^((18.678 - 25/234.5) × 25/(257.14+25))
P = 3.169 kPa (23.8 mmHg)
Example: Boiling Point
Given:
- Temperature: 100°C
- Atmospheric pressure: 101.325 kPa
Solution:
At 100°C, vapor pressure equals atmospheric pressure
P = 101.325 kPa (760 mmHg)
This is why water boils at 100°C at sea level
Limitations and Considerations
⚠️ When Calculations May Not Apply
- • Very high pressures (non-ideal behavior)
- • Near critical point (374°C, 22.1 MPa)
- • Saltwater or solutions (lower vapor pressure)
- • Temperature ranges outside formula validity
- • Non-equilibrium conditions
✓ Assumptions Made
- • Pure water (no dissolved substances)
- • Equilibrium conditions
- • Standard atmospheric composition
- • Flat surface (no curvature effects)
- • Temperature is uniform
📚 Official Data Sources
⚠️ Disclaimer: This calculator uses empirical formulas (Antoine, Buck, Magnus, Tetens) for water vapor pressure. For precise work consult NIST Steam Tables, IAPWS-IF97 for water properties, and IUPAC Gold Book for thermodynamic definitions.