Vapour Pressure Calculator from Temperature and Relative Humidity
Enter air temperature and relative humidity to calculate saturation vapour pressure, actual vapour pressure, vapour pressure deficit, and dew point.
Results
Enter values and click Calculate Vapour Pressure.
How to calculate vapour pressure from temperature and relative humidity
If you work with weather data, greenhouse control, HVAC, drying processes, or building moisture analysis, learning to calculate vapour pressure from temperature and relative humidity is one of the most practical psychrometric skills you can develop. Relative humidity alone can be misleading because it is temperature dependent. Vapour pressure gives you an absolute measure of how much water vapor is in the air. Once you know vapour pressure, you can compare moisture conditions across very different temperatures, estimate dew point behavior, and evaluate evaporation potential with far better confidence.
In simple terms, air can hold only a limited amount of water vapor before condensation begins. That limit at a given temperature is called saturation vapour pressure. Relative humidity tells you what percentage of that limit the air currently holds. So if you have temperature and relative humidity, you can convert directly to actual vapour pressure. This is exactly what the calculator above does.
Why vapour pressure is often better than relative humidity alone
- Relative humidity changes when temperature changes, even if the actual moisture content does not.
- Actual vapour pressure is tied to absolute moisture in the air, which is often what engineers and meteorologists need.
- Vapour pressure helps compare indoor and outdoor moisture loads for ventilation decisions.
- It is useful for crop water stress, drying rates, and mold risk analysis in buildings.
The core equations used in this calculator
To calculate actual vapour pressure, we first estimate saturation vapour pressure from temperature. A common approach is the Magnus form:
es = 0.61094 × exp((17.625 × T) / (T + 243.04))
where T is air temperature in Celsius and es is in kilopascals (kPa).
Then actual vapour pressure is:
ea = (RH / 100) × es
where RH is relative humidity percent. The calculator also computes vapour pressure deficit (VPD):
VPD = es – ea
VPD is widely used in agriculture and plant physiology because it tracks the drying power of air.
Step by step manual method
- Measure air temperature and convert to Celsius if needed.
- Measure relative humidity as a percent from 0 to 100.
- Use a saturation equation (Magnus or Buck) to get es.
- Multiply es by RH/100 to get actual vapour pressure ea.
- Subtract ea from es if you need VPD.
- Optionally estimate dew point for condensation planning.
Reference comparison table: saturation vapour pressure by temperature
The values below are standard approximate psychrometric reference values in kPa and show how strongly saturation vapour pressure rises with temperature. This is why warm air can carry far more water vapor than cold air.
| Temperature (°C) | Saturation Vapour Pressure es (kPa) | Saturation Vapour Pressure (hPa) | Saturation Vapour Pressure (Pa) |
|---|---|---|---|
| 0 | 0.611 | 6.11 | 611 |
| 5 | 0.872 | 8.72 | 872 |
| 10 | 1.228 | 12.28 | 1228 |
| 15 | 1.705 | 17.05 | 1705 |
| 20 | 2.339 | 23.39 | 2339 |
| 25 | 3.169 | 31.69 | 3169 |
| 30 | 4.243 | 42.43 | 4243 |
| 35 | 5.628 | 56.28 | 5628 |
| 40 | 7.375 | 73.75 | 7375 |
Comparison table: actual vapour pressure at 30°C under different humidity levels
Since saturation vapour pressure at 30°C is about 4.243 kPa, actual vapour pressure changes directly with relative humidity. This makes a good practical benchmark for warm season comfort, greenhouse control, and HVAC latent load discussions.
| Relative Humidity (%) | Actual Vapour Pressure ea (kPa) | VPD (kPa) | Interpretation |
|---|---|---|---|
| 30 | 1.273 | 2.970 | Very drying air, high evaporation demand |
| 50 | 2.122 | 2.121 | Moderate drying potential |
| 70 | 2.970 | 1.273 | Humid conditions, reduced drying |
| 90 | 3.819 | 0.424 | Near saturated, condensation risk rises |
Worked example
Suppose your sensor reports 25°C and 60% RH. First compute saturation vapour pressure at 25°C, approximately 3.17 kPa. Then actual vapour pressure equals 0.60 × 3.17 = 1.90 kPa. This means the air currently carries about 1.90 kPa of water vapor partial pressure. Vapour pressure deficit is 3.17 – 1.90 = 1.27 kPa. If you are evaluating plant transpiration, 1.27 kPa is a moderate to fairly active transpiration environment for many crops, depending on species and growth stage.
Practical use cases across industries
Agriculture and greenhouse management
Growers monitor vapour pressure deficit to balance transpiration and disease pressure. Low VPD can suppress transpiration and encourage pathogen conditions, while excessively high VPD can stress plants and increase irrigation demand. Temperature and RH sensors tied to automatic control systems often use these vapour pressure calculations in real time.
HVAC and building science
Indoor comfort is not just about dry bulb temperature. Moisture load affects occupant comfort, odor, and material durability. By converting indoor and outdoor readings to vapour pressure, engineers can estimate latent transfer direction. Moisture generally moves from higher vapor pressure zones toward lower vapor pressure zones, especially when airflow and diffusion pathways exist.
Drying, storage, and process control
Food processing, pharmaceutical manufacturing, and archival storage all rely on moisture control. Relative humidity targets are useful, but actual vapour pressure helps compare conditions during temperature swings. This is vital for preventing condensation on products and minimizing moisture driven degradation.
Common mistakes when calculating vapour pressure
- Using Fahrenheit directly in Celsius based equations without conversion.
- Entering RH as a decimal when the equation expects percent, or the reverse.
- Confusing saturation vapour pressure with actual vapour pressure.
- Ignoring sensor calibration drift, especially in low cost RH probes.
- Comparing values in mixed units such as hPa and kPa without conversion.
Unit conversions you should memorize
- 1 kPa = 10 hPa
- 1 kPa = 1000 Pa
- 1 hPa = 100 Pa
Many weather reports use hPa or millibars for pressure context, while psychrometric engineering calculations often use kPa. Keeping units consistent is essential for reliable interpretation.
Accuracy notes on formulas
Most online calculators use Magnus type approximations because they are simple and robust over common atmospheric ranges. The Buck equation often provides slightly improved fit near near-surface meteorological conditions. In professional or scientific workflows, exact standards can vary by organization, but the practical differences are usually small for routine applications in comfort, building operation, and field agriculture decisions.
Authoritative references and further reading
For deeper background on humidity, dew point, and weather moisture metrics, review these trusted resources:
- National Weather Service (.gov): Humidity and Dew Point overview
- NOAA Education (.gov): Dew point fundamentals
- UCAR Center for Science Education (.edu): Humidity learning guide
Final takeaway
To calculate vapour pressure from temperature and relative humidity, always start with saturation vapour pressure from temperature, then scale by RH fraction. This gives you an absolute moisture measure that is more stable and more decision ready than RH alone. Whether you are tuning greenhouse controls, diagnosing indoor condensation, or interpreting weather station data, vapour pressure and VPD provide a clear and physically meaningful view of air moisture behavior.
Educational note: The calculator provides engineering grade estimates suitable for most practical planning and monitoring tasks. For critical research or compliance calculations, align formulas and constants with your governing standard.