Vapor Pressure Calculator (Temperature + Relative Humidity)
Enter air temperature and relative humidity to calculate actual vapor pressure instantly. Built for weather analysis, HVAC checks, greenhouse control, and scientific workflows.
How to Calculate Vapor Pressure Given Temperature and Relative Humidity
If you work in meteorology, indoor air quality, agriculture, data center management, or process engineering, calculating vapor pressure from temperature and relative humidity is one of the most useful practical skills you can have. It tells you how much water vapor is actually in the air as a pressure value, rather than only as a percentage. This gives you a direct physical measure of moisture conditions, helps explain evaporation behavior, and supports better technical decision making.
Why vapor pressure matters in real operations
Relative humidity alone can be misleading when temperatures change. For example, 50% RH at 10°C and 50% RH at 30°C do not represent the same moisture content. Warm air can hold much more water vapor than cool air, so the same RH value can correspond to very different actual moisture levels. Vapor pressure solves this by expressing moisture in pressure units such as kPa or hPa.
- HVAC and building science: assess condensation risk and comfort control.
- Greenhouse management: tune transpiration conditions and disease pressure.
- Weather and climate analytics: compare moisture loads across locations and seasons.
- Industrial environments: optimize drying, coating, storage, and corrosion prevention.
The core equation used by this calculator
To compute actual vapor pressure from temperature and RH, you first estimate the saturation vapor pressure at the given temperature, then multiply by relative humidity fraction:
- Saturation vapor pressure: es(T)
- Actual vapor pressure: e = (RH / 100) × es(T)
This calculator uses a standard Tetens/Magnus-style approximation for temperatures in Celsius:
es(kPa) = 0.61078 × exp[(17.27 × T) / (T + 237.3)]
Then:
e(kPa) = (RH / 100) × es(kPa)
This method is widely used in environmental calculations, agronomy, and hydrology because it is accurate over common atmospheric temperature ranges and computationally simple.
Step-by-step example calculation
Suppose your measured air temperature is 25°C and RH is 60%.
- Calculate saturation vapor pressure at 25°C: es ≈ 3.17 kPa.
- Convert RH to fraction: 60% = 0.60.
- Multiply: e = 0.60 × 3.17 ≈ 1.90 kPa.
This means the air currently contains water vapor exerting about 1.90 kPa of partial pressure. That is the actual vapor pressure value you can compare across conditions, regardless of the RH percentage context.
Reference comparison table: saturation vapor pressure by temperature
The table below shows widely used physical values for saturation vapor pressure over liquid water at selected temperatures. These values are practical benchmarks used in atmospheric and engineering contexts.
| Temperature (°C) | Saturation Vapor Pressure (kPa) | Saturation Vapor Pressure (hPa) | Saturation Vapor Pressure (mmHg) |
|---|---|---|---|
| 0 | 0.611 | 6.11 | 4.58 |
| 10 | 1.228 | 12.28 | 9.21 |
| 20 | 2.338 | 23.38 | 17.54 |
| 30 | 4.243 | 42.43 | 31.83 |
| 40 | 7.384 | 73.84 | 55.38 |
You can see the non-linear rise very clearly: from 20°C to 30°C, saturation vapor pressure jumps from about 2.34 kPa to 4.24 kPa, nearly doubling. This is exactly why RH must be interpreted together with temperature.
Comparison table: how RH changes actual vapor pressure at 25°C
At 25°C, saturation vapor pressure is approximately 3.17 kPa. The table below shows actual vapor pressure values at different RH levels.
| Relative Humidity (%) | Actual Vapor Pressure (kPa) | Actual Vapor Pressure (hPa) | Practical Interpretation |
|---|---|---|---|
| 30% | 0.95 | 9.5 | Dry indoor air, higher evaporation demand |
| 50% | 1.58 | 15.8 | Typical comfort range in conditioned spaces |
| 60% | 1.90 | 19.0 | Moderate moisture, often acceptable in many climates |
| 80% | 2.54 | 25.4 | Humid conditions, lower evaporation potential |
| 95% | 3.01 | 30.1 | Near saturation, high condensation risk on cool surfaces |
Unit handling and conversions
Different industries use different pressure units. Meteorology often uses hPa (or mbar), engineering often uses Pa and kPa, and some lab or legacy contexts use mmHg.
- 1 kPa = 1000 Pa
- 1 kPa = 10 hPa
- 1 kPa ≈ 7.50062 mmHg
This calculator computes internally in kPa and converts the output to your selected unit to minimize rounding errors.
Common mistakes to avoid
- Using RH as a whole number in multiplication: Always divide RH by 100 first.
- Ignoring temperature unit conversion: Most formulas expect Celsius. Convert from °F or K before calculation.
- Comparing RH values across different temperatures: Compare vapor pressure values instead when evaluating absolute moisture load.
- Assuming linear behavior: Saturation vapor pressure increases exponentially with temperature.
How this relates to dew point and vapor pressure deficit
Actual vapor pressure is directly tied to dew point temperature, because dew point is the temperature at which saturation vapor pressure equals the current actual vapor pressure. In plant science, vapor pressure deficit (VPD) is the difference between saturation and actual vapor pressure: VPD = es – e. A higher VPD means stronger drying power of the air, which can increase transpiration in crops and also increase evaporation from soils and wet surfaces.
That is why serious environmental monitoring systems typically store temperature, RH, vapor pressure, and often dew point or VPD together. The combined view is much more actionable than any single metric by itself.
Authoritative references and further reading
For high-quality scientific and educational background, review these trusted sources:
- U.S. National Weather Service (.gov)
- NOAA educational material on moisture and dew point (.gov)
- Penn State meteorology educational resource (.edu)
These references help validate the physical interpretation of humidity, dew point, and moisture-pressure relationships used in practical calculations.
Bottom line
To calculate vapor pressure given temperature and relative humidity, find the saturation vapor pressure at that temperature and scale it by RH fraction. This gives you a physically meaningful moisture quantity that is better for technical decisions than RH alone. Use the calculator above for instant results, unit conversions, and a chart that visualizes how actual vapor pressure tracks below the saturation curve at your selected humidity level.