Vapor Pressure Calculator from Temperature and Humidity
Compute saturation vapor pressure, actual vapor pressure, and vapor pressure deficit using validated atmospheric formulas.
How to Calculate Vapor Pressure from Temperature and Humidity
Vapor pressure is one of the most useful and most misunderstood variables in weather science, HVAC design, agriculture, building diagnostics, and industrial process control. If you already know air temperature and relative humidity, you can estimate how much moisture is actually present in the air by calculating actual vapor pressure. This value is more physically meaningful than relative humidity alone because it represents the real partial pressure of water vapor molecules in air.
In practical terms, this means vapor pressure helps you answer questions that relative humidity cannot answer by itself. Two locations can both show 60% relative humidity, yet one may contain much more moisture simply because it is warmer. Temperature controls how much moisture air can hold at saturation. Relative humidity tells you the fraction of that capacity currently used. Combining both gives vapor pressure, which is often the variable you need for engineering and environmental decisions.
Core Concepts You Need First
To calculate vapor pressure correctly, you need three basic terms:
- Saturation vapor pressure (eₛ): The maximum possible water vapor pressure at a given temperature.
- Relative humidity (RH): The ratio of actual vapor pressure to saturation vapor pressure, expressed as a percentage.
- Actual vapor pressure (eₐ): The true partial pressure of water vapor in the air.
The relationship is simple and foundational:
eₐ = (RH / 100) × eₛ
So the real challenge is obtaining a reliable value for saturation vapor pressure from temperature. A standard and highly accurate approach for common atmospheric conditions is the Magnus-type equation over water:
eₛ (hPa) = 6.1094 × exp((17.625 × T) / (T + 243.04)), where T is in °C.
For ice surfaces, a variant constant set is often used to improve cold-weather accuracy. This calculator supports both options.
Step by Step Calculation Workflow
- Convert your input temperature to Celsius if it is in Fahrenheit or Kelvin.
- Compute saturation vapor pressure using a Magnus formulation.
- Multiply saturation vapor pressure by RH fraction to get actual vapor pressure.
- Optionally compute vapor pressure deficit (VPD): VPD = eₛ – eₐ.
- Convert to your preferred unit (kPa, hPa, Pa, mmHg, or inHg).
This process is standard in meteorology, crop science, and indoor environmental engineering. VPD is especially valuable in greenhouse management because it describes the drying power of air and is directly linked to plant transpiration response.
Reference Data: Saturation Vapor Pressure by Temperature
The table below shows representative saturation vapor pressure values over liquid water. These values are consistent with standard psychrometric approximations and illustrate the non-linear rise in moisture-holding capacity with temperature.
| Temperature (°C) | Saturation Vapor Pressure (kPa) | Saturation Vapor Pressure (hPa) |
|---|---|---|
| 0 | 0.611 | 6.11 |
| 5 | 0.872 | 8.72 |
| 10 | 1.228 | 12.28 |
| 15 | 1.705 | 17.05 |
| 20 | 2.338 | 23.38 |
| 25 | 3.169 | 31.69 |
| 30 | 4.243 | 42.43 |
| 35 | 5.628 | 56.28 |
| 40 | 7.384 | 73.84 |
This table highlights an important physical fact: going from 20°C to 30°C does not produce a small linear increase. Saturation vapor pressure jumps from about 2.34 kPa to 4.24 kPa, an increase of about 81%. That is why hot conditions can feel dramatically more humid even at similar RH percentages.
Applied Comparison: Same Temperature, Different Relative Humidity
At a fixed temperature of 30°C (where eₛ is about 4.243 kPa), actual vapor pressure and VPD vary strongly by RH. This directly affects comfort, drying rates, and plant stress.
| Relative Humidity (%) | Actual Vapor Pressure eₐ (kPa) | VPD (kPa) |
|---|---|---|
| 20 | 0.849 | 3.394 |
| 40 | 1.697 | 2.546 |
| 60 | 2.546 | 1.697 |
| 80 | 3.394 | 0.849 |
| 100 | 4.243 | 0.000 |
The symmetry in this table makes interpretation easier. At 30°C, every 20% RH increment shifts roughly 0.849 kPa between VPD and actual vapor pressure. As RH approaches saturation, VPD falls toward zero and evaporative demand weakens.
Why Vapor Pressure Is Better Than Relative Humidity Alone
Relative humidity is useful but contextual. A value of 50% does not tell you moisture mass content unless you also know temperature. Vapor pressure solves that by expressing moisture in pressure terms that can be compared directly between environments.
- Building science: Helps evaluate condensation risk and moisture migration across envelopes.
- Agriculture: Supports irrigation timing, disease pressure forecasting, and transpiration control through VPD.
- Meteorology: Improves interpretation of dew point behavior, boundary-layer moisture, and thunderstorm potential.
- HVAC: Enables psychrometric calculations used in latent load sizing and dehumidification strategy.
Common Mistakes and How to Avoid Them
- Using temperature in the wrong unit: Most empirical equations expect Celsius. Convert first.
- Forgetting RH is a percent: Divide by 100 before multiplying by saturation pressure.
- Mixing pressure units: hPa, kPa, and Pa differ by factors of 10 or 100. Keep a clean conversion chain.
- Applying one formula outside intended conditions: Use ice-specific constants in sustained sub-zero contexts if needed.
- Rounding too early: Keep full precision until final display to avoid compounding errors.
Unit Conversions You Will Use Often
- 1 kPa = 10 hPa = 1000 Pa
- 1 hPa = 100 Pa
- 1 hPa ≈ 0.75006 mmHg
- 1 hPa ≈ 0.02953 inHg
Most atmospheric datasets use hPa, while many engineering workflows prefer kPa. Laboratory and process environments may switch to Pa. Meteorological reports in some regions still use inHg or mmHg.
Interpreting Results in Real Environments
If your calculation returns a high actual vapor pressure, that means the air contains a substantial amount of water vapor regardless of whether RH appears moderate. For indoor air quality, this can indicate increased risk of moisture accumulation in cool surfaces or concealed cavities. For crops, the same result at a moderate RH may still be acceptable if VPD remains inside target range for the growth stage.
If VPD is very high, evaporation and transpiration rates increase. This can accelerate plant stress, skin dryness, and water demand. If VPD is too low, moisture removal slows and pathogen risk can rise in controlled environments. Therefore, many growers and facility managers monitor both actual vapor pressure and VPD as paired metrics.
Scientific and Educational References
For deeper background, these sources provide high-quality explanations of humidity, water vapor physics, and practical meteorological calculations:
- U.S. National Weather Service (weather.gov): Humidity fundamentals
- U.S. National Weather Service (weather.gov): Vapor pressure calculator resources
- Penn State (.edu): Meteorology lesson on vapor pressure and humidity relationships
Practical Example
Suppose air temperature is 77°F and RH is 55%. Convert 77°F to 25°C. Using the water-phase Magnus equation, saturation vapor pressure is about 31.7 hPa. Actual vapor pressure is 0.55 × 31.7 = 17.4 hPa, equivalent to 1.74 kPa. VPD is 31.7 – 17.4 = 14.3 hPa, or 1.43 kPa. This is a moderate drying environment, often acceptable for many indoor comfort and greenhouse conditions, depending on your exact target range.
Final Takeaway
Calculating vapor pressure from temperature and humidity is straightforward, but the impact of that calculation is significant. It gives you a physically grounded measure of atmospheric moisture content and a direct route to VPD. Whether you work in weather analysis, HVAC engineering, agriculture, or indoor environmental management, this is one of the highest-value calculations you can automate. Use the calculator above for fast results, review the chart for trend context, and keep unit consistency to maintain technical accuracy.