In short: warmer water molecules escape into the air faster, so vapor pressure rises steeply with temperature — from about 0.611 kPa at freezing to 2.338 kPa at room temperature (20 °C) and over 100 kPa at boiling. If the air isn't fully saturated, the actual vapor pressure is lower than this maximum — multiply by the relative humidity to find it.
These figures are planning and reference aids based on published equations. For calibrated instrumentation or safety-critical HVAC design, confirm with a qualified engineer.
How to calculate water vapor pressure
The saturation vapor pressure of water is the maximum pressure that water vapor can exert above a liquid surface at a given temperature. This calculator uses the Tetens equation, a well-tested empirical fit recommended by the World Meteorological Organization:
Worked example. At 25 °C: 17.27 × 25 = 431.75; divide by (25 + 237.3) = 262.3 to get 1.646; e1.646 ≈ 5.186; multiply by 0.6108 to get 3.168 kPa — the saturation vapor pressure at 25 °C. If the air is only 60% saturated, the actual vapor pressure is 3.168 × 0.60 = 1.901 kPa, and the vapor pressure deficit is 3.168 − 1.901 = 1.267 kPa.
Saturation vs. actual, in one line. Saturation vapor pressure is the ceiling at a given temperature; actual (partial) vapor pressure is how much water vapor is really present. The gap between the two — the vapor pressure deficit — drives evaporation, drying and plant transpiration.
Why temperature changes the answer
Heat speeds up water molecules at the liquid surface, so more of them have enough energy to break free into the air. That escape rate grows roughly exponentially with temperature, which is why saturation vapor pressure climbs so much faster than temperature itself — doubling roughly every 10–12 °C in the middle of the range shown here.
Relative humidity describes how close the air is to that ceiling: 100% RH means the air is fully saturated and can hold no more moisture at its current temperature (this is exactly the air temperature at the dew point). Below 100%, the actual vapor pressure is only a fraction of the maximum, and the shortfall — the vapor pressure deficit — is what pulls moisture out of skin, soil, laundry and open water. You can read the full method on our methodology page.
Saturation vapor pressure of water by temperature
The values this calculator uses at 100% relative humidity, from freezing to boiling. Notice how quickly the pressure accelerates in the upper half of the range.
| Temperature | kPa | mmHg | psi |
|---|---|---|---|
| 0 °C / 32 °F (freezing) | 0.6108 | 4.581 | 0.0886 |
| 4 °C / 39 °F | 0.8133 | 6.100 | 0.1180 |
| 10 °C / 50 °F | 1.228 | 9.210 | 0.1781 |
| 20 °C / 68 °F (room) | 2.338 | 17.539 | 0.3391 |
| 25 °C / 77 °F | 3.168 | 23.760 | 0.4594 |
| 30 °C / 86 °F | 4.243 | 31.826 | 0.6154 |
| 37 °C / 99 °F (body) | 6.275 | 47.065 | 0.9101 |
| 50 °C / 122 °F | 12.337 | 92.533 | 1.789 |
| 60 °C / 140 °F | 19.933 | 149.5 | 2.891 |
| 80 °C / 176 °F | 47.525 | 356.5 | 6.893 |
| 100 °C / 212 °F (boiling) | 102.2 | 766.7 | 14.825 |
Quick reference: actual vapor pressure at 20 °C
How relative humidity scales the actual vapor pressure at a fixed room temperature of 20 °C, where saturation is 2.338 kPa.
| Relative humidity | Actual vapor pressure (kPa) | mmHg |
|---|---|---|
| 100% (saturated) | 2.338 | 17.539 |
| 80% | 1.871 | 14.031 |
| 60% | 1.403 | 10.523 |
| 50% (typical indoor) | 1.169 | 8.769 |
| 40% | 0.9353 | 7.015 |
| 30% (dry) | 0.7015 | 5.262 |
| 20% | 0.4677 | 3.508 |
Worked examples
Typical indoor air
1.169 kPa
At 20 °C and 50% relative humidity — a comfortable indoor reading — the actual vapor pressure is 1.169 kPa, exactly half the 2.338 kPa saturation ceiling at that temperature.
Fog or dew forming
3.168 kPa
At 25 °C and 100% relative humidity the air is fully saturated: actual and saturation vapor pressure are equal at 3.168 kPa, and the vapor pressure deficit drops to zero — condensation begins.
Greenhouse growing conditions
4.392 kPa
At 37 °C and 30% relative humidity the vapor pressure deficit is a high 4.392 kPa — air pulling moisture out of leaves quickly. Growers usually aim well below this to avoid plant stress.
What else affects the result
Dissolved substances. Salts and other dissolved solutes lower vapor pressure slightly (Raoult's law) — seawater's vapor pressure runs a percent or two below fresh water at the same temperature. This calculator models pure water.
Ambient (barometric) pressure. Saturation vapor pressure is a property of the water itself and does not depend on surrounding air pressure. Barometric pressure does determine the boiling point, though — see our boiling point at altitude calculator for how that works at elevation.
Ice vs. liquid. Below 0 °C this equation still describes supercooled liquid water; the saturation vapor pressure over ice is slightly lower than over liquid water at the same sub-zero temperature.
Airflow and surface area. These change how fast a system reaches equilibrium — the rate of evaporation — but not the saturation vapor pressure itself, which depends only on temperature. For absolute humidity in mass terms, see our humidity to water content calculator.
Frequently asked questions
What is water vapor pressure?
Water vapor pressure is the partial pressure exerted by water molecules that have evaporated into the air above a water surface. At any given temperature, air can hold water vapor up to a maximum called the saturation vapor pressure; beyond that, excess vapor condenses back to liquid as dew, fog or rain.
What is the saturation vapor pressure of water at room temperature?
At 20 °C (68 °F), the saturation vapor pressure of water is about 2.338 kPa (17.5 mmHg, 0.339 psi). This is the maximum vapor pressure air can hold at that temperature before water starts condensing out.
What is the difference between saturation and actual vapor pressure?
Saturation vapor pressure is the maximum possible at a given temperature, reached at 100% relative humidity. Actual (or partial) vapor pressure is the true amount present in less-than-saturated air, found by multiplying the saturation value by the relative humidity. At 20 °C and 50% RH, saturation is 2.338 kPa but the actual vapor pressure is only 1.169 kPa.
Why does vapor pressure rise so quickly with temperature?
Water molecules escape the liquid surface at a rate that grows roughly exponentially with temperature, so a modest temperature rise evaporates far more molecules into the air. That is why saturation vapor pressure at 100 °C is more than 40 times higher than at 20 °C, even though the absolute temperature has only about tripled.
What is vapor pressure deficit (VPD) and why does it matter?
Vapor pressure deficit is the difference between saturation and actual vapor pressure — how much more moisture the air could hold before becoming saturated. A high VPD pulls moisture rapidly from plants, skin or open water; a low VPD near zero means the air is close to saturated and evaporation slows. Growers commonly target roughly 0.4 to 1.6 kPa for most greenhouse crops.
How accurate is the Tetens equation used here?
The Tetens equation is accurate to within about 1% of reference data across most everyday conditions from 0 to 50 °C, and is recommended by the World Meteorological Organization for general use. It becomes slightly less precise approaching 100 °C, where more elaborate equations are preferred for exact engineering work.
Sources & references
- Tetens, O. (1930). Über einige meteorologische Begriffe. Zeitschrift für Geophysik, 6, 297–309.
- World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)