How do you calculate saturation vapor pressure?

This calculator uses the Magnus-Tetens formula: eₛ = 6.112 × exp(17.625 × T / (243.04 + T)), where T is temperature in °C and eₛ is in hectopascals (hPa). Relative humidity does not affect it — saturation pressure depends only on temperature.

What is the difference between actual and saturation vapor pressure?

Actual vapor pressure is simply saturation vapor pressure multiplied by relative humidity: e = eₛ × (RH ÷ 100). At 100% RH the two are equal — the air is saturated and dew forms.

What units is vapor pressure measured in?

The SI unit is the pascal (Pa); meteorology uses hectopascals (hPa), which equal millibars (mb). Other common units are kilopascals (kPa = hPa ÷ 10) and millimeters of mercury (mmHg = hPa × 0.750). This tool reports hPa, kPa, and mmHg.

Why does vapor pressure matter?

Vapor pressure drives evaporation, transpiration, and drying. Growers use the gap between saturation and actual vapor pressure — the vapor pressure deficit (VPD) — to manage plant health, while HVAC engineers use it to size dehumidification.

Vapor Pressure Calculator · Relative Humidity Calculator

How to use this calculator

Enter an air temperature and relative humidity above and press Calculate. In the psychrometric state-point block you'll find both Saturation Vapor Pressure (eₛ) and Actual Vapor Pressure (e), reported in hectopascals. Divide by 10 for kPa, or multiply by 0.750 for mmHg — or read the values straight from the table below.

What is vapor pressure?

Air is a mixture of gases, and each contributes part of the total pressure. The share contributed by water vapor is the vapor pressure. When the air holds as much moisture as it possibly can at a given temperature, it has reached its saturation vapor pressure — add any more and the excess condenses into dew, fog, or cloud. Because warmer air can hold exponentially more moisture, saturation vapor pressure climbs steeply with temperature.

The formula

Saturation vapor pressure comes from the Magnus-Tetens approximation (Alduchov & Eskridge, 1996), and the actual vapor pressure is just that scaled by relative humidity:

eₛ = 6.112 × exp( 17.625 × T / (243.04 + T) )
e  = eₛ × (RH / 100)

eₛ = saturation vapor pressure (hPa)
e  = actual vapor pressure (hPa)
T  = air temperature (°C)
RH = relative humidity (%)

Saturation vapor pressure table

Saturation vapor pressure depends only on temperature. These values are computed at build time from the same engine that powers the calculator, so they match its output exactly.

Temp (°C)	Temp (°F)	eₛ (hPa)	eₛ (kPa)	eₛ (mmHg)
-10°	14°	2.87	0.287	2.15
-5°	23°	4.22	0.422	3.17
0°	32°	6.11	0.611	4.58
5°	41°	8.72	0.872	6.54
10°	50°	12.27	1.227	9.20
15°	59°	17.03	1.703	12.77
20°	68°	23.34	2.334	17.51
25°	77°	31.63	3.163	23.72
30°	86°	42.38	4.238	31.79
35°	95°	56.20	5.620	42.15
40°	104°	73.78	7.378	55.34
45°	113°	95.95	9.595	71.97
50°	122°	123.66	12.366	92.75

Worked example

At 25°C (77°F) and 60% relative humidity: the saturation vapor pressure is 31.63 hPa (see the table), so the actual vapor pressure is 31.63 × 0.60 ≈ 18.98 hPa. The 12.65 hPa gap between them is the vapor pressure deficit, which tells you how briskly water will evaporate.

Related calculations

Once you know the vapor pressure you can derive almost every other moisture metric. Learn how in How to Calculate Relative Humidity from Vapor Pressure, or jump to the absolute humidity calculator and the dew point calculator.

Frequently Asked Questions

What is vapor pressure?: Vapor pressure is the partial pressure exerted by water vapor in the air. Actual vapor pressure (e) is how much the water molecules currently present are pushing; saturation vapor pressure (eₛ) is the maximum the air can hold at that temperature before condensing.
How do you calculate saturation vapor pressure?: This calculator uses the Magnus-Tetens formula: eₛ = 6.112 × exp(17.625 × T / (243.04 + T)), where T is temperature in °C and eₛ is in hectopascals (hPa). Relative humidity does not affect it — saturation pressure depends only on temperature.
What is the difference between actual and saturation vapor pressure?: Actual vapor pressure is simply saturation vapor pressure multiplied by relative humidity: e = eₛ × (RH ÷ 100). At 100% RH the two are equal — the air is saturated and dew forms.
What units is vapor pressure measured in?: The SI unit is the pascal (Pa); meteorology uses hectopascals (hPa), which equal millibars (mb). Other common units are kilopascals (kPa = hPa ÷ 10) and millimeters of mercury (mmHg = hPa × 0.750). This tool reports hPa, kPa, and mmHg.
Why does vapor pressure matter?: Vapor pressure drives evaporation, transpiration, and drying. Growers use the gap between saturation and actual vapor pressure — the vapor pressure deficit (VPD) — to manage plant health, while HVAC engineers use it to size dehumidification.