Water Vapor Pressure Calculator

Calculator Inputs

The page uses a single-column flow. The calculator fields switch to three columns on large screens, two on smaller screens, and one on mobile.

Calculation mode

Formula model

Temperature

Relative humidity

%

Actual vapor pressure

Total system pressure

Reset

Plotly Graph

The chart shows the Buck saturation vapor pressure curve around the current working range. When available, the result point and actual pressure line are also displayed.

Example Data Table

These sample values use the Buck saturation equation for water.

Temperature (°C)	Saturation Vapor Pressure (kPa)	Saturation Vapor Pressure (mmHg)	Approx. Saturation Mole Fraction at 101.325 kPa
0	0.61121	4.584452	0.00603217
10	1.22786	9.209709	0.01211804
25	3.168531	23.76594	0.03127097
40	7.38236	55.372251	0.07285822
60	19.945145	149.600894	0.19684328
80	47.410267	355.60625	0.46790296
100	101.307781	759.870849	0.99983006

Formula Used

1) Buck saturation vapor pressure equation

e_s(T) = 0.61121 × exp[(18.678 − T/234.5) × T / (257.14 + T)]

Here, T is temperature in °C and e_s is saturation vapor pressure in kPa. This is the default broad-range model in the calculator.

2) Antoine equation for water

log₁₀(P) = A − B / (C + T)

The calculator uses standard water coefficients and converts the Antoine result from mmHg to kPa. This model is common in chemistry references.

3) Actual vapor pressure from relative humidity

e = (RH / 100) × e_s

4) Relative humidity from actual pressure

RH = 100 × e / e_s

5) Vapor pressure deficit

VPD = e_s − e

6) Mole fraction of water vapor

y = e / P_total

7) Absolute humidity

ρ_v = 216.7 × e(hPa) / T(K)

8) Dew point

The dew point is obtained by numerically inverting the selected saturation relation until the saturation pressure equals the entered actual vapor pressure.

How to Use This Calculator

Select the calculation mode that matches your chemistry task.
Choose either the Buck equation or Antoine equation.
Enter temperature when the selected mode requires it.
Enter relative humidity or actual vapor pressure if needed.
Optionally enter total system pressure for mole fraction outputs.
Press Calculate to show the result above the form.
Review the converted units, dew point, humidity, and notes.
Use the CSV or PDF buttons to download results.

Frequently Asked Questions

1) What is water vapor pressure?

Water vapor pressure is the partial pressure created by gaseous water molecules. At saturation, it depends mainly on temperature for pure water.

2) Does vapor pressure increase with temperature?

Yes. Higher temperature gives water molecules more kinetic energy, so more molecules escape into the gas phase and vapor pressure rises.

3) What is the difference between saturation and actual vapor pressure?

Saturation vapor pressure is the maximum equilibrium value at a temperature. Actual vapor pressure is the real partial pressure currently present in the gas mixture.

4) Why is relative humidity linked to vapor pressure?

Relative humidity compares actual vapor pressure with saturation vapor pressure. It tells you how close the air or gas mixture is to saturation.

5) What does vapor pressure deficit mean?

Vapor pressure deficit is the gap between saturation and actual vapor pressure. It is useful when studying evaporation, drying, and gas-water equilibrium behavior.

6) Which model should I choose?

Choose Buck for a practical broad-range estimate. Choose Antoine when you want a classic chemistry correlation within its coefficient temperature range.

7) Can relative humidity exceed 100%?

It can appear above 100% during supersaturation, rapid cooling, condensation onset, or inconsistent inputs. The calculator flags this so you can review conditions.

8) Is this valid for salt water or other solutions?

No. Dissolved solutes reduce water activity and lower vapor pressure. This calculator is intended for pure water behavior unless you add solution corrections separately.