Vapor Pressure Calculator
Calculate vapor pressure from temperature or reverse-solve temperature from vapor pressure using Antoine or Clausius-Clapeyron equations.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Select the equation method, unknown variable, and liquid preset.
Enter the known values
Only fields required by the selected method and solve mode are active.
Visual Check
The curve shows vapor pressure versus temperature, valid preset range, and external pressure check.
Solution
Live result, converted units, warnings, and full solution steps.
Converted results and checks
- Check—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
Source/standard information updates based on the selected method.
- Assumptions will appear after a valid calculation.
On this page
Calculator Guide
How to Use the Vapor Pressure Calculator
The Vapor Pressure Calculator above estimates the equilibrium vapor pressure of a liquid at a selected temperature or solves for the temperature needed to reach a target vapor pressure. It supports Antoine equation inputs, Clausius-Clapeyron inputs, pressure unit conversions, temperature conversions, liquid presets, and step-by-step calculation checks.
Use the calculator when you need a quick engineering or chemistry estimate for vapor pressure, boiling behavior, vacuum conditions, or temperature-pressure relationships. The most important rule is that vapor pressure is an absolute pressure, and Antoine constants must match the exact pressure unit, temperature unit, and valid temperature range used by the constant set. When vapor pressure equals the surrounding external pressure, the liquid is at a boiling condition for that pressure.
Quick Answer
Vapor pressure is the pressure exerted by vapor in equilibrium with its liquid phase at a given temperature. To calculate it with the Antoine equation, enter the liquid temperature and matching constants \(A\), \(B\), and \(C\). To estimate pressure change from a known reference point, use the Clausius-Clapeyron method with \(P_1\), \(T_1\), \(T_2\), and \(\Delta H_{vap}\).
Do not rely on a simplified calculator when…
Do not use a simplified vapor pressure estimate as the only basis for pressure vessel design, relief sizing, flammable vapor evaluation, hazardous material handling, regulated process safety work, or final equipment selection. For safety-critical work, use verified thermodynamic property data, the correct valid range, and professional engineering review.
Inputs and Outputs Used by the Calculator
The calculator changes the active inputs based on the selected method. Antoine mode uses compound constants, while Clausius-Clapeyron mode uses a known reference pressure, reference temperature, final temperature or pressure, and enthalpy of vaporization.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Temperature, \(T\) | Liquid temperature used to estimate equilibrium vapor pressure. | °C, K, °F, °R |
| Input | Target Vapor Pressure, \(P\) | Pressure used when solving for the temperature that produces that vapor pressure. | kPa, atm, mmHg, torr |
| Input | Antoine Constants, \(A\), \(B\), \(C\) | Empirical constants for a specific compound, unit basis, and temperature range. | Unit-set dependent |
| Input | Known Pressure, \(P_1\) | Reference vapor pressure for Clausius-Clapeyron calculations. | Pa, kPa, atm, mmHg |
| Input | Known Temperature, \(T_1\) | Reference temperature corresponding to \(P_1\). | K or °C |
| Input | Enthalpy of Vaporization, \(\Delta H_{vap}\) | Energy required to vaporize one mole of liquid, used in the simplified Clausius-Clapeyron equation. | kJ/mol |
| Input | External Pressure | Surrounding absolute pressure used to check whether the liquid is near boiling. | kPa, atm, psi |
| Output | Vapor Pressure | Estimated equilibrium vapor pressure at the selected temperature. | Pa, kPa, bar, atm, mmHg, torr, psi |
| Output | Temperature / Boiling Point Check | Estimated temperature where the liquid reaches the target vapor pressure. | °C, K, °F, °R |
Vapor Pressure Formula
The two most useful vapor pressure formulas for a calculator are the Antoine equation and the integrated Clausius-Clapeyron equation. Use Antoine when valid compound constants are available; use Clausius-Clapeyron when you know one reference pressure, one reference temperature, and enthalpy of vaporization.
Antoine Equation
This is the most common calculator form. The pressure unit and temperature unit are not universal; they are built into the Antoine constants.
Antoine Equation Solved for Pressure
Use this form when temperature and constants \(A\), \(B\), and \(C\) are known.
Antoine Equation Solved for Temperature
Use this rearranged form to estimate the temperature or boiling point at a target vapor pressure.
Clausius-Clapeyron Equation
This form assumes \(\Delta H_{vap}\) is approximately constant over the temperature range. Temperatures must be absolute temperatures in Kelvin.
Clausius-Clapeyron Solved for Vapor Pressure
Use this form when you know the reference vapor pressure \(P_1\), the reference temperature \(T_1\), the new temperature \(T_2\), and \(\Delta H_{vap}\).
Antoine constants are not interchangeable
A, B, and C must come from the same parameter set. Do not mix constants from one source with a different pressure unit, temperature unit, or valid temperature range. Some sources use \(P\) in mmHg and \(T\) in °C, while others use \(P\) in bar and \(T\) in K.
Important formula detail
The Antoine equation often uses \(\log_{10}\), while Clausius-Clapeyron uses natural logarithm \(\ln\). Accidentally switching logarithm types will produce the wrong answer even if the rest of the inputs are correct.
What the Variables Mean
Every variable has a unit requirement. Vapor pressure equations are especially sensitive to whether temperature is entered in Celsius or Kelvin and whether pressure is entered in mmHg, torr, bar, kPa, or another absolute pressure unit.
| Symbol | Meaning | How to Enter It |
|---|---|---|
| \(P\) | Vapor pressure calculated from the Antoine equation. | Use the pressure unit required by the selected Antoine constant set, then convert to the desired output unit. |
| \(T\) | Temperature used in the Antoine equation. | Use the temperature basis required by the constants, such as °C or K. |
| \(A\), \(B\), \(C\) | Compound-specific Antoine constants. | Use constants from the same source, same unit basis, and same valid temperature range. |
| \(P_1\) | Known vapor pressure in the Clausius-Clapeyron equation. | Enter as an absolute pressure. \(P_1\) and \(P_2\) must be in the same pressure basis before forming the pressure ratio. |
| \(P_2\) | New vapor pressure or target vapor pressure. | Use the same pressure basis as \(P_1\) internally; the calculator converts units. |
| \(T_1\) | Known reference temperature corresponding to \(P_1\). | Enter in any supported temperature unit; calculation converts to Kelvin. |
| \(T_2\) | New temperature or calculated temperature. | Use Kelvin internally for Clausius-Clapeyron calculations. |
| \(\Delta H_{vap}\) | Molar enthalpy of vaporization. | Usually entered in kJ/mol; calculation converts to J/mol. |
| \(R\) | Universal gas constant. | \(R=8.314462618\;J/(mol \cdot K)\) |
How to Use the Calculator
Start by choosing the equation method that matches your available data. Then select whether you want to calculate vapor pressure or solve for temperature.
Choose the method
Select Antoine if you have valid constants for the liquid. Select Clausius-Clapeyron if you know \(P_1\), \(T_1\), \(\Delta H_{vap}\), and either \(T_2\) or \(P_2\).
Select the solve mode
Use vapor pressure mode when temperature is known. Use temperature mode when a target vapor pressure or boiling pressure is known.
Check the units and valid range
For Antoine constants, make sure the selected constant format matches the constants and check the valid range warning. For Clausius-Clapeyron, make sure all pressure values are absolute and all temperature values can be converted to Kelvin.
Review the result, conversions, and warnings
Compare the main result to converted units, the external pressure check, the valid range warning, and the solution steps.
How to Interpret Vapor Pressure Results
Vapor pressure increases strongly with temperature. A small temperature increase can cause a large pressure increase, especially near the normal boiling point.
| Result Pattern | What It Means | What to Check Next |
|---|---|---|
| Very low vapor pressure | The liquid is less volatile at the selected temperature. | Check whether the temperature is far below the boiling point. |
| Moderate vapor pressure | The liquid produces measurable vapor but may still be far below boiling at atmospheric pressure. | Compare the result with external pressure and application limits. |
| Near 101.325 kPa | The liquid is near its normal boiling point only when the external pressure is 1 atm. | Check whether the external pressure is actually 1 atm, high elevation, or a vacuum condition. |
| Above external pressure | The liquid would be expected to boil under the selected external pressure. | Review pressure basis, temperature, valid range, and process conditions. |
| Large disagreement between methods | Inputs, constants, or assumptions may not be compatible. | Verify Antoine range, \(\Delta H_{vap}\), reference values, and units. |
What to do with the result
Use the result to compare volatility, check boiling conditions, estimate vapor pressure at process temperatures, or convert between pressure units. If the result is used for equipment sizing, safety analysis, or chemical handling decisions, verify it against a trusted property database or original source data.
What changes the result most?
Temperature usually dominates the result because vapor pressure changes nonlinearly with temperature. The second major factor is the constant set or property method. Two Antoine constant sets for the same liquid can produce different results if they cover different temperature ranges or use different pressure units.
Quick sanity check
For water near \(25^\circ C\), vapor pressure should be about \(3.2\,kPa\). For water near \(100^\circ C\), vapor pressure should be close to \(101.3\,kPa\). If your result is many orders of magnitude away from these reference points, check the unit basis and constants first.
Input Quality Checklist
Vapor pressure formulas are easy to calculate but easy to misuse. Check these inputs before relying on the answer.
Antoine Constant Format
Confirm whether the constants expect pressure in mmHg, torr, bar, kPa, or another unit and whether temperature is in °C or K.
Valid Temperature Range
Do not extrapolate Antoine constants far outside their fitted range unless you only need a rough educational estimate.
Absolute Pressure
Vapor pressure is absolute pressure. Gauge pressure must be converted to absolute pressure before use.
Temperature Scale
Clausius-Clapeyron requires Kelvin internally. Antoine temperature basis depends on the selected constants.
Enthalpy Units
For Clausius-Clapeyron, make sure \(\Delta H_{vap}\) is molar, such as kJ/mol, not kJ/kg unless converted.
Liquid Identity
Use constants for the exact chemical species. Similar names, mixtures, hydrates, and solutions may not share the same vapor pressure curve.
Step-by-Step Worked Example
The most common use case is calculating vapor pressure from temperature with Antoine constants. This example estimates the vapor pressure of water at \(25^\circ C\).
Formula
Substitute Values
Calculate Pressure
Convert to kPa
Result
Vapor pressure of water at \(25^\circ C\): approximately \(3.16\,kPa\) using this Antoine constant set.
Is the answer reasonable?
Yes. Water at room temperature has a vapor pressure of only a few kilopascals, which is much lower than standard atmospheric pressure. That is why water evaporates at room temperature but does not boil until its vapor pressure approaches the surrounding pressure.
Vapor Pressure Diagram
Vapor pressure is the equilibrium pressure created when molecules leave and return to the liquid surface inside a closed space. At equilibrium, evaporation and condensation still occur, but their rates balance.
Typical Vapor Pressure Reference Values
Reference values are useful for checking whether a result is plausible. Values vary by source and method, so use these as quick reasonableness checks rather than final design data.
| Temperature | Approximate Vapor Pressure | Reasonableness Check |
|---|---|---|
| \(0^\circ C\) | \(0.61\,kPa\) | Low, near freezing conditions. |
| \(20^\circ C\) | \(2.34\,kPa\) | Typical room-temperature reference value. |
| \(25^\circ C\) | \(3.17\,kPa\) | Good check for the calculator default water example. |
| \(50^\circ C\) | \(12.3\,kPa\) | Much higher than room temperature, still below 1 atm. |
| \(80^\circ C\) | \(47.4\,kPa\) | Approaching boiling under reduced pressure. |
| \(100^\circ C\) | \(101.3\,kPa\) | Near standard atmospheric pressure, so water boils at about this point. |
Water vapor pressure note
These are saturation vapor pressure values for pure water. Humidity, partial pressure of water vapor in air, saltwater, glycol mixtures, and other nonideal solutions require additional analysis.
Boiling Checks and Practical Ranges
A vapor pressure result can be mathematically valid but practically misleading if the equation is used outside its assumptions. Always compare the result with the liquid, temperature range, external pressure, and intended use.
Below External Pressure
If vapor pressure is below the surrounding pressure, the liquid can evaporate but should not boil under that external pressure.
Near External Pressure
If vapor pressure is near external pressure, the liquid is near a boiling condition for that pressure.
Above External Pressure
If vapor pressure exceeds external pressure, boiling is expected unless system constraints or non-equilibrium conditions prevent it.
Vapor pressure and boiling point
A liquid does not boil at one fixed temperature under all conditions. It boils when its vapor pressure equals the surrounding external pressure. At \(1\,atm\), water boils near \(100^\circ C\). Under vacuum, the same water can boil at a lower temperature because the external pressure is lower.
Engineering judgment note
Do not use a vapor pressure estimate alone to evaluate closed-container pressure, flammability, relief sizing, vacuum operation, or process safety. Those applications may require mixture behavior, nonideal vapor-liquid equilibrium, temperature gradients, transient effects, and verified property data.
Vapor Pressure Units and Conversions
Vapor pressure can be reported in many pressure units. The calculator can convert the final result, but the equation constants still need the correct original unit basis.
| Unit | Equivalent | Common Use |
|---|---|---|
| \(1\,kPa\) | \(1000\,Pa\) | Engineering and SI calculations. |
| \(1\,MPa\) | \(1000\,kPa\) | High-pressure engineering systems. |
| \(1\,bar\) | \(100\,kPa\) | Thermodynamic property data and process engineering. |
| \(1\,atm\) | \(101.325\,kPa\) | Normal boiling point comparisons. |
| \(1\,mmHg\) | \(\approx 0.133322\,kPa\) | Many older Antoine constant sets. |
| \(1\,torr\) | \(\approx 1\,mmHg \approx 0.133322\,kPa\) | Chemistry, vacuum, and vapor pressure references. |
| \(1\,psi\) | \(\approx 6.89476\,kPa\) | U.S. customary pressure checks. |
Most common unit mistake
The most common mistake is using Antoine constants intended for \(P\) in mmHg and \(T\) in °C while treating the result as kPa, bar, or Kelvin-based data. Always keep the constants and equation format together.
Antoine Equation vs. Clausius-Clapeyron Equation
The best method depends on what data you have. Antoine is usually better for quick compound-specific estimates when valid constants are available. Clausius-Clapeyron is useful when estimating a pressure change from a known reference point.
| Method | Best For | Inputs Needed | Main Caution |
|---|---|---|---|
| Antoine Equation | Fast vapor pressure estimates for pure liquids with known constants. | \(T\), \(A\), \(B\), \(C\) | Constants are valid only for their specific unit basis and fitted temperature range. |
| Rearranged Antoine | Finding boiling temperature or temperature at a target vapor pressure. | \(P\), \(A\), \(B\), \(C\) | Reverse solving is only as good as the constant set and valid range. |
| Clausius-Clapeyron | Estimating pressure change from \(P_1\), \(T_1\), \(T_2\), and \(\Delta H_{vap}\). | \(P_1\), \(T_1\), \(T_2\), \(\Delta H_{vap}\) | Assumes \(\Delta H_{vap}\) is approximately constant. |
| Property Tables or Databases | Final design, safety work, and high-accuracy calculations. | Compound and state conditions | Requires verified data and correct state/property interpretation. |
Vapor pressure vs. partial pressure
Vapor pressure is the equilibrium pressure a pure liquid can exert at a given temperature. Partial pressure is the actual pressure contribution of one gas or vapor in a gas mixture. In air, the partial pressure of water vapor may be below saturation vapor pressure unless the air is saturated.
Common Mistakes When Calculating Vapor Pressure
Most bad vapor pressure results come from unit mismatch, extrapolation, or using the wrong equation for the available data.
Common Mistakes
- Using Antoine constants with the wrong pressure or temperature units.
- Using constants outside the stated temperature range.
- Entering gauge pressure instead of absolute pressure.
- Using Celsius instead of Kelvin in Clausius-Clapeyron calculations.
- Using \(\log_{10}\) where \(\ln\) is required, or the reverse.
- Assuming a pure-liquid result applies to a mixture or solution.
Better Practice
- Keep each Antoine constant set tied to its source, units, and valid range.
- Convert all pressures to absolute pressure before calculation.
- Use Kelvin internally for Clausius-Clapeyron calculations.
- Compare results to known reference values when possible.
- Use verified property data for safety-critical or final design work.
- Check whether the liquid is pure, a mixture, or a nonideal solution.
Troubleshooting Unexpected Results
If the calculator result looks wrong, first check the unit basis and the valid range. Those two issues explain many vapor pressure calculation errors.
| Problem | Likely Cause | Fix |
|---|---|---|
| Result is much too high | Antoine constants may be used with the wrong pressure unit or temperature basis. | Verify whether the constants expect mmHg/°C, bar/K, kPa/°C, or another format. |
| Temperature result is impossible | Target pressure or constants may be incompatible. | Check that pressure is positive, absolute, and within the equation’s practical range. |
| Water result does not match reference values | Wrong constant set, wrong temperature scale, or rounding difference. | Test water at \(25^\circ C\) and \(100^\circ C\) as sanity checks. |
| Clausius result differs from Antoine | \(\Delta H_{vap}\) may not be constant over the temperature span. | Use a smaller temperature range or verified property data. |
| Result changes wildly after a small temperature change | The liquid may be near boiling, or the formula may be used outside its valid range. | Check the valid range and compare against a property table or database. |
| Result says boiling but the situation seems stable | External pressure may be entered incorrectly or the system may not be at equilibrium. | Check external pressure, absolute pressure basis, and actual system conditions. |
Suspicious result check
A vapor pressure that changes by orders of magnitude from a small temperature adjustment may be real near boiling, but it can also indicate a unit or constant mismatch. Recheck the equation format before trusting extreme results.
Assumptions, Sources, and Limitations
This calculator is intended for educational use, preliminary engineering checks, and quick vapor pressure estimates. It does not replace verified thermodynamic property data for safety-critical work.
Pure Substance Assumption
Antoine constants generally apply to pure compounds, not mixtures, solutions, or contaminated liquids.
Valid Range Assumption
Antoine constants are empirical fits and should not be used far outside their stated temperature range.
Constant \(\Delta H_{vap}\) Assumption
Clausius-Clapeyron calculations assume enthalpy of vaporization is approximately constant over the temperature span.
Equilibrium Assumption
Vapor pressure is an equilibrium property. Rapid heating, open systems, and transient processes may behave differently.
Source note
For final property data, verify constants and valid ranges against a trusted property database such as the NIST Chemistry WebBook. NIST and similar references provide compound-specific thermodynamic data and parameter sets, but users must still select the correct unit basis, valid range, and equation form. Some references use \(P\) in bar and \(T\) in K, while many older Antoine tables use \(P\) in mmHg and \(T\) in °C.
Final design caution
Vapor pressure affects boiling, closed-container pressure, flammable vapor generation, vacuum operation, and relief design. For those applications, use verified data, appropriate safety factors, applicable codes or standards, and qualified engineering judgment.
Glossary of Terms
These terms help explain the calculator output and the vapor pressure formulas.
Vapor Pressure
The pressure exerted by vapor in equilibrium with its liquid or solid phase at a given temperature.
Equilibrium
A condition where evaporation and condensation continue, but their rates are balanced.
Antoine Constants
Empirical constants \(A\), \(B\), and \(C\) used to estimate vapor pressure over a limited temperature range.
Normal Boiling Point
The temperature where a liquid’s vapor pressure equals one standard atmosphere.
Enthalpy of Vaporization
The energy required to convert a liquid to vapor, usually reported in kJ/mol for Clausius-Clapeyron calculations.
Absolute Pressure
Pressure measured relative to a perfect vacuum. Vapor pressure calculations use absolute pressure, not gauge pressure.
Partial Pressure
The pressure contribution of one gas in a gas mixture. Vapor pressure is an equilibrium property of a liquid, while partial pressure describes the amount of vapor actually present in a gas mixture.
Frequently Asked Questions
What does the Vapor Pressure Calculator calculate?
The calculator estimates vapor pressure from temperature or solves for the temperature needed to reach a target vapor pressure. It can use the Antoine equation with compound constants or the Clausius-Clapeyron equation with a known pressure, temperature, and enthalpy of vaporization.
What is the Antoine equation for vapor pressure?
The Antoine equation is \(\log_{10}(P)=A-B/(T+C)\), where \(P\) is vapor pressure, \(T\) is temperature, and \(A\), \(B\), and \(C\) are compound-specific constants. The constants must match the pressure unit, temperature unit, and valid temperature range.
What happens when vapor pressure equals atmospheric pressure?
When vapor pressure equals the surrounding external pressure, the liquid reaches a boiling condition. At \(1\,atm\), this is the normal boiling point. Under vacuum, the same liquid can boil at a lower temperature.
Is vapor pressure the same as partial pressure?
No. Vapor pressure is an equilibrium property of a liquid at a given temperature. Partial pressure is the actual pressure contribution of one gas or vapor in a gas mixture. A vapor’s partial pressure may be lower than saturation vapor pressure if the gas mixture is not saturated.
Why do different Antoine constants give different vapor pressure results?
Different Antoine constant sets may use different pressure units, temperature units, fitted temperature ranges, and data sources. Using a constant set outside its valid range or with the wrong unit basis can produce incorrect results.
When should I use Clausius-Clapeyron instead of Antoine?
Use Clausius-Clapeyron when you know one vapor pressure, the corresponding temperature, a second temperature or target pressure, and the enthalpy of vaporization. Use Antoine when you have valid Antoine constants for the liquid and temperature range.