How this instrument works
The Antoine equation is an empirical formula for a substance's vapor pressure as a function of temperature: log10(P) = A - B / (C + T), where A, B, and C are constants fitted specifically to that substance's own measured vapor pressure data, and are only valid over the specific temperature range they were fitted to. Unlike the Clausius-Clapeyron relation, which derives from thermodynamic theory and needs a heat-of-vaporization value, the Antoine equation's constants come purely from curve-fitting real measurements — which is why, within its valid range, it tends to track a substance's actual vapor pressure more precisely than a theory-derived approximation.
For water specifically, over the 1-100 degC range, the standard Antoine coefficients are A=8.07131, B=1730.63, and C=233.426, with pressure P in mmHg and temperature T in degC — the coefficient set this calculator uses. These are the coefficients cited across chemistry references (including the NIST Chemistry WebBook's data compilations) as the standard low-temperature-range fit for water, distinct from a separate, different coefficient set that covers water's higher-temperature range from 100 to 374 degC.
Because these coefficients were fitted to data across 1-100 degC, using them outside that range produces an unreliable result — this calculator is deliberately restricted to that same 1-100 degC window rather than extrapolating past it. For any water vapor pressure calculation above 100 degC (into superheated steam territory), a different coefficient set fitted to that higher range is required, which this calculator doesn't include.
- Enter the water temperature, in degrees Celsius, into Water temperature (degC, valid 1-100).
- Read Vapor pressure (mmHg) and Vapor pressure (kPa) below the input — both update instantly as the temperature changes.
- Stay within the 1-100 degC range this specific coefficient set is valid for; temperatures outside that window need a different Antoine coefficient set this calculator doesn't use.
Worked example — water at 25 degC
Enter 25 into Water temperature (degC, valid 1-100). Vapor pressure (mmHg) reads about 23.69 mmHg, and Vapor pressure (kPa) reads about 3.158 kPa.
That roughly 23.7 mmHg figure is the actual pressure water vapor exerts in equilibrium with liquid water at ordinary room temperature — it's also, not coincidentally, the same correction chemists apply when collecting a gas over water in the lab: the total pressure measured in a gas-collection setup includes both the gas of interest and this water-vapor contribution, so subtracting water's vapor pressure at the working temperature is a standard step in getting the dry gas's own pressure.
Questions
Why use the Antoine equation instead of the Clausius-Clapeyron relation?
Because the Antoine equation's constants are fitted directly to a substance's real measured vapor pressure data, rather than derived from a simplified thermodynamic approximation that assumes a constant heat of vaporization — within its valid temperature range, that makes it noticeably more accurate for a well-characterized substance like water. The trade-off is that Antoine coefficients only exist (and are only valid) for substances and temperature ranges someone has already measured and fitted; Clausius-Clapeyron works for any substance as long as you know its heat of vaporization, at some cost to precision.
What happens if I enter a temperature outside 1-100 degC?
This calculator flags it as outside the valid range for these specific coefficients and won't return a result, because the A=8.07131, B=1730.63, C=233.426 coefficient set was fitted to data within 1-100 degC and isn't reliable outside it. Water's vapor pressure above 100 degC (superheated steam) follows a different Antoine coefficient set fitted to that higher-temperature range instead.
Does water's vapor pressure equal 760 mmHg exactly at 100 degC?
By definition, yes for the real physical quantity — 100 degC (at standard atmospheric pressure) is defined as water's normal boiling point, where vapor pressure equals exactly 1 atmosphere, 760 mmHg. This Antoine fit lands very close to that reference point at T=100 degC, which is a useful built-in sanity check that the coefficient set is behaving correctly right at the edge of its valid range.
Why do I need vapor pressure of water specifically, rather than just any liquid?
Because water vapor pressure shows up constantly as a correction factor in lab chemistry — most commonly when collecting a gas over water, where the measured total pressure includes both the gas of interest and water vapor from the collection container. Subtracting water's vapor pressure at the working temperature (exactly what this calculator provides) isolates the dry gas's own partial pressure, a standard step in gas-law calculations done this way.
How is Vapor pressure (kPa) related to Vapor pressure (mmHg)?
They're the same physical pressure expressed in two different units, related by a fixed conversion factor: 1 mmHg equals 0.133322 kPa. This calculator computes the mmHg figure directly from the Antoine equation (since that's the unit the equation's coefficients were fitted for) and then multiplies by that conversion factor to also report the kPa figure, so both units are available without a separate manual conversion.