Boiling Point Calculator at Different Pressures

This boiling point calculator estimates water’s boiling temperature from the surrounding absolute pressure, using the Antoine equation. Enter pressure in kilopascals and get the estimated boiling point in degrees Celsius; the result is for pure water near its liquid–vapor equilibrium.

Formula

The calculator uses the Antoine equation for water over its applicable temperature range:

T = A / (B − log₁₀(P_mmHg)) − C

• T is the estimated boiling temperature in degrees Celsius.
• P_mmHg is absolute pressure in millimeters of mercury (mmHg).
• A = 1730.63, B = 8.07131, and C = 233.426 are Antoine constants for water, with pressure in mmHg and temperature in °C.
• The calculator accepts pressure p in kilopascals (kPa) and converts it using 1 kPa ≈ 7.50062 mmHg.

Boiling occurs when a liquid’s vapor pressure reaches the pressure acting on its surface. The equation estimates the temperature at which that happens. The constants are an empirical fit, not a universal formula for all temperatures or pressures; this set is suitable roughly from 1 to 100°C, corresponding to about 0.66–101.3 kPa. Do not extrapolate it to high-pressure cooking or unusual conditions.

Worked Example

With the default pressure, p = 101.325 kPa:

1. Convert to mmHg: 101.325 × 7.50062 ≈ 760.00 mmHg.
2. Take the base-10 logarithm: log₁₀(760.00) ≈ 2.88081.
3. Substitute: T = 1730.63 / (8.07131 − 2.88081) − 233.426.
4. Calculate: T ≈ 99.97°C, which rounds to 99.97°C.

This is the familiar boiling point near standard atmospheric pressure. Small differences from 100°C result from the chosen constants, rounding, and the definition of standard conditions.

How Pressure Changes the Boiling Point

Lower external pressure means water needs a lower vapor pressure to boil, so it boils at a lower temperature. Higher pressure raises the boiling temperature. This is why water boils below 100°C at high elevations and above 100°C in a sealed pressure cooker. The calculator uses absolute pressure, which measures pressure relative to a perfect vacuum.

Illustrative values from the same equation are:

• 101.325 kPa: about 99.97°C (212°F), near standard atmospheric pressure.
• 80 kPa: about 93.5°C (200.3°F).
• 70 kPa: about 90.0°C (194.0°F).
• 50 kPa: about 81.3°C (178.3°F).

These are estimates for pure water. Fahrenheit conversions use °F = (°C × 9/5) + 32. At standard atmospheric pressure, the usual rounded reference is 100°C or 212°F; the exact equilibrium value depends on the conditions and reference convention.

Choosing and Measuring Pressure

Use the pressure at the liquid surface, not a number from a weather report unless you know whether that report gives station pressure or pressure adjusted to sea level. Local atmospheric pressure changes with altitude and weather. A pressure measurement must be absolute to work in this calculator.

• Absolute pressure: pressure measured from vacuum. Enter this directly in kPa.
• Gauge pressure: pressure measured above the surrounding atmosphere. Convert it before use: absolute pressure = gauge pressure + local atmospheric pressure, using the same units.
• Unit conversions: 1 bar = 100 kPa; 1 atmosphere = 101.325 kPa; 1 psi ≈ 6.89476 kPa; and 1 mmHg ≈ 0.133322 kPa.

For example, a gauge reading of 50 kPa in an environment at 101.3 kPa absolute corresponds to about 151.3 kPa absolute. That pressure is outside the stated useful range of this calculator’s Antoine constants, so a broader-range steam table or equation of state is more appropriate.

When the Estimate Is Useful—and Its Limits

This calculation is useful for understanding altitude effects, checking a low-pressure water process, or estimating the boiling point from a measured absolute pressure. It assumes pure water and equilibrium, and it describes the temperature at which boiling begins under the entered pressure—not how quickly water heats or how long it takes to boil.

Real liquids can behave differently. Dissolved salts and other nonvolatile substances generally raise water’s boiling point; mixtures can have more complicated behavior. Impurities, changing pressure, temperature gradients, and measurement uncertainty also affect practical results. For precise work, use validated steam tables or property data covering the full pressure and temperature range, and account for the actual composition of the liquid.

A common misconception is that water always boils at exactly 100°C. That value applies approximately at one atmosphere, not at every location or pressure. Also, boiling does not mean a liquid must be heated to a fixed temperature: a liquid boils when its vapor pressure matches the local external pressure.

Frequently Asked Questions

What pressure should I enter for boiling water?
Enter the absolute pressure at the water’s surface, in kPa. Near sea level, 101.325 kPa is a useful standard reference; actual local atmospheric pressure varies.

Why does water boil at a lower temperature at high altitude?
Atmospheric pressure is lower at higher elevations. Water’s vapor pressure therefore matches the surrounding pressure at a lower temperature.

Can I enter gauge pressure?
Not directly. Add local atmospheric pressure to gauge pressure first, converting both values to the same units. The calculator expects absolute pressure.

Does this calculate the boiling point of any liquid?
No. The constants and formula here are for water. Other liquids need their own vapor-pressure data or an appropriate property model.

Why might my measured boiling temperature differ?
Local pressure, dissolved substances, thermometer placement and calibration, and departures from equilibrium can all change the observed value. The equation also has a limited range and rounds its output.

Related

• Activation Energy Calculator: Formula and Example
• Actual Yield Calculator: Formula and Example
• Activation Energy Calculator From Slope: Arrhenius Method

in p | Absolute pressure | kPa | 101.325 | 0
out t | Water boiling point | °C | 1730.63 / (8.07131 - log10(p * 7.50062)) - 233.426 | 2