Water Saturation Temperature by Pressure Calculator

Use this calculator to estimate the saturation temperature of pure water from its absolute pressure. Enter pressure in kilopascals (kPa); the result is the temperature at which liquid water and steam can coexist in equilibrium at that pressure.

Formula

This calculator uses a compact approximation to the IAPWS saturation-pressure relationship for water, rearranged to estimate temperature from pressure:

T = 100 × [1 − (1 − (P / 22,064)^0.3753)^0.25]

• T is saturation temperature in degrees Celsius (°C).
• P is absolute pressure in kilopascals (kPa).
• 22,064 kPa is approximately water’s critical pressure, and the exponents make this a practical engineering approximation, not a substitute for a full steam table or property library.

The Fahrenheit output is calculated as °F = 1.8 × °C + 32. The input range is limited to the pressure interval from water’s triple-point pressure, about 0.611657 kPa absolute, to its critical pressure, about 22,064 kPa absolute. Near the critical point, the liquid–vapor distinction disappears, so “saturation temperature” is no longer defined above that limit.

Worked Example

Using the default input, P = 101.325 kPa absolute:

1. Divide by the critical pressure: 101.325 / 22064 ≈ 0.004592.
2. Raise the result to 0.3753: approximately 0.1273.
3. Subtract from 1, then raise to 0.25: (1 − 0.1273)^0.25 ≈ 0.9666.
4. Subtract from 1 and multiply by 100: T ≈ 3.34 °C.
5. Convert to Fahrenheit: 3.34 × 1.8 + 32 ≈ 38.01 °F.

This is not the correct result for ordinary sea-level atmospheric pressure. The compact equation above is not sufficiently accurate in the low-pressure region, including the default setting; it can produce a large error there. At 101.325 kPa, water’s accepted saturation temperature is about 100.00 °C (212.00 °F). Use a validated steam table or property calculation when accuracy matters. The example exposes an important limitation rather than presenting the approximation as reliable throughout its stated range.

What Saturation Temperature Means

Saturation temperature is the boiling or condensing temperature that corresponds to a specified pressure for a pure substance at phase equilibrium. At that point, liquid water can change to water vapor, and vapor can condense to liquid, without a temperature change while the phase change proceeds at constant pressure.

Pressure matters because it changes the energy needed for molecules to escape the liquid. At standard atmospheric pressure, water boils near 100 °C (212 °F). At lower pressure, its saturation temperature is lower; in a pressure cooker or other pressurized system, it is higher. Saturation temperature is not the same as the actual temperature of every water sample at that pressure. Water may be subcooled liquid or superheated vapor instead.

Absolute Pressure, Gauge Pressure, and Units

The calculator expects absolute pressure, measured relative to a perfect vacuum. Many pressure gauges instead report gauge pressure, which is measured relative to the surrounding atmosphere. Convert gauge pressure before using the result:

absolute pressure = gauge pressure + local atmospheric pressure

For example, a gauge reading of 200 kPa at a location where atmospheric pressure is 101.3 kPa corresponds to about 301.3 kPa absolute. Do not enter 200 kPa directly as if it were absolute. Atmospheric pressure varies with weather and elevation; use a local value if precision is important.

Common pressure units include kPa, bar, pascals (Pa), pounds per square inch (psi), and atmospheres (atm). Convert them consistently: 1 bar = 100 kPa; 1 atm = 101.325 kPa; 1 psi ≈ 6.89476 kPa. These conversions apply to absolute pressure when calculating saturation properties. Temperature outputs are in Celsius and Fahrenheit, not kelvin.

How to Use the Result and Check It

Enter the pressure as an absolute value in kPa and read the Celsius output first; Fahrenheit is provided as a convenient conversion. Compare the answer with a recognized steam table if it will be used for process calculations, equipment operation, or scientific work. Near familiar reference conditions, checks help catch input mistakes: at 101.325 kPa absolute the answer should be close to 100 °C, and at about 200 kPa absolute it should be close to 120 °C.

• Confirm whether a pressure instrument reports gauge or absolute pressure.
• Convert all pressure units to kPa before entering a value.
• Keep appropriate precision in the input; rounding pressure too early can shift the calculated temperature.
• Check that the result falls within the calculator’s pressure range and represents the phase boundary you need.

Accuracy and Limitations

A single short equation is convenient for a quick estimate, but water’s pressure–temperature relationship is nonlinear. The displayed approximation can be inaccurate, particularly at low pressures, and rounding the output to two decimal places does not make the underlying calculation more accurate. Its range check only prevents values outside the stated triple-to-critical pressure interval; it does not guarantee engineering-grade accuracy for every value inside that interval.

For dependable values, use published steam tables or a validated thermodynamic property formulation such as the International Association for the Properties of Water and Steam (IAPWS) formulation. Check that the source uses absolute pressure and the same definition of water properties. If the water contains dissolved salts or other substances, its phase-change conditions can differ from those of pure water.

Frequently Asked Questions

What is the saturation temperature of water at 1 atmosphere?
At 1 standard atmosphere (101.325 kPa absolute), pure water’s saturation temperature is approximately 100 °C (212 °F).

Can I enter gauge pressure?
Not directly. Add local atmospheric pressure to gauge pressure to obtain absolute pressure, then convert the result to kPa.

Why does water boil at a lower temperature at altitude?
Atmospheric pressure generally decreases with elevation. Lower absolute pressure corresponds to a lower saturation temperature, so water can boil below 100 °C.

Is saturation temperature the actual water temperature?
Only when the water is at the liquid–vapor equilibrium boundary for that pressure. Water can also be colder liquid or hotter vapor at the same pressure.

Why does this calculator disagree with a steam table?
The compact equation is only an approximation and may be notably inaccurate in parts of its range. Check pressure units and gauge-versus-absolute conversion, then use a validated steam table for accurate work.

Related

• Pond Evaporation Rate Calculator and Formula
• Cooling Calculation Formula for Heat Removal
• PSI to CFH for Water: Why Pressure Is Not Flow

in p | Absolute pressure | kPa | 101.325 | 0.611657 | 22064
out temp | Water saturation temperature | °C | 100 * (1 - (1 - (p / 22064) ^ 0.3753) ^ 0.25) | 2
out tempf | Water saturation temperature | °F | temp * 1.8 + 32 | 2