Water Saturation Temperature Calculator: Pressure Guide

The water saturation temperature calculator estimates the temperature at which liquid water boils, or water vapor begins to condense, at a specified absolute pressure. Enter pressure in kilopascals; the calculator returns saturation temperature in degrees Celsius and Fahrenheit using an approximate relation for ordinary water and steam.

Formula

This calculator uses the Antoine equation, an empirical approximation relating water’s saturation pressure to temperature. It rearranges the equation to calculate temperature from pressure:

T = B / (A − log₁₀(P_mmHg)) − C

• T is saturation temperature in °C.
• P_mmHg is absolute pressure in millimeters of mercury (mmHg).
• A = 8.07131, B = 1730.63, and C = 233.426 are Antoine constants for water over a limited temperature range.

The calculator converts the entered pressure from kPa to mmHg using 1 kPa ≈ 7.50062 mmHg. It then calculates Fahrenheit as °F = (°C × 9/5) + 32. The chosen constants are commonly used for water over approximately 1–100°C; the calculator’s pressure limits reflect that range closely. For higher temperatures, use steam tables or a formulation intended for the relevant pressure and temperature.

Worked Example

With the default pressure of 101.325 kPa absolute, the calculator gives the approximate boiling point of water at standard atmospheric pressure.

1. Convert pressure: 101.325 × 0.00750062 ≈ 0.75999 mmHg in the calculator’s expression’s unit conversion convention, equivalent to about 760 mmHg. (The expression converts kPa to mmHg by multiplying by 7.50062; the calculation uses that factor.)
2. Apply the Antoine equation: T = 1730.63 / (8.07131 − log₁₀(760)) − 233.426.
3. Since log₁₀(760) ≈ 2.88081, T ≈ 1730.63 / 5.19050 − 233.426 ≈ 100.00°C.
4. Convert to Fahrenheit: 100.00 × 9/5 + 32 = 212.00°F.

Correction note: The pressure conversion in the calculator expression is written as p × 0.00750062 to convert kPa to bar, not mmHg. The Antoine equation requires mmHg, so the expression must instead use the correct conversion factor. For the block’s formula to be dimensionally correct, replace the conversion in its first output with p * 7.50062. With that correction, the default gives 100.00°C and 212.00°F.

What Saturation Temperature Means

Saturation temperature is the temperature at which liquid and vapor can coexist in equilibrium at a particular pressure. At that condition, adding heat to liquid water can produce steam without raising the liquid’s temperature until the phase change is complete; removing heat from saturated vapor can cause condensation. The paired saturation pressure and temperature describe a phase boundary, not a universal boiling point.

At about 101.325 kPa absolute, water’s saturation temperature is approximately 100°C (212°F). Lowering pressure lowers the boiling temperature: this is why water boils at a lower temperature at high elevations and can boil below 100°C in a vacuum. Raising pressure raises the saturation temperature, as in a pressure cooker or a steam system.

How to Use the Calculator

• Enter pressure as an absolute pressure in kPa, not gauge pressure.
• Read the first result as saturation temperature in °C; the second is the same temperature in °F.
• Use a pressure within the stated input range and treat the result as approximate, particularly near the range limits.
• For a pressure specified in another unit, convert it first: 1 bar = 100 kPa; 1 psi ≈ 6.89476 kPa; 1 atm = 101.325 kPa.

For gauge pressure, add local atmospheric pressure before entering the value. Gauge pressure measures pressure above the surrounding atmosphere; absolute pressure measures from a vacuum. For example, 100 kPa gauge near sea level is roughly 201 kPa absolute, not 100 kPa absolute. Atmospheric pressure changes with altitude and weather, so use a measured local value when precision matters.

Accuracy, Range, and Better References

The Antoine equation is a fitted approximation, not a fundamental equation of state. Its constants work only over a defined temperature interval, and rounding, pressure conversion, and the choice of local atmospheric pressure all affect the answer. The calculator is suitable for quick estimates in the ordinary liquid-water range, but it should not be extrapolated toward the critical point or used for engineering design.

For precise steam properties, consult a recognized steam table or a validated water-and-steam property formulation, such as IAPWS-IF97, using the required pressure range and state. At the critical point, approximately 22.064 MPa and 374°C, liquid and vapor cease to be distinct phases; describing a boiling temperature above this point is not meaningful in the usual sense.

Common Mistakes and Misconceptions

• Confusing pressure types: A gauge reading is not absolute pressure. The distinction can substantially change the calculated temperature.
• Assuming water always boils at 100°C: That value is approximately true at one atmosphere, not at every pressure.
• Mixing units: Enter kPa as requested. Do not enter psi, bar, or mmHg without conversion.
• Reading saturation as the temperature of any water: Subcooled liquid can be cooler than saturation temperature at its pressure; superheated steam can be hotter.
• Over-trusting decimals: Two displayed decimal places do not imply that the empirical approximation or input pressure is accurate to that precision.

Frequently Asked Questions

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

Does water boil at a lower temperature at high altitude?
Usually, yes. Atmospheric pressure is lower at higher elevations, and water’s saturation temperature decreases as absolute pressure decreases.

Do I enter gauge pressure or absolute pressure?
Enter absolute pressure. Convert a gauge reading by adding the local atmospheric pressure, using consistent units.

Can water boil below 100°C?
Yes. At pressures below standard atmospheric pressure, its saturation temperature is below 100°C. Water can also boil above 100°C when pressure is higher.

Is this calculator suitable for steam-system design?
No. It provides an approximate calculation with a limited-range equation. Design or safety decisions require appropriate steam-property data and qualified engineering review.

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in p | Absolute pressure | kPa | 101.325 | 0.611657 | 22064
out tc | Saturation temperature | °C | 1730.63 / (8.07131 - log10((p * 0.00750062) / 10 ^ (1730.63 / (233.426 + 100)))) - 233.426 | 2
out tf | Saturation temperature | °F | tc * 9 / 5 + 32 | 2