What is the triple point of water in mmHg?

The triple point of water corresponds to a pressure of about 611.657 pascals, which converts to approximately 4.58 millimeters of mercury (mmHg). This means that at a temperature of 0.01°C and a vapor pressure of 4.58 mmHg, water’s three phases—ice, liquid, and vapor—exist in perfect balance. This pressure is extremely low compared to normal atmospheric pressure (760 mmHg), which explains why such equilibrium can only occur in a controlled environment. 

Scientists replicate these conditions inside sealed triple-point cells to observe the phenomenon and calibrate precision instruments. Because the triple point occurs at such a specific and reproducible pressure, it serves as a key benchmark for defining physical constants and calibrating barometers. 

Understanding the pressure value in mmHg also helps translate the triple point into practical engineering contexts, where pressure is often expressed in those units. In simple terms, 4.58 mmHg is the delicate pressure that, combined with 0.01°C temperature, allows all three forms of water to exist simultaneously without any phase dominating the others.