A substance's triple point is the temperature and pressure at which it exists simultaneously as a solid, liquid and gas. The formula used to determine this point, called the Equation of State, is different for every substance and must be determined from experimental data and computer simulations. These equations are often very complex; therefore, people outside the scientific community usually use phase diagrams, which show the results of the Equation of State plotted on a graph, to calculate triple points.
Examine a phase diagram for the substance whose triple point you want to know. Note where the boundaries lie between the solid, liquid and gas phases.
Find the point where the lines separating the phases meet. This represents the triple point.
Read the temperature on the x-axis directly below that point. This is the temperature at which the triple point occurs.
Read the temperature on the y-axis to the left of that point. This is the pressure at which the triple point occurs.