Phase Diagram Calculator

• •Use the Phase Diagram Calculator to check your hand calculations on homework or exam problems — comparing its result with your own working helps identify arithmetic errors before they propagate.
• •Use it during the early design phase to rapidly iterate on parameters and narrow down feasible configurations before committing time to detailed finite element simulations or full design packages.
• •Use it when reviewing a colleague's calculation or checking a vendor's data sheet for plausibility — a quick sanity check can prevent costly downstream errors.
• •Use it to generate reference data for a technical report or presentation without manual computation, ensuring consistent, reproducible numbers throughout the document.
• •Use it in the field when a quick estimate is needed and a full engineering software package is not available.

The Phase Diagram Calculator is a precision engineering calculation tool designed for students, engineers, and technical professionals. Interactive P-T phase diagram for water with IAPWS saturation, sublimation and melting curves, triple and critical points, region lookup and process path plotting All calculations are performed using established engineering formulas from the relevant scientific literature and standards. Follow the units printed beside each field. Where a unit selector is provided, the selected unit is converted internally. Results are computed in your browser without sending data to a server, which keeps your inputs private. This calculator is intended as a supplementary tool for learning and design exploration; always verify results against authoritative references for safety-critical applications.

A phase diagram for a pure substance shows the stable phase (solid, liquid, vapor, supercritical) as a function of two independent variables, most commonly pressure and temperature. The key features on a P-T diagram are: the sublimation line (solid-vapor coexistence at low pressures), the melting line (solid-liquid coexistence, nearly vertical because solid and liquid have similar specific volumes, so the Clapeyron slope dP/dT = Δs/Δv is steep), the vaporization line or saturation curve (liquid-vapor coexistence, curving upward from the triple point to the critical point), the triple point (where all three phases coexist, a unique P and T for each substance), and the critical point (the end of the liquid-vapor coexistence line, above which the substance is 'supercritical' with continuous density variation between liquid-like and vapor-like). Water has an unusual negative slope to its ice Ih melting line: increasing pressure lowers the melting point, by about 0.074 K per MPa near 0 °C (the IAPWS melting curve rises about 13.5 MPa per kelvin of cooling). That is far too little for the few megapascals under a skate blade to melt ice at −10 °C; frictional heating and a thin premelted surface layer are the accepted explanations of why ice is slippery. Most other substances have positive melting lines. The critical point for water is T_c = 373.95°C and P_c = 22.064 MPa. The triple point of water (where ice, liquid, and vapor coexist) is T = 0.01°C and P = 611.657 Pa, which has been used as a temperature reference. Above the critical temperature, water is a supercritical fluid with no distinct phase boundaries — industrially important for supercritical power plants (efficient steam cycles above the critical pressure) and for chemistry (supercritical water oxidation for waste treatment, supercritical CO₂ for extraction). The calculator draws the saturation curve from IAPWS-IF97 and the sublimation and ice Ih, III, V and VI melting curves from IAPWS R14-08, and classifies any (P, T) point against those same curves.

• •Cooking at high altitude: at lower atmospheric pressure (higher altitude), water boils at lower temperature. At 3000 m elevation, where the standard-atmosphere pressure is about 70 kPa, water boils at about 90°C — which changes cooking times significantly. Phase diagrams explain this effect directly.
• •Freeze drying (lyophilization): food and pharmaceutical preservation uses low-pressure sublimation. Ice is converted directly to vapor without passing through the liquid phase, avoiding heat damage. Operating at 0.1 kPa keeps water in the solid-vapor regime.
• •Supercritical CO₂ extraction: industrial-scale extraction of caffeine, essential oils, and other compounds uses supercritical CO₂ (T > 31°C, P > 7.4 MPa). The supercritical fluid penetrates like a gas but dissolves like a liquid.
• •High-pressure ice and liquid: below 0 °C water stays liquid only above the ice Ih melting pressure (about 60 MPa at −5 °C), and below 0 °C remains liquid between the ice Ih melting pressure and the melting pressure of a denser ice. That upper limit rises from about 209 MPa at −22 °C (the Ih–III–liquid triple point) to about 632 MPa at 0.16 °C (the V–VI–liquid triple point); the upper limit is about 534 MPa at −5 °C. These are the temperature-dependent boundaries drawn from IAPWS R14-08.
• •Rankine cycle design: boiler and condenser pressures must be chosen to place the cycle within safe operating regions on the phase diagram. Below the triple-point pressure liquid water cannot exist, and above the critical pressure the boiler has no boiling plateau (a supercritical cycle).