Written and maintained by CASRAI Editorial Board
Last updated
Short answer
The triple point of water is the single temperature and pressure at which solid ice, liquid water and water vapor coexist in equilibrium. Its temperature is 273.16 K, which is 0.01 °C. Since the 2019 revision of the SI it is no longer the definition of the kelvin, but it remains the reference point labs use to calibrate precision thermometers.
Definition and scope
A triple point is the one condition where three phases of a pure substance are in equilibrium together. For water, NIST describes it as the point where solid, liquid and vapor meet, and gives the temperature at that interface as 273.16 K. On the Celsius scale, which NIST describes as an interval of 1 K with 0 °C equal to 273.15 K, this is 0.01 °C, a hundredth of a degree above the point at which ordinary water freezes.
Triple point definition, in plain terms
Because three phases can only coexist at one pressure-temperature combination, the triple point is a fixed, repeatable reference and not a range. NIST notes that for a precise formulation of water at a specific pressure, the triple point always occurs at exactly the same temperature.
Triple point graph (phase diagram)
On a pressure-temperature phase diagram, the solid-liquid, liquid-vapor and solid-vapor boundary lines each separate two phases. The triple point is the single dot where all three lines meet. Every pure substance has its own; our guide to liquid nitrogen temperature covers the equivalent point for nitrogen, which sits far lower on the temperature axis.
How a triple point cell is used for calibration
NIST describes the standard as a special cell: an evacuated glass cylinder containing pure water. When the cell is cooled enough that a mantle of ice forms around the reentrant well, the temperature at the interface of solid, liquid and vapor is 273.16 K. Thermometers to be calibrated are placed in that reentrant well.
The NIST Standard Platinum Resistance Thermometer Calibration Laboratory (SPRTCL) uses such cells as part of its national temperature standards. It calibrates standard platinum resistance thermometers (SPRTs), the interpolating instruments of the International Temperature Scale of 1990 (ITS-90), against fixed-point cells that enclose materials of 99.9999% purity or better. Its published photographs show a water triple point cell and a maintenance bath holding SPRTs. Calibrations are made at combinations of fixed points matched to the temperature subrange the customer needs.
Triple point of water compared with other ITS-90 fixed points
The water triple point is one of several fixed points that NIST realizes. The table below is taken directly from the SPRTCL page; the last column is the laboratory’s stated expanded uncertainty (k=2).
| Fixed point | T (K) | T (°C) | Uncertainty (mK, k=2) |
|---|---|---|---|
| Triple point of argon | 83.8058 | -189.3442 | 0.14 |
| Triple point of mercury | 234.3156 | -38.8344 | 0.15 |
| Triple point of water | 273.16 | 0.01 | 0.09 |
| Melting point of gallium | 302.9146 | 29.7646 | 0.13 |
| Freezing point of indium | 429.7485 | 156.5985 | 0.25 |
| Freezing point of tin | 505.078 | 231.928 | 0.27 |
| Freezing point of zinc | 692.677 | 419.527 | 0.55 |
Read across the uncertainty column and the water cell stands out: at 0.09 mK it has the smallest stated uncertainty of any point NIST lists with a value. The long-stem SPRT service spans the triple point of argon up to the freezing point of silver (961.78 °C), and the water cell sits within that range.
Why the 2019 redefinition did not retire the water cell
In 1954 the kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water, according to NIST. When the revised SI was approved in November 2018, the kelvin was redefined by fixing the Boltzmann constant at exactly 1.380649 × 10-23 J/K. The unit is therefore no longer tied to a particular substance, but ITS-90 still names the water triple point as a fixed point, and NIST’s calibration laboratory still maintains and realizes it at 273.16 K. In practice, the cell moved from defining the unit to being the most carefully realized reference for thermometers. For the way SI units are written in manuscripts, see SI units in scientific writing.
Where it matters for labs and clinics
- Traceability of thermometers. NIST says its SPRT calibrations give traceability to ITS-90, and that commercial devices adopted to support regular calibration services are traceable to its national standards. A calibration certificate for a reference thermometer can therefore be traced back to cells like these.
- Calibration at a known point. A thermometer placed in the cell’s reentrant well is checked against the 273.16 K interface that NIST describes.
- Different scope than everyday probes. The cell serves reference thermometers. Routine temperature-sensing hardware in a process line, such as a thermowell, is a separate topic from the primary standard that sits above it in the calibration chain.
Triple point versus ice point and other common misunderstandings
It is not the same as the freezing point
NIST gives the widely used Celsius reference as water freezing at 0 °C (273.15 K), while the triple point is 273.16 K (0.01 °C). The triple point is realized in an evacuated cell of pure water, and the two values differ by 0.01 K, which matters at calibration-laboratory uncertainty levels but not for everyday use.
It is not the boiling point or a range
The triple point is a single point on the phase diagram, not a band of temperatures, and it is unrelated to the boiling point (about 100 °C on NIST’s reference table).
It no longer defines the kelvin
Many older references still say the kelvin is defined as 1/273.16 of the water triple point. That was true before the 2019 revision of the SI; the current definition rests on the Boltzmann constant.
Not every substance’s triple point is a thermometer reference
ITS-90 fixed points include triple points (argon, mercury, water) but also melting and freezing points of metals such as gallium, indium, tin and zinc, as the NIST table above shows.
Frequently asked questions
What is the triple point of water?
It is the condition at which ice, liquid water and water vapor coexist in equilibrium. NIST gives its temperature as 273.16 K (0.01 °C).
What is the triple point definition?
A triple point is the unique temperature and pressure at which three phases of a pure substance are in equilibrium at once. For water the phases are solid, liquid and vapor.
What does the triple point graph look like?
On a pressure-temperature phase diagram, it is the point where the solid-liquid, liquid-vapor and solid-vapor boundaries meet.
What is water at the triple point used for?
It is realized in a sealed cell used to calibrate thermometers, including the SPRTs that interpolate the International Temperature Scale of 1990.
What is the triple point of water in Celsius?
0.01 °C, which equals 273.16 K. Zero degrees Celsius is 273.15 K, according to NIST.
Is the triple point of water still used to define the kelvin?
No. Since the revised SI approved in November 2018, the kelvin is defined by fixing the Boltzmann constant, though the water triple point remains an ITS-90 fixed point. For background on the physics, see our overview of thermodynamics.
Sources
- NIST, SI Units: Temperature, checked 2026-10-29.
- NIST, Standard Platinum Resistance Thermometer Calibration Laboratory (SPRTCL), checked 2026-10-29.
- NIST, Kelvin: Introduction, checked 2026-10-29.
