Three scales dominate temperature measurement: Celsius (used globally for everyday life and science), Fahrenheit (used primarily in the United States), and Kelvin (the SI unit used in all scientific thermodynamics). Kelvin starts at absolute zero (0 K = −273.15 °C), the theoretically coldest possible temperature where all thermal motion ceases. Rankine is an absolute scale that uses Fahrenheit-sized degrees, used in some US engineering disciplines.
Celsius was proposed in 1742 using water's freezing (0 °C) and boiling (100 °C) points at standard pressure. Fahrenheit (1724) set 0 °F at a freezing brine mixture and 96 °F at body temperature. Kelvin (1848) is indispensable for thermodynamics — equations like the ideal gas law (PV = nRT) only produce correct results when temperature is expressed in Kelvin.
| Water freezes | = 0 °C / 32 °F / 273.15 K |
| Body temp | = 37 °C / 98.6 °F |
| Water boils | = 100 °C / 212 °F |
| Absolute zero | = −273.15 °C / −459.67 °F / 0 K |
| Room temp | ≈ 20 °C / 68 °F |