Absolute zero
0 K = −273.15 °C = −459.67 °F
Absolute zero is the lowest temperature possible: the point where a substance has the least thermal energy it can have. It is 0 K on the Kelvin scale and 0 °R on the Rankine scale, which is why those two are called absolute scales. Celsius and Fahrenheit put their zeros somewhere more convenient for daily life, so absolute zero shows up there as a large negative number.
Four scales, one starting line
Pick a landmark. The bars show how far above absolute zero it sits; each scale just labels that same distance differently.
Dashed ticks mark where Celsius (0 °C) and Fahrenheit (0 °F) put their own zeros. Anything left of those ticks reads negative on that scale.
Why Kelvin has no degree sign
Since 1967 the SI unit has been "the kelvin" (symbol K), not "degrees Kelvin". It is a unit of measurement like the metre, and since the 2019 SI redefinition it is fixed by the Boltzmann constant rather than by the triple point of water. A kelvin is exactly the same size as a Celsius degree, so converting Celsius to Kelvin is just adding 273.15.
The Rankine scale
Rankine does for Fahrenheit what Kelvin does for Celsius: it keeps Fahrenheit-sized degrees but starts at absolute zero. Water freezes at 491.67 °R and boils at 671.67 °R. US engineers still use it in thermodynamics and gas calculations. See Rankine to Fahrenheit and Rankine to Celsius, or the full Rankine scale guide.
Cold places on the scale
Deep space, lit only by the leftover glow of the Big Bang, sits at about 2.7 K. Much closer to home, the Moon's equator heats to roughly 120 °C at lunar noon and drops to around −130 °C at night, and NASA's Lunar Reconnaissance Orbiter has measured about 26 K inside permanently shadowed craters near the poles. Those lunar swings come down to which half of the Moon is facing the Sun, the same geometry behind its phases, which ahaboo covers in a narrated explainer.
Questions
What is absolute zero in Celsius?
Absolute zero is −273.15 °C, which is 0 K, −459.67 °F and 0 °R.
What is 0 Kelvin in Celsius?
0 K is −273.15 °C.
Can anything reach absolute zero?
No. The third law of thermodynamics means it can be approached but never reached. Laboratories have cooled atoms to within a billionth of a kelvin of it.
What is the Rankine scale?
Rankine is an absolute scale that starts at absolute zero but uses Fahrenheit-sized degrees, so °R = °F + 459.67. It was proposed by the Scottish engineer William Rankine in 1859.