What is 100 °C in Fahrenheit?

100 °C = (100 × 9/5) + 32 = 180 + 32 = 212 °F — the boiling point of water at sea level.

At what temperature are Celsius and Fahrenheit equal?

−40 °. Set °F = °C: x = (x × 9/5) + 32 → x − 9x/5 = 32 → −4x/5 = 32 → x = −40.

Why is absolute zero the coldest possible temperature?

Absolute zero (0 K, −273.15 °C) is the temperature at which particles have minimum possible thermal energy. The third law of thermodynamics states that no finite series of processes can reduce a system's temperature to absolute zero.

Is there a 'hottest' temperature?

The Planck temperature (~1.416 × 10³² K) is theorized to be the upper limit where current physics breaks down. Beyond this, quantum gravitational effects dominate and existing models no longer apply.

Temperature Conversion Explained: Celsius, Fahrenheit, and Kelvin

Temperature scales are unlike most unit conversions — they involve addition and subtraction as well as multiplication. This guide explains why the three major scales exist, what zero means on each, the exact conversion formulas, and the most common mistakes people make when converting temperatures.

Published March 19, 2026

Key takeaways

Temperature conversions are affine (not purely multiplicative) because the scales have different zero points.
Celsius and Kelvin have the same degree size; their zeros differ by 273.15.
Fahrenheit uses a smaller degree (5/9 of a Celsius degree) and its zero is −17.78 °C.
Absolute zero (0 K) is −273.15 °C and −459.67 °F — the coldest possible temperature.
When converting temperature differences (not absolute temperatures), ignore the offsets and multiply by the scale factor only.

Why temperature conversion is different

Most unit conversions are purely multiplicative: multiply meters by 3.28084 to get feet, and you're done. Temperature scales are different because each has an arbitrary zero point that does not represent the same physical state.

Celsius sets 0 °C at the freezing point of water and 100 °C at boiling (at 1 atm). Fahrenheit sets 32 °F at water's freezing point and 212 °F at boiling. Kelvin starts at absolute zero — the theoretical point at which all molecular motion stops.

Because the zeros differ, converting requires adding or subtracting an offset in addition to scaling. This type of conversion is called affine.

The exact conversion formulas

Formula

Celsius → Fahrenheit:  °F = (°C × 9/5) + 32
Fahrenheit → Celsius:  °C = (°F − 32) × 5/9
Celsius → Kelvin:       K = °C + 273.15
Kelvin → Celsius:       °C = K − 273.15
Fahrenheit → Kelvin:   K = (°F − 32) × 5/9 + 273.15
Kelvin → Fahrenheit:   °F = (K − 273.15) × 9/5 + 32

Worked examples

Body temperature (37 °C to °F): °F = (37 × 9/5) + 32 = 66.6 + 32 = 98.6 °F

Oven temperature (350 °F to °C): °C = (350 − 32) × 5/9 = 318 × 5/9 = 176.7 °C

Liquid nitrogen (−196 °C to K): K = −196 + 273.15 = 77.15 K

Surface of the Sun (5778 K to °C): °C = 5778 − 273.15 = 5504.85 °C

Freezer temperature (0 °F to °C): °C = (0 − 32) × 5/9 = −17.78 °C — typical food-safe freezer range.

Cold winter day (−30 °C to °F): °F = (−30 × 9/5) + 32 = −54 + 32 = −22 °F — extreme cold in Canada or Siberia.

Converting temperature differences vs absolute temperatures

Important

A common error: if a temperature rises by 10 °C, is that a rise of 18 °F or 50 °F?

It is 18 °F — because a difference of 10 °C equals a difference of 10 × 9/5 = 18 °F. The +32 offset cancels out when you subtract two absolute temperatures in °F.

Rule: when converting a temperature difference (Δ), only apply the scale factor — not the offset. ΔT(°F) = ΔT(°C) × 9/5.

Kelvin and thermodynamic calculations

Kelvin is the SI base unit for temperature. In thermodynamics, gas laws (PV = nRT), black-body radiation (Stefan-Boltzmann law), and chemical equilibrium constants all require absolute temperature in Kelvin — not Celsius or Fahrenheit.

Absolute zero (0 K) is −273.15 °C and −459.67 °F. No physical object can reach 0 K because doing so would require removing all thermal energy. The coldest temperatures ever achieved in a laboratory are fractions of a nanokelvin above absolute zero.

Temperature reference table

Formula

Common reference temperatures across all three scales:

°C       °F       K          Event
−273.15  −459.67   0          Absolute zero
−196     −320.8   77.15      Liquid nitrogen boiling point
−40      −40      233.15     Celsius = Fahrenheit crossover
0        32       273.15     Water freezing point (1 atm)
20       68       293.15     Comfortable room temperature
37       98.6     310.15     Normal human body temperature
100      212      373.15     Water boiling point (1 atm)
232      449.6    505.15     Tin melting point
660      1220     933.15     Aluminium melting point
1538     2800.4   1811.15    Iron melting point

When to use Celsius, Fahrenheit, or Kelvin

Use Celsius when: communicating with most of the world about everyday temperatures — weather, cooking, body temperature, and science at the laboratory level. Celsius is the everyday scientific standard outside the US.

Use Fahrenheit when: working with US weather reports, US cooking recipes, or communicating with US audiences. Fahrenheit is rarely used in scientific contexts outside the US.

Use Kelvin when: performing thermodynamic calculations (gas laws, heat transfer, black-body radiation), working in astrophysics, or whenever the physics requires an absolute temperature scale. Kelvin is never negative and is the correct scale for equations involving temperature ratios.