Three scales, one physical quantity
Celsius, Fahrenheit and Kelvin all measure the same underlying physical quantity, thermodynamic temperature, but they use different zero points and different step sizes ('degrees'), which is why the same physical temperature has three different numeric values depending on which scale is used to express it. None of the three scales is more 'real' than the others in a physical sense -- they are different labeling conventions applied to the same continuous thermal quantity.
Where Fahrenheit and Celsius came from
The Fahrenheit scale was devised by the physicist Daniel Gabriel Fahrenheit in the early 1720s. Fahrenheit's scale eventually settled on two now-familiar reference points, the freezing point of water at 32 °F and the boiling point of water at 212 °F, a 180-degree span between them, and it remains the everyday temperature scale in the United States.
The Celsius scale, proposed by the astronomer Anders Celsius in 1742, originally set 0 at the boiling point of water and 100 at its freezing point -- the reverse of the modern convention -- and was later reversed to the familiar arrangement of 0 °C at freezing and 100 °C at boiling, both under standard atmospheric pressure. Celsius (in its modern, reversed orientation) is the everyday scale used in most of the world today.
Kelvin: the scale that starts at absolute zero
Kelvin is the SI base unit of thermodynamic temperature and the scale used throughout science. It shares the same size of 'degree' as Celsius -- a one-kelvin change is identical in magnitude to a one-degree-Celsius change -- but its zero point is set at absolute zero, the theoretical point at which a physical system holds minimum thermal energy, rather than at a property of water. Because Celsius and Kelvin share the same step size, the two scales are related by a simple fixed offset (273.15), not a scaling multiplication, unlike the relationship between Celsius and Fahrenheit.
Absolute zero equals exactly 0 K, −273.15 °C and −459.67 °F, and no physical system can go below it -- it is a genuine lower bound on temperature, not merely a very cold measured value. Because Kelvin starts at this true physical zero, ratios of Kelvin values are physically meaningful in a way that ratios of Celsius or Fahrenheit values are not: 200 K genuinely represents twice the thermal-energy scale of 100 K, whereas 200 °C is not twice as hot as 100 °C in the same physical sense, since 0 °C is an arbitrary reference point (water's freezing point) rather than a true zero.
The exact conversion formulas
Converting between Fahrenheit and Celsius uses an exact linear formula fixed by the two scales' defined reference points, not an approximation: °F = °C × 9/5 + 32, and in the reverse direction, °C = (°F − 32) × 5/9. Because Kelvin and Celsius share the same degree size and differ only by a fixed offset, converting between them is simple addition or subtraction: K = °C + 273.15.
The Celsius-to-Kelvin offset of 273.15 is an exact defined value, not a rounded or experimentally measured figure -- following the 2019 revision of the International System of Units, the kelvin's magnitude is fixed by an exact defined value of the Boltzmann constant, which makes the numeric relationship K = °C + 273.15 a matter of definition rather than measurement.
Why Kelvin has no degree sign
Unlike Celsius and Fahrenheit, which are written with a degree symbol (°C, °F), the kelvin is written simply as 'kelvin,' symbol K, with no degree word and no ° symbol -- a convention fixed by the 13th General Conference on Weights and Measures (CGPM) in 1967/68. This reflects Kelvin's status as an absolute scale anchored to a fixed physical zero point, in contrast to Celsius and Fahrenheit, both of which are defined relative to an arbitrary reference (a property of water, in Celsius's case) rather than to a true physical zero.
Writing 'degrees Kelvin' or a ° symbol before K is therefore a common but outdated usage error, inconsistent with the unit's definition since 1967/68 -- correct modern usage is simply '300 K,' not '300 °K' or '300 degrees Kelvin.'
Verified conversion table
The table below lists familiar physical reference points across all three scales, useful for sanity-checking any individual conversion result against a known anchor point.
| Reference point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15 °C | −459.67 °F | 0 K |
| Water freezes (1 atm) | 0 °C | 32 °F | 273.15 K |
| Typical human body temperature | ≈37 °C | ≈98.6 °F | ≈310.15 K |
| Water boils (1 atm) | 100 °C | 212 °F | 373.15 K |
Часто задаваемые вопросы
What is the exact formula to convert Celsius to Fahrenheit?
°F = °C × 9/5 + 32. For example, 20 °C converts to 20 × 1.8 + 32 = 68 °F. This is an exact linear relationship fixed by the two scales' defined reference points, not an approximation.
How do you convert Celsius to Kelvin?
Add 273.15: K = °C + 273.15. For example, 0 °C (water's freezing point) equals 273.15 K, and 100 °C (water's boiling point) equals 373.15 K. This offset is an exact defined value under the SI, not a rounded figure.
What is absolute zero and why does it matter?
Absolute zero is the theoretical lower limit of temperature, the point at which a physical system holds minimum thermal energy, equal to exactly 0 K, −273.15 °C and −459.67 °F. It matters because Kelvin uses it as its zero point, which is why Kelvin values (unlike Celsius or Fahrenheit) can be meaningfully compared as ratios -- and why no physically valid temperature can fall below it.
Why is it written 'kelvin' and not 'degrees Kelvin'?
The 13th General Conference on Weights and Measures (1967/68) redefined the unit as simply 'kelvin' (symbol K), removing the word 'degree' and the ° symbol that Celsius and Fahrenheit retain, to reflect Kelvin's status as an absolute scale tied to a fixed physical zero point rather than an arbitrary reference like the Celsius or Fahrenheit zero.
When were the Fahrenheit and Celsius scales created?
Daniel Gabriel Fahrenheit devised his scale in the early 1720s. Anders Celsius proposed his scale in 1742, originally with the reference points reversed (0 at boiling, 100 at freezing) from the modern convention, which was later reversed to today's familiar 0 °C freezing / 100 °C boiling arrangement.
Источники
- National Institute of Standards and Technology (NIST) -- Special Publication 811: Guide for the Use of the International System of Units (SI), 2008 Edition.
- Bureau International des Poids et Mesures (BIPM) -- The International System of Units (SI Brochure), 9th edition, 2019.
- Bureau International des Poids et Mesures -- Resolution 3 of the 13th General Conference on Weights and Measures (CGPM), 1967/68, definition of the unit kelvin.