Temperature Conversion Tools
Last updated: February 28, 2026
Accurate temperature conversion tools grounded in the SI definition of the Kelvin and its defined relationships to Celsius, Fahrenheit, and Rankine.
Free online temperature conversion calculators. Convert between Celsius, Fahrenheit and Kelvin with instant results and step-by-step formulas.
Understanding Temperature Scales
Celsius (°C) is the most widely used temperature scale in the world. It sets 0° at the freezing point and 100° at the boiling point of water at standard atmospheric pressure.
Fahrenheit (°F) is primarily used in the United States. It sets 32° at the freezing point and 212° at the boiling point of water.
Kelvin (K) is the SI base unit of temperature used in science. It starts at absolute zero (0 K = −273.15 °C), the lowest theoretically possible temperature.
Rankine (°R) is the Fahrenheit-based absolute scale, where 0 °R equals absolute zero (−459.67 °F).
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Temperature Guides & Resources
- Body Temperature Chart — Normal, fever & hypothermia ranges in °C and °F
- Oven Temperature Guide — Celsius to Fahrenheit conversion chart for cooking
- Countries That Use Fahrenheit — Complete list and history of adoption
- Celsius vs Fahrenheit — Differences, history and comparison
Quick Reference Formulas
| Conversion | Formula |
|---|---|
| Celsius to Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit to Celsius | °C = (°F − 32) × 5/9 |
| Celsius to Kelvin | K = °C + 273.15 |
| Kelvin to Celsius | °C = K − 273.15 |
| Fahrenheit to Kelvin | K = (°F − 32) × 5/9 + 273.15 |
| Kelvin to Fahrenheit | °F = (K − 273.15) × 9/5 + 32 |
History of Temperature Scales
The measurement of temperature has a rich history spanning over three centuries, driven by the need to quantify heat in a reliable and reproducible way.
Daniel Gabriel Fahrenheit developed his eponymous scale in 1724 while working with mercury thermometers in Amsterdam. He established three reference points: the temperature of a brine solution (0 °F), the freezing point of pure water (32 °F), and human body temperature (96 °F on his original scale). His use of mercury — rather than the alcohol common at the time — allowed far greater precision, and his 180-degree interval between freezing and boiling water remained the standard in English-speaking countries for centuries.
Anders Celsius, a Swedish astronomer, proposed his centigrade scale in 1742. His original design placed 0 at the boiling point of water and 100 at the freezing point. Shortly after his death, fellow Swedish scientists Carl Linnaeus and Marten Stromer inverted the scale to the form used today, with 0 °C at freezing and 100 °C at boiling under standard atmospheric pressure of 1 atm. The scale was officially renamed from "centigrade" to "Celsius" in 1948 by the General Conference on Weights and Measures to avoid confusion with the centesimal grading system used in some countries.
William Thomson, later known as Lord Kelvin, introduced the absolute thermodynamic temperature scale in 1848. Building on the theoretical work of Guillaume Amontons and the gas law experiments of Jacques Charles, Kelvin recognized that there must be a natural lower bound to temperature where molecular kinetic energy reaches its minimum. He defined absolute zero as the starting point of his scale and sized each Kelvin unit to match one Celsius degree. Today the Kelvin is the SI base unit for thermodynamic temperature and is defined through the Boltzmann constant (k = 1.380649 × 10−23 J/K) rather than any physical substance.
William John Macquorn Rankine, a Scottish engineer, proposed an absolute scale based on the Fahrenheit degree in 1859. The Rankine scale sets 0 °R at absolute zero (−459.67 °F) and maintains the same degree size as Fahrenheit. Although rarely encountered in everyday life, it remains relevant in some branches of American engineering, particularly in thermodynamic calculations involving the ideal gas law and specific enthalpy tables.
The existence of multiple temperature scales reflects different historical needs: Fahrenheit optimized for weather and everyday precision, Celsius for scientific clarity, and Kelvin and Rankine for thermodynamic calculations that require an absolute reference point.
The International Temperature Scale (ITS-90)
The International Temperature Scale of 1990 (ITS-90) is the current globally accepted standard for calibrating thermometers and reporting temperature measurements. It was adopted by the Consultative Committee for Thermometry under the International Bureau of Weights and Measures (BIPM) and has been in effect since January 1, 1990.
ITS-90 defines temperature through a series of fixed reference points — reproducible physical states such as the triple point of water (273.16 K exactly), the melting point of gallium (302.9146 K), and the freezing point of silver (1234.93 K). Between these fixed points, temperature is interpolated using standard platinum resistance thermometers (SPRTs) or, at higher temperatures, by radiation thermometry based on Planck's law.
The scale spans from 0.65 K to the highest temperatures measurable by optical pyrometry. Its direct relationship with the Kelvin makes it the foundation for all precision temperature work in physics, chemistry, engineering, and metrology. All conversion formulas used on this website produce results consistent with ITS-90 definitions.
For detailed technical documentation, see the official BIPM temperature measurement standards.
Absolute Zero and Thermodynamics
Absolute zero represents the theoretical lower limit of temperature, the point at which a system possesses its minimum possible thermal energy. In numerical terms:
- 0 Kelvin (0 K)
- −273.15 degrees Celsius (−273.15 °C)
- −459.67 degrees Fahrenheit (−459.67 °F)
- 0 degrees Rankine (0 °R)
At absolute zero, classical thermodynamics predicts that the entropy of a perfect crystal reaches zero, a principle formalized as the Third Law of Thermodynamics by Walther Nernst in 1906. In practical terms, atomic and molecular motion does not cease entirely — quantum mechanics dictates that particles retain a residual zero-point energy even at 0 K — but no heat can be extracted from a system at this temperature.
Reaching absolute zero exactly is physically impossible, a consequence of the Nernst unattainability principle. However, modern cryogenic laboratories have cooled matter to within billionths of a kelvin above absolute zero using techniques such as laser cooling and adiabatic demagnetization. These ultra-cold experiments have revealed remarkable quantum phenomena including Bose-Einstein condensation and superfluidity.
Understanding absolute zero is essential for any temperature conversion involving the Kelvin or Rankine scales, since both are defined with their zero point at this fundamental physical limit.
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Data Accuracy
All formulas on this site reflect the SI-defined relationships between temperature scales. Calculations are performed in double-precision floating point (IEEE 754) and displayed results are rounded to one decimal place. The values produced are consistent with the International Temperature Scale of 1990 (ITS-90), the globally accepted metrological standard for thermometer calibration and temperature reporting.
Author
Content maintained by the CelsiusFahrenheit.co team. All conversions follow internationally defined temperature scale relationships and are validated against the ITS-90 standard.