Temperature is a physical quantity describing how hot or cold an object is, microscopically representing the average kinetic energy of molecules. Humans have developed multiple temperature scales to quantify temperature; this tool covers the five most commonly used: Celsius (°C), Fahrenheit (°F), Kelvin (K), Rankine (°R), and Réaumur (°Ré). Conversion between these scales requires handling both scaling and zero-point offset—these are affine transformations, not pure multiplication.
The Celsius scale (°C), proposed by Swedish astronomer Anders Celsius in 1742, defines water's freezing point as 0°C and boiling point as 100°C at standard atmospheric pressure, divided evenly into 100 increments (hence the alternative name centigrade). Celsius is the official temperature unit in the vast majority of countries worldwide, and is used alongside Kelvin in the SI system for daily life, weather forecasts, healthcare, and cooking.
The Fahrenheit scale (°F), proposed by German physicist Gabriel Fahrenheit in 1724, originally used an ice-brine mixture temperature as 0°F and approximate human body temperature as 96°F; it was later standardized with water freezing at 32°F and boiling at 212°F, an interval of 180 degrees. Fahrenheit remains in everyday use primarily in the United States, the Bahamas, Cayman Islands, and a few other regions; American recipes, oven dials, and Fahrenheit thermometers all use °F.
The Kelvin (K) is the only thermodynamic temperature unit in the International System of Units (SI), starting at absolute zero (0 K = -273.15°C) with degree increments identical to Celsius (1 K = 1°C temperature difference). Kelvin does not use a "degree" symbol—values are written directly as number + K. It is the standard temperature scale for physics, chemistry, and engineering thermodynamics. All physical formulas involving temperature (PV=nRT, Stefan-Boltzmann law, Carnot efficiency, etc.) require absolute temperature (Kelvin or Rankine).
The Rankine scale (°R), proposed by Scottish engineer William Rankine, is the absolute temperature scale based on Fahrenheit: 0°R = absolute zero, with degree increments identical to Fahrenheit (1°R = 1°F temperature difference), hence °R = °F + 459.67 = K × 9/5. Rankine remains in use in U.S. aerospace and thermodynamic engineering, serving as the "Kelvin" of the Fahrenheit world. The Réaumur scale (°Ré), proposed by 18th-century French scientist René Réaumur, defines water freezing at 0°Ré and boiling at 80°Ré. It was widely used in 19th-century Europe and now appears only in some old recipes, cheesemaking, and historical literature.
Temperature conversion is fundamentally based on the affine transformation T_target = a × T_source + b, where a is the scaling factor and b is the zero-point offset. For example, °C→°F: a=9/5, b=32; °C→K: a=1, b=273.15. Converting °F→K requires two steps: first subtract 32, multiply by 5/9, then add 273.15. This tool includes direct conversion formulas between all scales to avoid floating-point errors from chained conversions.
