Celsius (°C) - the everyday metric scale
The Celsius scale is the everyday temperature scale used in Saudi Arabia and almost every country except the US and a few small dependencies. It's defined relative to the Kelvin scale: 0°C is exactly 273.15 K. Originally, 0°C was the freezing point of pure water at 1 atmosphere and 100°C the boiling point - but since 2019 the official definition is purely in terms of Kelvin, with freezing and boiling points being measured quantities.
Fahrenheit (°F) - the US scale
Fahrenheit was proposed by Daniel Gabriel Fahrenheit in 1724. He originally set 0°F as the freezing point of brine (water + ammonium chloride) and 96°F as roughly human body temperature. Modern Fahrenheit is defined relative to Celsius: water freezes at 32°F, boils at 212°F, with each Fahrenheit degree being 5/9 of a Celsius degree. The US and Liberia still use Fahrenheit for daily weather and cooking; Saudi Arabia uses Celsius.
Kelvin (K) - the SI absolute scale
The Kelvin scale starts at absolute zero - the theoretical lowest possible temperature, where atoms have minimum kinetic energy. 0 K = −273.15°C = −459.67°F. There is no degree symbol with Kelvin; you write '300 K' not '300°K'. Since 2019 the Kelvin has been defined in terms of the Boltzmann constant - a fundamental physical constant. This is the temperature scale used in physics, chemistry, and astronomy.
The conversion formulas
°F → °C: subtract 32, multiply by 5/9. °C → °F: multiply by 9/5, add 32. °C → K: add 273.15. K → °C: subtract 273.15. °F → K: add 459.67, multiply by 5/9. K → °F: multiply by 9/5, subtract 459.67. Rankine (°R) is the Fahrenheit equivalent of Kelvin - same step size as Fahrenheit, anchored to absolute zero: °R = °F + 459.67, or °R = K × 9/5.
Common use cases
Recipes: US cookbooks specify oven temperatures in °F (350°F → 177°C); Saudi cooks need to convert. Weather: international weather forecasts and US-based apps may report °F; you convert to °C for your local sense. Fever: a normal body temperature is 37°C (98.6°F); a fever starts around 38°C (100.4°F). Oven settings: gas-mark or US-style °F appliances vs metric thermostats. Air-conditioning: most Saudi ACs display Celsius; some imported units show Fahrenheit.
Common Arabic terms
Celsius is «مئوي» or «درجة مئوية» - both standard. Fahrenheit is «فهرنهايت» (direct transliteration). Kelvin is «كلفن» or «درجة كلفن». Rankine is «رانكين» but rarely used outside scientific contexts. In Saudi everyday life, all weather and indoor temperatures are quoted in Celsius; Fahrenheit comes up mostly when reading imported cookbooks or US news sources.
Frequently asked questions
Subtract 30, then divide by 2 - close enough for daily weather. Example: 70°F − 30 = 40, ÷ 2 = 20°C (actual: 21.1°C). The shortcut is off by ~1° for typical room temperatures but works for quick estimates.
°F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9. Both are exact linear relations defined by international standard.
Because it starts at absolute zero - the lowest possible temperature, where there's no thermal motion. Many physical laws (gas laws, blackbody radiation, thermodynamics) are simpler when expressed in Kelvin because the scale has a true zero rather than an arbitrary one tied to water.
Both - they're the same temperature. 37°C × 9/5 + 32 = 98.6°F exactly. Fever is generally considered to start at 38°C (100.4°F). Many people have a baseline slightly below 37°C; modern medical thinking treats 36.5–37.5°C as the normal adult range.
Because Kelvin is the SI base unit of thermodynamic temperature, not a 'degree' on an arbitrary scale. The degree symbol survives on Celsius and Fahrenheit for historical reasons; Kelvin is written as a plain unit, like meters or kilograms. So '300 K' is correct, '300°K' is not.
Sources
- The International System of Units (SI) - Kelvin definition— Bureau International des Poids et Mesures (BIPM)
- ITS-90 - The International Temperature Scale of 1990— Bureau International des Poids et Mesures (BIPM)
- Normal body temperature - modern clinical reference— Mayo Clinic
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