Article — Centigrade to Celsius Converter
Centigrade to Celsius: Same Scale, Different Name
Centigrade and Celsius are the same temperature scale. The 9th General Conference on Weights and Measures (CGPM) officially renamed centigrade to Celsius in 1948 to honor Swedish astronomer Anders Celsius and to avoid collision with the centesimal degree (a French surveying unit of angle). The conversion is the simplest of any temperature pair: x centigrade equals x Celsius, no math, no offset. 100 centigrade equals 100 Celsius equals 212 Fahrenheit equals 373.15 Kelvin.
The Celsius name has been the standard since 1948, but centigrade still appears in older textbooks, kitchen thermometers, and casual speech. The two terms are interchangeable; this calculator confirms the identity and shows the Fahrenheit and Kelvin equivalents alongside.
Centigrade vs Celsius identity
There is nothing to convert. A reading of 25 centigrade is the same physical temperature as 25 Celsius. The two terms refer to the same thermodynamic scale, with the same zero point (ice melting at standard atmospheric pressure), the same 100 point (water boiling at standard atmospheric pressure), and the same degree size between them.
The CGPM resolved on October 22, 1948, that "the degree centigrade is renamed degree Celsius, with no change in value." Every official metric and SI document since then has used Celsius. The centigrade name persists colloquially but never appears in modern scientific publications.
Anders Celsius proposed his original scale in 1742 with the freezing point of water at 100 degrees and the boiling point at 0 degrees — inverted relative to the modern scale. Carl Linnaeus and Daniel Ekström independently flipped it to today's orientation within a year. Celsius himself died in 1744, before the scale bearing his name took its current form.
History of the centigrade scale
The word centigrade comes from Latin: centum (hundred) and gradus (step or degree). It described the 100-degree division between the freezing and boiling points of water. The scale was developed during the late 18th century when scientists wanted a decimal alternative to the older Fahrenheit (180 degrees between the same reference points) and Réaumur (80 degrees) scales.
By the mid-1800s, the centigrade name had become standard in English- and French-speaking science. The scale was used in meteorology, medicine, and laboratory work for over a century before the 1948 rename.
Why centigrade was renamed to Celsius
Two reasons. First, to honor a specific person — most temperature scales bear an inventor's name (Fahrenheit, Kelvin, Rankine, Réaumur), and Celsius had not previously been formally credited. Second, the word centigrade collided with the centesimal degree (gradian), a unit of angle equal to 1/100 of a right angle, used by French and continental European surveyors and gunners. Calling 25 degrees of angle and 25 degrees of temperature both "centigrade" caused real confusion in scientific publications.
The CGPM's 1948 decision was unanimous and effective immediately. Schoolbooks took two decades to catch up; by the 1970s, "Celsius" had replaced "centigrade" in nearly all formal contexts.
Celsius to Fahrenheit and Kelvin
The Celsius-to-Fahrenheit conversion is the formula most non-American travelers learn. Multiply Celsius by 9/5 (i.e. 1.8) and add 32. So 20°C = 68°F, 30°C = 86°F, 35°C = 95°F. Going the other way: subtract 32, multiply by 5/9.
The Celsius-to-Kelvin conversion is even simpler: add 273.15. Kelvin and Celsius share the same degree size; only the zero point differs. So 20°C = 293.15 K, 100°C = 373.15 K, and -273.15°C = 0 K (absolute zero).
°F = °C × 9/5 + 32 °C = (°F - 32) × 5/9K = °C + 273.15 °C = K - 273.15°R = (°C + 273.15) × 9/5 (Rankine)Common temperature reference points
Knowing a handful of landmark temperatures helps you sense-check any conversion. Water freezes at 0°C / 32°F and boils at 100°C / 212°F. Human body temperature is 37°C / 98.6°F. A comfortable indoor room is 20-22°C / 68-72°F. The hottest recorded outdoor air temperature on Earth (Death Valley, 1913) was 56.7°C / 134°F.
- Absolute zero = -273.15°C = -459.67°F = 0 K
- Dry ice sublimates at -78.5°C = -109.3°F
- Water freezes at 0°C = 32°F at sea-level pressure
- Room temperature conventionally 20°C = 68°F
- Body temperature = 37°C = 98.6°F (oral)
- Water boils at 100°C = 212°F at sea-level pressure
- -40 crossover: -40°C = -40°F (only point of agreement)
Celsius vs Fahrenheit, Kelvin, Rankine
Four temperature scales survive in active use. Celsius dominates worldwide and in all science outside the US. Fahrenheit is the everyday scale in the United States, the Bahamas, and a few small territories. Kelvin is the SI base unit for thermodynamic temperature, used in physics, astronomy, and engineering. Rankine is Kelvin scaled to Fahrenheit degrees — used occasionally in US engineering thermodynamics.
Absolute zero and the kelvin definition
Absolute zero is the lowest possible temperature, defined as -273.15°C exactly. No physical system can be cooled to absolute zero — quantum-mechanical zero-point motion remains. The closest experimental realizations have reached a few picokelvin (10⁻¹² K) in laser-cooled atom traps.
Since 2019, the kelvin is defined via a fixed value of the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K exactly), replacing the older definition based on the triple point of water (273.16 K). The change made the kelvin a derived unit from physical constants rather than a material-property reference.
Below absolute zero is physically impossible in normal thermodynamics, so the calculator refuses any centigrade value below -273.15. If you encounter a "negative kelvin" reference in physics papers, it refers to systems with inverted population distributions, which formally have negative temperatures but are actually hotter than any positive temperature — a separate concept entirely.
Common centigrade/Celsius mistakes
The biggest mistake is assuming centigrade and Celsius are different. They aren't — there is no conversion factor, no offset, no scaling. They are the same scale, just renamed. Any calculator that gives a non-identity result for centigrade-to-Celsius is wrong.
A reading of 35° could mean a hot summer day (35°C = 95°F) or a mild winter day (35°F = 1.7°C). Always check whether the scale shows °C or °F. Digital thermometers usually have a toggle; old glass thermometers may show only one scale.
Another common slip is forgetting the 0.15 in the Kelvin offset. K = °C + 273.15, not + 273. The 0.15 is the gap between the ice point (used by Celsius) and the triple point of water (273.16 K, used as the historical Kelvin anchor). At everyday temperatures the 0.15 K rounding error is invisible; in cryogenics or radiometry it matters.