The Celsius scale is used for weather reports, medical temperatures, food storage, ovens outside the United States, laboratory measurements, climate data, HVAC settings, and most international technical documentation. It is easy to read in daily life because common water-related reference points fall at round numbers: 0 C for freezing and 100 C for boiling under standard pressure.
Celsius quick facts
- Scale name
- Celsius
- Symbol
- C
- Quantity
- Temperature
- System
- Metric, SI-compatible temperature scale
- Water freezing point
- 0 C at standard atmospheric pressure
- Water boiling point
- 100 C at standard atmospheric pressure
- Kelvin relationship
- kelvin = celsius + 273.15
- Fahrenheit relationship
- fahrenheit = celsius x 9/5 + 32
Definition and scale
Celsius is a temperature scale named after Anders Celsius. In practical use, it is anchored around the phase behavior of water: 0 C is the freezing point of water and 100 C is the boiling point of water at standard atmospheric pressure. Modern temperature measurement is tied to thermodynamic definitions, but these water points remain the most familiar way to understand the scale.
A Celsius degree has the same interval size as a kelvin. That means a change of 1 C is equal to a change of 1 K. The difference is zero point: 0 K is absolute zero, while 0 C is 273.15 K.
Symbol and writing rules
On Xunits, Celsius values are written with the ASCII symbol C, such as 20 C or -5 C. The symbol does not take a plural s. In formal typography, many publications write degrees Celsius with a degree sign before C, but the unit relationship and conversion formulas are the same.
Celsius in temperature systems
Celsius is metric-friendly but is not the SI base temperature unit. The SI temperature unit is the kelvin. Celsius is accepted for use with SI because it is directly related to Kelvin by a fixed offset and has the same interval size.
| Scale | Zero point | Interval size | Best use |
|---|---|---|---|
| Celsius | 0 C near water freezing | 1 C = 1 K interval | Everyday metric temperature |
| Kelvin | 0 K at absolute zero | 1 K = 1 C interval | Scientific and thermodynamic calculations |
| Fahrenheit | Offset from Celsius | 1 F = 5/9 C interval | US everyday temperature |
Celsius vs Fahrenheit
Celsius and Fahrenheit both measure temperature, but their zero points and degree sizes differ. Water freezes at 0 C and 32 F. Water boils at 100 C and 212 F. The interval between these points is 100 Celsius degrees and 180 Fahrenheit degrees, so 1 C of temperature change equals 1.8 F of temperature change.
For an actual reading, do not multiply by 1.8 alone. The correct Celsius-to-Fahrenheit formula includes the offset: fahrenheit = celsius x 9/5 + 32.
Celsius vs Kelvin
Celsius and Kelvin are closer relatives. Their intervals are the same size, but Kelvin starts at absolute zero. This makes Kelvin essential in physics and engineering equations where absolute temperature matters. A value of 0 C equals 273.15 K, and 100 C equals 373.15 K.
Kelvin is written without degrees. Use 293.15 K, not degrees K, for the Kelvin equivalent of 20 C.
Common uses
Weather and climate
Celsius is the standard weather scale in most countries and is widely used in climate records, meteorology, and environmental reporting.
Medicine and health
Body temperature, storage temperature, fever thresholds, and laboratory handling instructions commonly use Celsius outside US-specific consumer contexts.
Cooking and storage
Refrigerators, freezers, food safety guidance, and many ovens outside the United States use Celsius temperature settings.
Science and engineering
Celsius is often used for reporting measured temperature, while Kelvin is used when equations require absolute temperature.
How Celsius is measured
Celsius readings come from thermometers and temperature sensors, including liquid-in-glass thermometers, thermistors, resistance temperature detectors, thermocouples, infrared sensors, and digital probes. The displayed unit is only one part of the measurement. Sensor type, calibration, response time, placement, and contact with the measured object all affect the result.
For reliable records, note the unit, measurement location, instrument type, and whether the reading is air temperature, surface temperature, liquid temperature, or internal temperature.
Precision and rounding
Everyday Celsius values are often rounded to whole numbers, such as 22 C for room temperature. Medical, laboratory, and industrial values may need one decimal place or more. Rounding should happen after conversion, not before, when the converted value will be used in a calculation or specification.
Conversion formulas
fahrenheit = celsius x 9/5 + 32celsius = (fahrenheit - 32) x 5/9kelvin = celsius + 273.15celsius = kelvin - 273.15Celsius conversion table
| Celsius | Fahrenheit | Kelvin | Reference point |
|---|---|---|---|
| -40 C | -40 F | 233.15 K | same numeric value in Celsius and Fahrenheit |
| 0 C | 32 F | 273.15 K | water freezing point at standard pressure |
| 20 C | 68 F | 293.15 K | comfortable room-temperature range |
| 37 C | 98.6 F | 310.15 K | typical human body temperature reference |
| 100 C | 212 F | 373.15 K | water boiling point at standard pressure |
Common mistakes
- Using celsius x 9/5 without adding 32 when converting an actual Celsius reading to Fahrenheit.
- Applying the +32 offset to a temperature interval, such as a change of 10 C.
- Confusing Celsius with Kelvin in scientific equations that require absolute temperature.
- Writing Kelvin as degrees K when the correct unit symbol is simply K.
- Assuming 100 C is always the boiling point of water without considering atmospheric pressure.
- Rounding Celsius before conversion when the final Fahrenheit or Kelvin value needs precision.
Related Xunits pages
FAQ
What is Celsius?
Celsius is a temperature scale where water freezes at 0 C and boils at 100 C at standard atmospheric pressure. It is the most common everyday temperature scale in much of the world.
How do you convert Celsius to Fahrenheit?
Use fahrenheit = celsius x 9/5 + 32. For example, 20 C x 9/5 + 32 = 68 F.
How do you convert Celsius to Kelvin?
Use kelvin = celsius + 273.15. For example, 20 C equals 293.15 K.
Is Celsius an SI unit?
The SI base unit for thermodynamic temperature is Kelvin. Celsius is accepted for use with SI because Celsius intervals are equal in size to kelvin intervals and the two scales differ by a fixed offset.
Why can Celsius be negative?
Celsius zero is based near water's freezing point, not absolute zero. Temperatures below freezing water are negative on the Celsius scale.