Temperature is a physical quantity that describes thermal state: how hot or cold something is, and more precisely how it relates to the average kinetic energy of particles in a system. Temperature units are used in weather, medicine, cooking, manufacturing, HVAC, laboratory work, thermodynamics, and engineering specifications.
Unlike length or mass units, most everyday temperature scales are offset scales. Converting between Celsius and Fahrenheit requires both a scale change and an added offset. That is why a temperature conversion cannot always be handled by multiplying by a single factor.
What is temperature?
Temperature measures thermal condition, not the total heat energy stored in an object. A cup of boiling water and a large pot of boiling water can have the same temperature, even though the pot contains more total thermal energy. This distinction matters in cooking, climate data, laboratory measurement, and thermal engineering.
Temperature scales are not ordinary units
Many unit conversions are ratio conversions. For example, 1 meter equals 100 centimeters, so every meter value can be multiplied by 100 to get centimeters. Celsius and Fahrenheit do not work that way because their zero points are different. A 10 C change equals an 18 F change, but 10 C as an actual temperature equals 50 F.
Temperature value
20 C is an actual reading on a scale. To convert it to Fahrenheit, use both multiplication and an offset: fahrenheit = celsius x 9/5 + 32.
Temperature interval
A change of 20 C is a difference between two readings. For intervals, the Celsius-to-Fahrenheit scale factor is 9/5, with no +32 offset.
Celsius, Fahrenheit, and Kelvin
The three most common temperature scales on Xunits are Celsius, Fahrenheit, and Kelvin. Celsius and Fahrenheit are everyday temperature scales. Kelvin is the scientific absolute temperature scale used in thermodynamics and many engineering calculations.
| Scale | Symbol | Water freezing point | Water boiling point | Typical use |
|---|---|---|---|---|
| Celsius | C | 0 C | 100 C | Weather, medicine, cooking, science, global everyday use |
| Fahrenheit | F | 32 F | 212 F | US weather, US ovens, body temperature, legacy settings |
| Kelvin | K | 273.15 K | 373.15 K | Science, physics, engineering, thermodynamic formulas |
Temperature conversion formulas
Temperature formulas must preserve both the size of a degree and the location of zero on the scale. Celsius and Kelvin have equal-sized increments, but different zero points. Fahrenheit increments are smaller than Celsius increments, and the Fahrenheit zero point is offset from Celsius.
fahrenheit = celsius x 9/5 + 32celsius = (fahrenheit - 32) x 5/9kelvin = celsius + 273.15celsius = kelvin - 273.15Choosing a temperature scale
Use Celsius for most international weather, cooking, medical, and classroom contexts. Use Fahrenheit when communicating with US audiences or reading US appliances and weather reports. Use Kelvin when the calculation depends on absolute temperature, such as gas laws, blackbody radiation, heat transfer models, or thermodynamic efficiency.
Common temperature-unit mistakes
- Treating Celsius to Fahrenheit as a simple multiplication and forgetting the +32 offset.
- Using a Fahrenheit formula for a temperature interval instead of an actual temperature reading.
- Writing Kelvin with the word "degrees"; Kelvin values are written as K, not degrees K.
- Assuming water always boils at exactly 100 C without considering pressure and altitude.
- Mixing sensor precision with measurement accuracy; a display with 0.1 C resolution is not always accurate to 0.1 C.
Available temperature guides
The temperature Wiki will grow one hand-written unit page at a time. The first guide explains Celsius in detail, including definition, symbol rules, formulas, scale comparisons, common uses, and mistakes.