All Temperature Units

How to Convert Temperature Units?

Temperature conversion involves mathematical formulas to translate values between different temperature scales. The three main scales are Celsius, Fahrenheit, and Kelvin. Celsius and Fahrenheit are commonly used in daily life, while Kelvin is the standard unit in scientific applications.

The key conversion formulas are:
- Celsius to Fahrenheit: F = (C x 9/5) + 32
- Fahrenheit to Celsius: C = (F - 32) x 5/9
- Celsius to Kelvin: K = C + 273.15
- Kelvin to Celsius: C = K - 273.15

Units

Celsius

Celsius, also known as centigrade, is the most widely used temperature scale worldwide. It was developed by Swedish astronomer Anders Celsius in 1742. The scale is based on the freezing point of water at 0 degrees and the boiling point at 100 degrees under standard atmospheric pressure. Celsius is used in most countries for weather forecasts, cooking, and everyday temperature measurements.

Fahrenheit

Fahrenheit is a temperature scale proposed by German physicist Daniel Gabriel Fahrenheit in 1724. Water freezes at 32 degrees F and boils at 212 degrees F. It is primarily used in the United States and some Caribbean nations for weather forecasting, cooking, and indoor climate control. The scale offers more granularity for everyday temperatures since the degree intervals are smaller than Celsius.

Kelvin

Kelvin is the base unit of temperature in the International System of Units (SI). Unlike Celsius and Fahrenheit, Kelvin does not use degrees. It starts at absolute zero (0 K = -273.15 C), the theoretical point where all molecular motion stops. Kelvin is essential in scientific research, physics, chemistry, and engineering applications where precise temperature measurements are required.

Common Temperature Conversions

Frequently Asked Questions

Is 100 degrees Fahrenheit hot or cold and what does it feel like?

100 F is hot — about 37.8 C, slightly above normal human body temperature (98.6 F / 37 C). At this temperature you will feel uncomfortably warm, sweat heavily, and need shade and hydration. For reference: 80 F (27 C) is warm, 90 F (32 C) is summer-hot, 100 F (38 C) is heatwave territory, 110 F (43 C) is dangerous outdoor heat seen in deserts and during heatwaves. Going the other way: 70 F (21 C) is room temperature, 50 F (10 C) is jacket weather, 32 F (0 C) is the freezing point of water, 0 F (-18 C) is severely cold, -40 F is also -40 C (the only point where the two scales cross). Remember: F = C times 9/5 plus 32, or roughly double the Celsius and add 30 for a quick estimate.

What is the exact formula to convert Celsius to Fahrenheit?

The exact formula is F = (C times 9/5) + 32, or equivalently F = C times 1.8 + 32. Going backwards, C = (F - 32) times 5/9, or C = (F - 32) / 1.8. The two scales meet at -40 degrees (-40 F = -40 C) and diverge in opposite directions from there. Water freezes at 0 C / 32 F and boils at 100 C / 212 F at standard atmospheric pressure. Quick approximation for mental math: double the Celsius and add 30 (so 20 C becomes 70 F, actual is 68 F). The 9/5 ratio comes from the original definitions: Celsius divided the freezing-to-boiling range into 100 degrees, while Fahrenheit divided it into 180 degrees, so each Celsius degree equals 1.8 Fahrenheit degrees.

Why do scientists use Kelvin instead of Celsius?

Kelvin is the SI base unit of temperature and is essential in physics because it starts at absolute zero (-273.15 C / -459.67 F), the theoretical minimum where all molecular motion stops. This makes Kelvin an absolute scale where temperature is proportional to thermal energy — doubling the kelvin value really does mean doubling the energy. With Celsius or Fahrenheit you cannot do this because their zero points are arbitrary. Gas laws (PV = nRT), Stefan-Boltzmann radiation law, and most thermodynamic equations require absolute temperature, so Kelvin is mandatory. Kelvin uses the same degree size as Celsius (1 K change = 1 C change), so conversions are simple addition: K = C + 273.15. Room temperature (20 C) is 293.15 K. Note that Kelvin uses no degree symbol — just write 300 K, not 300 degrees K.

What is absolute zero and can anything actually be that cold?

Absolute zero is 0 K, equal to -273.15 C or -459.67 F, the temperature at which atoms and molecules have minimum quantum-mechanical motion (zero classical kinetic energy). The third law of thermodynamics states that absolute zero cannot be reached in a finite number of steps — you can approach it but never quite touch it. Researchers at MIT and elsewhere have cooled atomic gases to within billionths of a kelvin of absolute zero using laser cooling and evaporative cooling, reaching temperatures around 50 picokelvin (5 x 10^-11 K) as of 2024. The coldest natural place known is the Boomerang Nebula at about 1 K. Note that at extremely low temperatures, exotic quantum phases of matter appear: Bose-Einstein condensates, superfluids, and superconductors all emerge when thermal noise drops below quantum binding energies.

What is the Rankine scale and where is it used?

The Rankine scale (R or Ra) is an absolute temperature scale based on Fahrenheit degrees, where absolute zero is 0 R = -459.67 F. To convert: R = F + 459.67, and R = K times 9/5. Water freezes at 491.67 R and boils at 671.67 R. Rankine was proposed by Scottish engineer William Rankine in 1859 and is still used in some US engineering fields — particularly thermodynamics, combustion engineering, and aerospace — where engineers want to keep Fahrenheit-style degrees but need an absolute scale for gas-law and heat-transfer calculations. Outside the US, Rankine is essentially never used. The international scientific standard is Kelvin. If you encounter R or degrees R in older American engineering textbooks, just remember it is Fahrenheit shifted up by 459.67.

Why do weather forecasts say 'feels like' temperature different from the actual reading?

The 'feels like' temperature combines actual air temperature with wind chill (in cold weather) or heat index (in hot, humid weather) to estimate how the human body perceives the temperature. Wind chill formula: at 0 C with 30 km/h wind, exposed skin loses heat as if it were -8 C in still air. Heat index combines temperature and relative humidity: 32 C at 70% humidity feels like 41 C because sweat evaporates less efficiently. The exact 'feels like' formulas were developed by NOAA and the Canadian Meteorological Service in 2001. These are statistical models, not precise physics — actual perception varies by clothing, hydration, sun exposure, and acclimatization. For health warnings (heat stroke, hypothermia), the 'feels like' value matters more than the thermometer reading.

How accurate are home thermometers and what calibration standards exist?

Consumer digital thermometers typically claim +/- 0.5 to 1.0 C accuracy, while medical-grade thermometers achieve +/- 0.1 C in the body-temperature range. Mercury and alcohol thermometers have inherent limitations from glass expansion and sensor placement, generally +/- 0.5 to 1.0 C. For laboratory and industrial use, NIST (US) and NPL (UK) provide traceable calibration standards. The most accurate reference is the International Temperature Scale of 1990 (ITS-90), which defines fixed points like the triple point of water (273.16 K exactly by definition until 2019, now derived from the Boltzmann constant) and the freezing point of pure metals (silver 1234.93 K, gold 1337.33 K). Modern platinum resistance thermometers calibrated to ITS-90 achieve uncertainties below 0.001 K. For home use, place thermometers away from sunlight, vents, and drafts for the most representative reading.

Why is normal body temperature 98.6 F (37 C) and is that still considered accurate?

The 98.6 F / 37 C figure comes from German physician Carl Wunderlich's 1851 measurements of 25,000 patients — but recent research suggests his thermometers ran slightly hot and that today's healthy adults average closer to 97.5 F / 36.4 C. A 2020 Stanford study analyzed 677,000 oral temperature readings spanning 157 years and found a steady decline in average body temperature, likely due to less chronic inflammation, better dental health, and indoor climate control. Normal temperature also varies by individual, time of day (lowest at 4 AM, highest at 6 PM, range about 0.5 C), measurement site (rectal > oral > armpit by about 0.5 C each step), age (lower in elderly), and physical activity. Fever is generally defined as oral temperature above 38 C (100.4 F), but always interpret in context with other symptoms.