Convert between Celsius, Fahrenheit, and Kelvin in real time.
Temperature is a fundamental physical quantity that measures the average kinetic energy of particles in a substance. Three temperature scales dominate global usage: Celsius (used by most of the world for everyday purposes), Fahrenheit (used primarily in the United States), and Kelvin (the scientific standard). Converting between these scales is a common need for travelers, cooks, scientists, engineers, and anyone working with international data or weather information.
The Celsius scale, developed by Swedish astronomer Anders Celsius in 1742, is based on the properties of water. Zero degrees Celsius is the freezing point of water at standard atmospheric pressure, and 100 degrees Celsius is the boiling point. This clean, intuitive design makes Celsius practical for everyday use. Nearly every country in the world uses Celsius as its primary temperature scale. Room temperature is typically around 20 to 22 degrees Celsius, body temperature is approximately 37 degrees Celsius, and a comfortable summer day might reach 28 to 32 degrees Celsius.
The Fahrenheit scale was developed by German physicist Daniel Gabriel Fahrenheit in 1724. On this scale, water freezes at 32 degrees and boils at 212 degrees. The scale was originally calibrated using three reference points: the temperature of an ice, water, and ammonium chloride mixture (0 degrees F), the freezing point of pure water (32 degrees F), and the human body temperature (originally 96 degrees F, later revised to 98.6 degrees F). While less intuitive than Celsius, Fahrenheit provides finer granularity -- one degree Fahrenheit is smaller than one degree Celsius -- which some argue makes it better for describing weather and ambient temperatures in terms of human comfort.
The Kelvin scale is the absolute temperature scale used in science and engineering. It starts at absolute zero (the point where all molecular motion ceases, equal to -273.15 degrees Celsius or -459.67 degrees Fahrenheit) and uses the same degree increments as Celsius. This means that a change of 1 Kelvin equals a change of 1 degree Celsius. The Kelvin scale is essential in physics, chemistry, and astronomy because it provides an absolute reference point and eliminates negative temperatures, simplifying many thermodynamic calculations. Water freezes at 273.15 K and boils at 373.15 K.
The relationship between temperature scales involves both scaling and offset. The conversion formulas are as follows:
| Description | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 21 | 70 | 294.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
For rough mental conversions from Celsius to Fahrenheit, double the Celsius value and add 30. This gives a close approximation for temperatures between 0 and 40 degrees Celsius. For example, 20 degrees Celsius is roughly (20 x 2) + 30 = 70 degrees Fahrenheit (the exact answer is 68). Going the other direction, subtract 30 and divide by 2. Remember that -40 is the only temperature that reads the same on both Celsius and Fahrenheit scales, which serves as a useful anchor point.
Multiply by 9/5 (or 1.8) and add 32. For example, 25 C = (25 x 1.8) + 32 = 77 F.
Subtract 32, then multiply by 5/9. For example, 72 F = (72 - 32) x 5/9 = 22.2 C.
Absolute zero is the lowest possible temperature (0 K, -273.15 C, -459.67 F) where all molecular motion ceases. It has never been fully achieved in a laboratory.
At -40 degrees. Both -40 C and -40 F are exactly the same temperature.
Kelvin is the scientific standard. It starts at absolute zero, uses the same degree size as Celsius, and eliminates negative values, simplifying thermodynamic calculations.