Celsius and Fahrenheit use different interval sizes and zero points, while Kelvin shares the Celsius interval size but begins at absolute zero. The direction of the conversion matters.
The common formulas
Fahrenheit equals Celsius multiplied by 9/5 plus 32. Celsius equals Fahrenheit minus 32 multiplied by 5/9. Kelvin equals Celsius plus 273.15 under the standard definition.
Why 20 degrees is not 20 degrees
A temperature interval and a temperature reading are different ideas. A change of 10 Celsius degrees has the same size as a change of 10 Kelvin, but not 10 Fahrenheit degrees.
Use the right context
Cooking, weather, laboratory work, and body-temperature readings may use different scales and precision. Conversion cannot fix a reading taken with poor technique or an unsuitable device.
A worked example
Worked example: 20 Celsius converts to 68 Fahrenheit and 293.15 Kelvin. A change of 10 Celsius degrees is also a change of 10 Kelvin degrees, but equals 18 Fahrenheit degrees.
Use a temperature-reading formula for a reading and a scale-factor comparison for an interval. Do not use conversion to diagnose a measurement problem.
State whether you are converting a reading or a temperature difference. Adding an offset such as 32 applies to a reading, while a change between Celsius and Fahrenheit uses only the scale ratio.
A scale conversion changes the label on a measurement; it does not say whether the measured temperature is suitable, safe, or clinically concerning.
Temperature conversions are reliable when the scale, direction, and distinction between readings and intervals are clear.
COMMON QUESTIONS
Frequently asked questions
Can Kelvin be negative?
Under ordinary thermodynamic use, Kelvin starts at absolute zero and is not used for negative temperatures.
Is a Celsius degree the same size as a Fahrenheit degree?
No. A Celsius degree is larger; 1 C degree equals 1.8 F degrees as an interval.