Temperature Converter

How Temperature Scales Work

Temperature conversion is the process of translating a thermal reading from one measurement scale to another. Temperature itself measures the average kinetic energy of particles in a substance. According to the International Bureau of Weights and Measures (BIPM), the kelvin is one of the seven SI base units and is defined by fixing the numerical value of the Boltzmann constant to 1.380649 x 10-23 J/K. Three temperature scales are in common use: Celsius, Fahrenheit, and Kelvin.

Celsius, developed by Anders Celsius in 1742, sets water's freezing point at 0 degrees and boiling point at 100 degrees under standard atmospheric pressure (101.325 kPa). It is the standard scale in virtually every country except the United States. Fahrenheit, proposed by Daniel Gabriel Fahrenheit in 1724, sets water's freezing at 32 degrees and boiling at 212 degrees. According to the National Institute of Standards and Technology (NIST), only the US, the Bahamas, the Cayman Islands, Liberia, Palau, and the Marshall Islands still use Fahrenheit for everyday purposes.

Kelvin is the SI base unit of temperature, starting at absolute zero (0 K = -273.15 C = -459.67 F) where all thermal molecular motion ceases. Kelvin uses the same degree size as Celsius, so differences in kelvin and Celsius are identical. Scientists, engineers, and physicists use Kelvin for thermodynamic calculations, gas laws, and radiation equations where an absolute scale is required. Use our Cooking Converter when you need to translate oven temperatures from US recipes.

Temperature Conversion Formulas

The three core formulas for converting between temperature scales 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
• Fahrenheit to Kelvin: K = (F - 32) x 5/9 + 273.15

Worked example: Convert 72 F (a comfortable room temperature in the US) to Celsius and Kelvin. C = (72 - 32) x 5/9 = 40 x 5/9 = 22.22 C. K = 22.22 + 273.15 = 295.37 K. These formulas were standardized by the BIPM as part of the International System of Units.

Key Terms You Should Know

• Absolute Zero -- The lowest possible temperature (0 K, -273.15 C, -459.67 F) at which all molecular thermal motion ceases. It is a theoretical limit that can be approached but never reached.
• Standard Temperature and Pressure (STP) -- Defined by IUPAC as 0 C (273.15 K) and 100 kPa. Used as a reference point for gas law calculations and scientific comparisons.
• Degree (Celsius vs Fahrenheit) -- One Celsius degree is 1.8 times larger than one Fahrenheit degree. The Fahrenheit scale has 180 divisions between water's freezing and boiling points, while Celsius has 100.
• Thermodynamic Temperature -- Temperature measured on the Kelvin scale, where 0 K is absolute zero. It is the only scale where all values are non-negative.
• Triple Point of Water -- The temperature and pressure at which water exists simultaneously as solid, liquid, and gas: 273.16 K (0.01 C) at 611.657 Pa. It was historically used to define the kelvin.

Temperature Reference Points Compared

The table below provides common temperature reference points across all three scales, useful for everyday conversions and scientific work.

Practical Examples

Weather forecasting: A European weather report says tomorrow's high will be 35 C. Converting: F = 35 x 9/5 + 32 = 95 F. According to the National Weather Service, 95 F with 40% humidity produces a heat index of 99 F, which enters the "Caution" zone. This is why understanding temperature conversions matters for health and safety when traveling internationally.

Cooking across recipe systems: An American recipe calls for preheating the oven to 375 F. Converting: C = (375 - 32) x 5/9 = 190.6 C, which you would round to 190 C. Most European ovens are marked in Celsius. UK recipes sometimes use Gas Mark, where Gas Mark 5 equals 375 F or 190 C. Use our Cooking Converter for volume conversions in the same recipe.

Scientific laboratory: A chemistry experiment requires a reaction temperature of 350 K. Converting: C = 350 - 273.15 = 76.85 C, or F = 76.85 x 9/5 + 32 = 170.33 F. Labs typically use Kelvin because thermodynamic equations require an absolute temperature scale where 0 represents zero thermal energy.

Tips for Quick Temperature Conversions

• The "double and add 30" shortcut: For a rough Celsius-to-Fahrenheit estimate, double the Celsius value and add 30. For example, 20 C: 20 x 2 + 30 = 70 F (actual: 68 F). This works well between 0-35 C.
• Memorize key anchor points: 0 C = 32 F, 10 C = 50 F, 20 C = 68 F, 30 C = 86 F, 37 C = 98.6 F, 100 C = 212 F. Interpolate between these for quick estimates.
• Each Celsius degree = 1.8 Fahrenheit degrees: If the temperature increases by 5 C, it increases by 9 F. This 1.8:1 ratio helps with mental math for small temperature differences.
• For body temperature, know the fever threshold: Normal body temperature is 37 C (98.6 F). A fever is generally defined as 38 C (100.4 F) or above. Hypothermia begins below 35 C (95 F).
• Remember -40 is the crossover: At -40 degrees, Celsius and Fahrenheit scales read the same value. This is the only point where they intersect and a useful mental anchor for very cold temperatures.