Temperature Converter — Celsius, Fahrenheit, Kelvin

How Temperature Conversion Works

Temperature measures the average kinetic energy of particles in a substance. Three scales are in common use: Fahrenheit, Celsius, and Kelvin. Each was developed for different purposes, and understanding all three is essential for science, cooking, weather, and medicine.

Fahrenheit was developed by Daniel Gabriel Fahrenheit in 1724. He set 0°F as the coldest temperature achievable in his laboratory (a brine solution of ice, water, and ammonium chloride) and 96°F as approximate human body temperature. The reference points were later adjusted, landing on 32°F for water's freezing point and 212°F for boiling. The United States and its territories are now the only places that use Fahrenheit for everyday weather and home appliances.

Celsius (formerly centigrade) was designed around water: 0°C is the freezing point and 100°C is the boiling point at sea level. This makes it intuitive for everyday use and universal for science. It is used by virtually every country in the world for weather reporting, medical measurements, and cooking.

Kelvin is the SI base unit of temperature and starts at absolute zero — the lowest theoretically possible temperature, where all molecular motion stops at −273.15°C. Scientists always use Kelvin for thermodynamic calculations because it starts at true zero, making formulas involving temperature ratios valid. The kelvin has the same interval size as Celsius, so converting between them requires only adding or subtracting 273.15.

Key Facts About Temperature

Absolute zero: 0 K (−273.15°C / −459.67°F) is the coldest theoretically possible temperature. It has never been fully achieved; researchers have reached within billionths of a kelvin using laser cooling techniques.
Body temperature: Normal human body temperature is 37°C (98.6°F), but it varies by person, time of day, and measurement site. Oral readings average slightly lower; rectal readings slightly higher. A fever is generally defined as above 38°C (100.4°F).
The Sun's surface: The photosphere (visible surface) of the Sun is approximately 5,500°C (9,932°F). The core reaches about 15 million °C — hot enough for nuclear fusion.
The crossover point: −40° is the only temperature where Fahrenheit and Celsius scales are numerically equal: −40°F = −40°C. This is the algebraic intersection of the two scale equations.

Frequently Asked Questions

How do I convert Celsius to Fahrenheit?

To convert Celsius to Fahrenheit, multiply by 9/5 and then add 32. Formula: °F = (°C × 9/5) + 32. For example, 100°C = (100 × 9/5) + 32 = 180 + 32 = 212°F.

What is the formula for Fahrenheit to Celsius?

To convert Fahrenheit to Celsius, subtract 32 and then multiply by 5/9. Formula: °C = (°F − 32) × 5/9. For example, 98.6°F = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37°C.

What is absolute zero in Celsius and Fahrenheit?

Absolute zero is 0 Kelvin, which equals −273.15°C or −459.67°F. It is the lowest possible temperature, at which all thermal motion stops.

What temperature is the same in Celsius and Fahrenheit?

The temperature −40 is the same in both Celsius and Fahrenheit: −40°C = −40°F. This is the point where the two scales intersect.

How do I convert Celsius to Kelvin?

To convert Celsius to Kelvin, add 273.15. Formula: K = °C + 273.15. For example, 0°C = 273.15 K (the freezing point of water) and 100°C = 373.15 K (the boiling point of water).