Celsius vs Fahrenheit Explained
Daniel Fahrenheit built his scale in 1724 around brine (0°F), human body temperature (originally ~96°F), and water's freezing point (32°F). Anders Celsius took a simpler route in 1742: water freezes at 0°C and boils at 100°C — a clean 100-degree span.
The exact formulas
- Celsius to Fahrenheit: °F = °C × 9/5 + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
- Celsius to Kelvin: K = °C + 273.15
Mental math shortcuts
For weather-range temperatures, double the Celsius value and add 30 to get a Fahrenheit estimate within a few degrees (20°C → ~70°F, exact 68°F). Going the other way, subtract 30 and halve. For cooking, memorize the anchors: 180°C = 350°F and 200°C = 400°F cover most recipes.
A curiosity worth remembering: -40 is the only temperature where the two scales agree — -40°C is exactly -40°F.
Temperature changes are different from temperatures
An increase of 10°C is an increase of 18°F, not 50°F. The +32 offset applies when converting a temperature reading, but cancels out when converting a change between two readings. This distinction matters in weather forecasts, laboratory work, and instructions that say to raise or lower a setting by a certain number of degrees.
Kelvin has the same interval size as Celsius but starts at absolute zero: 0 K equals −273.15°C. Kelvin is written without a degree sign. If you are converting a difference rather than an absolute temperature, one kelvin of change equals one degree Celsius of change.