Two scales, two reference systems
Celsius anchors everyday temperatures to water freezing at 0°C and boiling at 100°C under standard atmospheric pressure. Fahrenheit places those same points at 32°F and 212°F.
Because the interval between those points is divided into 100 Celsius degrees but 180 Fahrenheit degrees, one Celsius degree represents a larger temperature change than one Fahrenheit degree.
How conversion works
To convert Celsius to Fahrenheit, multiply by 9/5 and add 32. To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9.
The scales meet at −40, so −40°C and −40°F describe the same temperature.
Which scale should you use?
Most countries and scientific work use Celsius. Fahrenheit remains common for everyday weather and cooking in the United States. Use the scale expected by your audience and always include the degree symbol and scale letter.