Celsius vs Fahrenheit
Both scales measure the same thing; they just disagree on two choices. Degree size: Celsius puts 100 degrees between water freezing and boiling, Fahrenheit puts 180, so one Celsius degree equals 1.8 Fahrenheit degrees. Zero point: water freezes at 0 °C but 32 °F. Everything else follows from those two facts.
Where the mental shortcut goes wrong
"Double it and add 30" is popular because it's easy. Slide the probe to see how far it drifts from the exact answer.
Exact 68 °F, shortcut 70 °F: off by 2 °F. The two lines only cross at 10 °C (50 °F); the error grows by 0.2 °F for every degree you move away.
The −40 crossover
Because Fahrenheit degrees are smaller and its zero is lower, the scales approach each other as it gets colder and meet exactly once: −40 °C is −40 °F. Below that, the Fahrenheit number is the lower of the two.
Where each scale came from
Daniel Gabriel Fahrenheit, a German-Dutch instrument maker, published his scale in 1724. Anders Celsius, a Swedish astronomer, proposed his in 1742, originally with 0 at boiling and 100 at freezing; it was flipped to today's direction shortly after his death. Since 1954 both have been defined through the kelvin, the SI unit, rather than through water.
Which feels more natural?
Fans of Fahrenheit like that 0 to 100 °F roughly covers the weather people live in. Fans of Celsius like that 0 and 100 land on water's freezing and boiling points, and that it lines up with Kelvin for science. Neither is more accurate; a thermometer is as precise in one as the other.
Questions
At what temperature are Celsius and Fahrenheit equal?
At −40. Solving C = C × 9/5 + 32 gives C = −40, so −40 °C is −40 °F.
Which is bigger, a Celsius or a Fahrenheit degree?
A Celsius degree. It is 1.8 Fahrenheit degrees, because Celsius spans water's freezing to boiling points in 100 steps and Fahrenheit in 180.
Which countries use Fahrenheit?
The United States and its territories, plus a handful of countries such as the Bahamas, Belize, the Cayman Islands, Palau, and Liberia. Most of the world uses Celsius day to day.