Temperature Converter

Celsius to Fahrenheit: C x 9 / 5 + 32. Fahrenheit to Celsius: (F - 32) x 5 / 9. Kelvin is Celsius shifted by 273.15, with the same step size, so K = C + 273.15. Kelvin has no degree sign because it starts at absolute zero rather than at an arbitrary point.

How do I convert Fahrenheit to Celsius in my head?

Subtract 30 and halve it. For 72 F that gives 21, against a true 22.2. It is within a degree or two across normal weather, which is all anyone needs when reading a foreign forecast.

At what temperature are Fahrenheit and Celsius the same?

Minus 40. It is the only point where the two scales cross, so minus 40 F and minus 40 C are the same temperature, which makes it a useful thing to remember and a poor day to be outside.

Why does Kelvin not use degrees?

Because it is an absolute scale, starting at absolute zero rather than at a chosen reference like the freezing point of water. It measures a quantity rather than a position on a scale, so the convention is 300 K, not 300 degrees K.