To convert from Fahrenheit to Celsius in your head you can subtract 30 from the Fahrenheit figure and halve the result. This is only a rough estimate and only to be used for values typically found in weather temperatures.
Fahrenheit is a thermodynamic temperature scale, where the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212°F (at standard atmospheric pressure). This puts the boiling and freezing points of water exactly 180 degrees apart. Therefore, a degree on the Fahrenheit scale is 1/180 of the interval between the freezing point and the boiling point of water. Absolute zero is defined as -459.67°F.
A temperature difference of 1°F is the equivalent of a temperature difference 0.556°C.
Although initially defined by the freezing point of water (and later the melting point of ice), the Celsius scale is now officially a derived scale, defined in relation to the Kelvin temperature scale.
Zero on the Celsius scale (0 °C) is now defined as the equivalent to 273.15 K, with a temperature difference of 1 deg C equivalent to a difference of 1 K, meaning the unit size in each scale is the same. This means that 100 °C, previously defined as the boiling point of water, is now defined as the equivalent to 373.15 K.
The Celsius scale is an interval system but not a ratio system, meaning it follows a relative scale but not an absolute scale. This can be seen because the temperature interval between 20 °C and 30 °C is the same as between 30 °C and 40 °C, but 40 °C does not have twice the air heat energy of 20 °C.
A temperature difference of 1 deg C is the equivalent of a temperature difference 1.8°F.