Gas Mark is a numeric temperature scale used on gas ovens and cookers in the United Kingdom, Ireland and several Commonwealth nations.
It is the most common temperature scale used for new gas ovens sold in the UK and the Commonwealth.
Very few UK and Commonwealth ovens are labeled in degrees Celsius or Fahrenheit.
Therefore, cooks in the USA and other countries, with their ovens marked in degrees Celsius or Fahrenheit, need to convert UK recipies from Gas Mark to degrees Celsius or Fahrenheit.
The reverse is also true, cooks in the UK and the Commonwealth need to convert some recipies from degrees Celsius or Fahrenheit to Gas Mark.
You can do both with this handy converter.