Discover

The 'Gas Mark' is a system and unit of marking temperatures on gas ovens and cookers in the United Kingdom and Commonwealth of Nations countries. It is still in use; however, it is not as widespread as it was in the last half of the twentieth century.
The draft 2003 edition of the Oxford English Dictionary lists the earliest known usage of the term as being in L. Chatterton's book "Modern Cookery" published in 1943: "Afternoon tea scones... Time: 20 minutes. Temperature: Gas, Regulo Mark 7".
This particular example also illustrates the term in combination with the word "regulo". This usage pattern is now almost obsolete, but still used by some of the older members of society.
The earliest printed evidence of use of the term "gasmark" (as a single word) appears to date from 1963, however
a search for earlier occurrences, launched jointly by the Oxford English Dictionary and the BBC in May of 2005, hopes to unearth earlier examples.

Conversion table

Gas mark	Celsius	Fahrenheit	Verbal
1/4	105°	225°	Very Slow/Very Low
1/2	120°	250°	Very Slow/Very Low
1	135°	275°	Slow/Low
2	150°	300°	Slow/Low
3	165°	325°	Moderately Slow/Warm
4	175°	350°	Moderate/Medium
5	190°	375°	Moderate/Moderately Hot
6	205°	400°	Moderately Hot
7	220°	425°	Hot
8	230°	450°	Hot/Very Hot
9	245°	475°	Very Hot

Different manufacturers and oven types do vary, so always refer to your cooker instruction book.
In general, the conversions between a Gas Mark temperature $T\_G$ and a Fahrenheit temperature $T\_F$ are given by

and

where the Fahrenheit constant $k\_f$ takes the value $k\_f\; =\; 275$.