Converts an oven temperature between Fahrenheit and Celsius, finds the nearest UK gas mark, and gives the standard fan-oven adjustment.
Published for review. The Founder is checking every card himself; anything he strikes comes down the same hour.
Try it
Inputs
temperature (number, required) : Oven temperature value
unit (string, required) one of: F, C : Unit of the given temperature
Outputs
celsius (C): Temperature in Celsius
fahrenheit (F): Temperature in Fahrenheit
gas_mark: Nearest UK gas mark from the cited table
fan_oven_celsius (C): Suggested fan (convection) oven temperature, conventional minus 20 C
Steps
If given in Fahrenheit: celsius = (fahrenheit - 32) * 5 / 9. If given in Celsius: fahrenheit = celsius * 9 / 5 + 32.
Round celsius and fahrenheit to the nearest whole degree.
gas_mark is the entry in the cited gas mark table whose Celsius value is nearest to the rounded celsius value.
Fan (convection) ovens run hotter than conventional (static) ovens; the standard adjustment is to reduce the conventional Celsius temperature by about 20 C (or the Fahrenheit temperature by about 25 F) for a fan oven.
fan_oven_celsius = celsius - 20, rounded to the nearest whole degree.
Constants
gas_mark_1_4_c = 110 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_1_2_c = 120 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_1_c = 140 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_2_c = 150 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_3_c = 170 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_4_c = 180 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_5_c = 190 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_6_c = 200 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_7_c = 220 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_8_c = 230 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
gas_mark_9_c = 240 C (source: Standard Fahrenheit/Celsius/gas mark oven temperature table)
Standard Fahrenheit/Celsius/gas mark oven temperature table (Scribner, 2026-09-30T19:55:00Z) - Source of the gas mark to Celsius/Fahrenheit correspondence used here.
Oven Temperature Conversion Table (Which? (Which? Ltd, UK Consumers' Association), 2026-09-30T22:29:13Z) - Confirmed table matches this card exactly: gas mark 4 = 350F/180C/160C fan, gas mark 6 = 400F/200C/180C fan, gas mark 7 = 425F/220C/200C fan; the page also states fan ovens run about 20C cooler than conventional ovens.