Your Oven Is Lying by Forty Degrees

Set a domestic oven to 180°C and the air inside will cycle roughly between 160 and 205°C, with a mean that is frequently nowhere near 180. A £9 oven thermometer sitting where the food sits is the cheapest improvement available to a home cook.

What four ovens actually did

Oven	Dial	Measured mean	Swing
Fan, 8 yrs old	180°C	172°C	±14°C
Fan, new	180°C	183°C	±9°C
Conventional gas	180°C	218°C	±26°C
Countertop convection	180°C	165°C	±21°C

The gas oven ran nearly forty degrees hot, which is enough to turn a 30-minute bake into a 20-minute one and to explain years of burnt bases. Knowing the offset is worth more than any technique adjustment.

Thermometer at shelf height rather than clipped to a rack: the reading changes by twelve degrees depending on where it sits.

Map the hot spots too

Toast test — a full tray of white bread slices at 200°C for six minutes. The browning pattern is your oven's map, and it is rarely symmetrical.
Rotate at the halfway point — front to back matters more than left to right in most fan ovens.
One tray at a time — two trays in a fan oven cost roughly 15°C at the lower shelf.

Write the offset on a sticker inside the cupboard door. You will not remember it in March.

Preheat longer than you think

The indicator light reports air temperature, and the air reaches temperature long before the walls and shelves do. For bread and pizza give it a further twenty minutes after the light goes out; for cakes the light is close enough. A cast-iron pot needs forty minutes to be genuinely at temperature, which is the single biggest difference between a loaf that springs and one that spreads.