Oven temperature converter

British recipes usually give the oven heat in three forms at once, written like "180°C/160°C fan/gas 4" in BBC Good Food's methods.

Each figure belongs to one kind of oven. The first is for a conventional oven, an electric oven that heats from elements at the top and bottom with no fan. The fan figure, here 160°C, is for an oven whose fan moves the hot air, so it sits about 20°C lower. The third is the number on the oven dial of a gas cooker, which needs no conversion. All three describe roughly the same heat, and only the one that matches the oven in use applies. The converter starts from whichever figure the recipe gives and asks which scale it is in. It then shows the same heat as a conventional setting, a fan setting, a gas mark and degrees Fahrenheit. Its results match the pairs that BBC Good Food and Which? print, as the cases under checked against known answers show.

With the starting values, 180°C conventional comes out as gas 4, with 160°C on the fan setting and 356°F beside it.

Gas mark 4. The conversion guide from BBC Good Food and the Which? conversion chart both pair 180°C with that mark, and give 160°C as the fan setting.

A gas mark is a step on a gas oven's dial, not a unit of temperature, and no UK body defines it. Makers' tables space the marks 25°F apart, from 225°F at ¼ to 475°F at 9 (Beko UK, Which?). The two fractional marks, ¼ and ½, sit below mark 1 at the cool end.

The numbered marks go back to the Regulo, a thermostat that Radiation Ltd of Birmingham fitted to its gas ovens. The Science Museum Group holds a Regulo thermostat from 1950-51 in its collection.

BBC Good Food rounds every mark to a tidy Celsius figure, and Which? prints the same values from mark 1 to 9. The last column converts each mark's Fahrenheit definition exactly, to show how far the rounding goes.

Mark 4 is 350°F, exactly 177°C, yet the tables print 180°C. When the recipe gives a mark, the converter shows that exact figure as a row of its own beside the rounded one. Makers do not all agree either. Beko UK's table puts marks 3 and 9 slightly higher than the one above, noting that some products have different gas mark temperature ratings.

Old recipes often name the heat in words instead. BBC Good Food ties each word to one mark, from very cool at ¼ to very hot at 9, while Which? groups the marks into bands.

• Cool: marks 1 and 2, 140°C to 150°C.
• Moderate: marks 3 and 4, 160°C to 180°C.
• Moderately hot: marks 5 and 6, 190°C to 200°C.
• Hot: marks 7 and 8, 220°C to 230°C.
• Very hot: mark 9, 240°C.

Because the fan circulates hot air around the food, which then cooks at a lower setting than in still air (Which?). Fan oven conversion is therefore a subtraction: Which? puts fan temperatures typically around 20°C below conventional ones, and Hoover UK says the same.

That figure is a rule of thumb, not a physical constant or a legal standard. An oven's own manual may give a different gap for a given dish, which is why the converter lets the offset be set anywhere from 0°C to 40°C.

Two designs share the name. In a true fan design, a circular element wrapped around the fan at the back heats the air as it is blown. A fan-assisted oven keeps the usual heating element at the top and another at the bottom, and the fan spreads their heat.

Neither Which? nor Hoover UK gives a separate offset for the two designs, so the converter applies one figure to both. Many ovens offer both settings on the same dial, and the symbols tell them apart.

A fan can shorten a long cooking time as well as lower the setting. Hoover UK says long cooking times can be cut by up to 10 minutes per cooking hour, adding that the food must still be cooked through.

That is a ceiling rather than a fixed saving. For a joint roasting for 2 hours it allows up to 20 minutes less, while a short bake of a quarter of an hour changes very little. Which? says only that fan cooking is quicker and gives no figure.

The converter changes the setting only and leaves the recipe's time as printed. None of its sources gives a time rule that holds for every dish, from baking a sponge to roasting meat.

American recipes give oven heat in degrees Fahrenheit and are written for a conventional oven. In US usage a fan oven is a convection oven, and a recipe meant for one normally says so.

The Fahrenheit scale converts by a fixed formula: °F = °C × 9 ÷ 5 + 32, and back, °C = (°F - 32) × 5 ÷ 9. Worked through, (350 - 32) × 5 ÷ 9 = 176.7, so 350°F in Celsius is about 177°C. The converter reads it as 177°C conventional, 157°C fan and gas 4.

From degrees Celsius, the converter's Fahrenheit figure is the exact conversion. That is why 180°C comes out as 356°F, not the 350°F that defines mark 4: the printed tables round, the formula does not. From a gas mark, it shows the mark's own Fahrenheit value instead.

Older British recipes and cookers may say centigrade, the earlier name of the Celsius scale; the two are the same degrees.

On a gas cooker the marks are the oven's own control, so a recipe given in marks is read straight off the dial.

Most gas ovens have no fan and heat by natural convection, as an electric oven's conventional setting does. Their marks therefore line up with the conventional Celsius column, not the fan one. A recipe's 160°C fan becomes 180°C conventional, and so mark 4 on the gas dial.

Without a fan to mix the air, heat rises, so the top of a gas oven usually runs hotter than the base. For a fan-assisted gas oven, which adds a fan, the sources give no separate rule and the maker's manual is the reference.

The converter reads any temperature to the nearest mark. One exactly halfway between two marks, such as 170°C, goes to the cooler one, mark 3. Anything beyond the ends of the scale is reported as off the dial: 260°C shows as "Above gas mark 9". Some cookers also carry a slow setting, marked S, below mark 1, with no temperature that could be converted.

Each symbol on the dial switches on a set of elements, and only some match a recipe's conventional or fan figure. Which? names the usual settings, and most dials draw them in a similar way.

• Two horizontal lines, top and bottom heat: the upper and lower heating elements with no fan, the conventional setting.
• A fan inside a circle: the true fan setting, with the circular element heating the air the fan blows.
• A fan on its own, or with lines above and below it: the fan spreading heat from the upper and lower elements.
• A single line at the bottom: the lower heat element only. Fan with lower heat adds the fan to it.
• Zigzag lines: full grill, part grill, grill and fan, and grill and lower heat.

Grill settings work by radiant heat from above and have no recipe temperature to convert. Brands draw the symbols in their own way, and the oven's manual is the final word on what each one switches on.

Behind the symbols, the energy label rules for ovens recognise two heating modes (Commission Delegated Regulation (EU) No 65/2014, Article 2). The conventional mode uses natural convection only, and in the fan-forced mode a built-in fan circulates heated air. The energy label states an oven's energy use per cycle in each mode.

An oven's thermostat switches the heat on and off around the set temperature, so the air inside swings either side of the dial.

Ovens can also run hotter or cooler than their dial overall, and the gap may change as an oven ages. An oven thermometer on the shelf shows the heat the food actually meets, independent of the dial. Hot spots, parts of the oven that run hotter than the rest, show up as uneven browning.

An oven temperature conversion only moves a number from one scale to another; it cannot correct a thermostat that runs hot or cold. If the dial runs a few degrees hot, every converted setting runs hot by the same margin.

How the converter's figures were chosen and tested is set out in the methodology, and every table and manual it draws on is listed under sources.