Most recipe disasters are conversion problems wearing a chef’s hat. A cup is not a unit of weight, a fan oven is not the temperature on the dial, and doubling a recipe does not double the cooking time. Each of those catches people who can cook perfectly well.
Cups measure space, not substance
This is the one that ruins baking. A cup is a volume — about 240 ml. What that weighs depends entirely on what you put in it, and on how you put it there.
A cup of water is 240 g. A cup of granulated sugar is around 200 g. A cup of plain flour is somewhere between 120 and 150 g depending on whether it was scooped, spooned or sifted — and that spread is up to 25% of the flour in the recipe. In bread or cake, where the ratio of flour to liquid decides the entire result, that is the difference between a good loaf and a brick.
For baking, weigh things. Kitchen scales remove the problem completely, which is why nearly every professional and European recipe is written by weight. For cooking, where a bit more onion harms nothing, cups are fine. The cooking measurement converter handles the per-ingredient densities rather than pretending one number fits all.
Two smaller traps live here too. American and British tablespoons differ — 15 ml against 17.7 ml for the old imperial one — and Australian tablespoons are 20 ml, a full third larger. And a US pint is 473 ml where an imperial pint is 568 ml, so a recipe calling for a pint of stock means noticeably different amounts either side of the Atlantic.
Oven temperatures, and the fan adjustment
The common conversions are worth knowing by heart: 180 °C is 350 °F is gas mark 4, and that is the default for most baking. 200 °C is 400 °F is gas mark 6. 160 °C is 325 °F is gas mark 3.
A fan oven runs hotter than the number suggests. Moving air transfers heat faster, so the standard adjustment is to set a fan oven about 20 °C below a conventional recipe — 180 °C conventional becomes 160 °C fan. Ignoring this is why things brown too fast on the outside while staying raw in the middle.
Gas marks are their own scale, not linear with either Celsius or Fahrenheit, so they have to be looked up rather than calculated. The oven temperature converter covers all four, including the fan adjustment.
Worth knowing regardless: most domestic ovens are out by 10 to 20 degrees, and many drift further with age. An oven thermometer costs very little and is the only way to find out what your oven actually does.
Scaling a recipe: what changes and what does not
Ingredients scale linearly. Double everything and you have twice the mixture. The recipe scaler does the arithmetic including the awkward fractions.
Three things do not scale, and this is where doubled recipes go wrong.
Cooking time. A cake twice the size does not take twice as long; it takes somewhat longer, because heat has to travel further inward while the surface is already cooking. Doubling the time burns the outside. Add a modest amount and test for doneness rather than trusting the clock.
Pan size. Tins scale by area, not by width. A 20 cm round tin has roughly 314 cm² of base; a 25 cm one has 491 cm² — a 25% wider tin holds about 56% more. Doubling a recipe needs a tin with double the area, which is only about 40% wider.
Strong seasonings. Salt scales, but chilli, spices and raising agents often do not scale cleanly, and a doubled quantity of baking powder can leave a metallic taste. Scale them, then taste.
Roasting times and the only reliable test
Roasting times are usually quoted per unit of weight, plus a fixed extra — something like 20 minutes per 500 g plus 20 minutes. That gets you into the right region, and no further.
Internal temperature is the only thing that actually tells you when meat is done. Weight-based timings assume a shape, a starting temperature and an accurate oven, and all three vary. A thermometer in the thickest part, away from bone, settles it — and resting matters too, since a large roast keeps rising several degrees after it leaves the oven.
The cooking time calculator gives both the estimate and the target internal temperatures. Use the first to plan the meal and the second to decide when it is ready.