Cups, Spoons and Ovens: Converting Recipes

A cup is not one size

The US customary cup is 236.6 ml, the US legal cup used on nutrition labels is 240 ml, the metric cup used in Australia and many other places is 250 ml, and older British recipes may mean an imperial cup of about 284 ml, while Japanese and Korean measuring cups are usually 200 ml. A recipe that says "1 cup" without saying which can be off by a fifth. Where the recipe comes from is usually the best clue.

Spoons differ less, but they do differ. A tablespoon is 15 ml almost everywhere (the US one is 14.8 ml), but in Australia it is 20 ml, four teaspoons instead of three. A teaspoon is about 5 ml. For small amounts of salt, baking powder or spices this matters more than it seems, since the ratio to flour is what counts.

Weigh what can pack

Liquids convert cleanly by volume, because a millilitre of milk is always a millilitre. Dry ingredients do not. All-purpose flour has a bulk density of roughly 0.53 g per ml, so a US cup holds about 125 g when spooned in and levelled, but flour scooped straight from the bag is compressed and can weigh 20% more. That is enough to make bread dense or a cake dry.

Brown sugar, cocoa, grated cheese and chopped herbs behave the same way. When a recipe gives both weight and volume, trust the weight. When it gives only cups, converting once to grams and writing the number down makes the next attempt repeatable. Remember too that fluid ounces measure volume and ounces measure weight; they match only for water.

Oven temperatures and gas marks

To turn Celsius into Fahrenheit, multiply by 9/5 and add 32: 180 °C is 356 °F, which recipes round to 350 °F. Going back, subtract 32 and multiply by 5/9: 350 °F is about 177 °C. Gas mark 4 corresponds to about 180 °C, and each step up or down is roughly 14 °C (25 °F).

A fan or convection oven moves hot air and cooks faster, so recipes written for a conventional oven are usually reduced by about 20 °C (25 °F) for a fan oven, or the time is shortened. Ovens also run hot or cold by 10 to 20 degrees more often than people expect; an inexpensive oven thermometer shows what yours really does.

Scaling a recipe and changing the tin

Doubling ingredients is simple; changing the tin is where recipes go wrong. Batter depth depends on the tin's area, and area grows with the square of the size. Moving from a 20 cm round tin to a 24 cm one multiplies the area by (24 ÷ 20)² = 1.44, so you need 44% more batter for the same depth. A 20 cm square tin holds about 27% more than a 20 cm round one (400 cm² against 314 cm²).

Baking times do not scale with the amount. A deeper cake needs longer at the same or a slightly lower temperature, while a thinner layer bakes faster. Check early and use the usual tests, such as a skewer coming out clean or the internal temperature, rather than multiplying the time. Eggs are the awkward ingredient to scale: when a calculation asks for 1.5 eggs, beat two and weigh out the share you need.

Small details that change results

Butter in American recipes is often counted in sticks: one stick is half a cup, about 113 g. Egg sizes vary by country, and a US large egg weighs about 57 g in the shell. Some ingredients do not swap one for one: dried yeast is used at roughly half the weight of fresh yeast, and fine salt fills a spoon with more salt than flaky salt does.

None of this needs to be memorised. A kitchen scale, a note of which cup a recipe uses, and a quick conversion before you start solve almost every problem.

• Check which cup the recipe means from where it was written
• Weigh flour, sugar and other dry ingredients when you can
• Convert oven temperatures and reduce about 20 °C for a fan oven
• Scale batter by the tin's area, not its width
• Test for doneness instead of scaling the baking time
• Write your converted gram amounts down for next time