A cup is not the same everywhere. Tablespoons and teaspoons, how filling flour changes results, why weighing helps, sticky ingredients and scaling recipes.
The cup in a recipe depends on where the recipe came from. Korean and Japanese recipes generally treat a cup as 200ml, recipes from the United States use roughly 240ml, and older British sources may use yet another value. If a foreign recipe followed exactly gives an odd result, the definition of a cup is a more likely culprit than the ingredients.
A tablespoon is commonly 15ml and a teaspoon 5ml, with three teaspoons to a tablespoon. Eating spoons vary in size and will not match measuring spoons. For salt or baking powder, where a small difference changes the outcome, use proper measuring spoons.
A cup of flour has no fixed weight. Scooping and packing puts in far more than sifting and spooning gently. Bread and cakes that keep coming out dense are often simply carrying too much flour. Spooning into the cup and levelling the top is at least consistent.
For baking, where ratios matter, weighing instead of measuring by volume removes almost all the error. A single scale also saves washing several cups and spoons. Water is close enough that 1ml weighs about 1g, but oil, honey and flour are not, and that is the only thing to remember. A kitchen scale that reads in 1 g steps is enough for most cooking.
Oven dials often differ from the real internal temperature, and even after the preheat signal it takes a few more minutes to settle. Converting a recipe written in Fahrenheit needs a subtraction and a division, so it is safer to check than to do it in your head.
For liquids such as water or milk, set the measuring jug on a flat surface and read the line at eye level. Looking down from above makes it seem like less or more than it is. Sticky ingredients such as honey, syrup or chilli paste leave a lot behind in the spoon, so less goes in than you think; lightly oil the spoon first and they slide off cleanly. If a recipe calls for both oil and honey, measuring the oil first works the same way. Butter often has markings on the wrapper; if not, weighing it is simplest.
Doubling every ingredient to turn a two-serving recipe into four is mostly fine, but a few things do not scale neatly. Taste at about one and a half times the salt and spicy seasoning before adding the full double, and with soups a wider pot changes how quickly liquid reduces, so adjust the water. In baking, changing the size or depth of the tin changes baking time and temperature, so using several tins of the same size gives more reliable results. Hard-to-split ingredients such as eggs can be beaten and divided by weight. If the arithmetic is tedious, a ratio calculator cuts mistakes.
Common measuring mistakes are treating a ladle or dinner spoon as a measuring tool, forgetting to zero the scale after putting the bowl on, and not checking the units in foreign recipes. Ounces (weight) and fluid ounces (volume) in particular sound alike but are different units and easily confused. Measuring is about safety as well as flavour. Too much baking soda or baking powder tastes bitter and can upset the stomach, and baby food and formula must follow the stated ratio exactly. Formula ratios vary by product, so always follow the instructions on the package and ask a paediatrician if you have questions.