A cup is not the same volume everywhere. The places measuring usually goes wrong.
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.
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.
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