A formula is a set way of doing something, often written down as specific instructions or rules to follow. In math and science, formulas are equations that show relationships between different quantities. The formula for the area of a rectangle, A = length × width, tells you exactly how to find the area every time. In chemistry, H₂O is the formula for water, showing that each water molecule contains two hydrogen atoms and one oxygen atom.

Formulas appear everywhere in daily life. A recipe is a formula for making food: it lists exact ingredients and steps to get the same delicious result each time. Baby formula is a carefully designed mixture of nutrients that can substitute for mother's milk. Racing cars compete in different classes based on technical formulas, or sets of rules, about engine size and weight.

The word can also mean a standard or predictable way of doing something, like a formula for writing a mystery story (introduce characters, present a crime, drop clues, reveal the culprit). In this sense, something formulaic follows a familiar pattern, which can be either comforting (because you know what to expect) or boring (because there are no surprises).