Ratios and proportions are foundational to student understanding across multiple topics in mathematics and science. Students are introduced to fundamental ratio problems in elementary school. In sixth through eigth grade, students focus on ratio and proportional relationships. A ratio is an ordered pair of numbers written a / b where b does not equal 0. A proportion is an equation in which two ratios are set equal to each other. For example, if there is 1 dog and 2 cats you could write the ratio as 1: 2 (for every 1 dog there are 2 cats). Linear equations represent proportional relationships. Proportions are foundational for high school algebra. Proportional and linear functions are almost identical in form. The only difference is the addition of the “​b​” constant to the linear function. Indeed, a proportional relationship is just a linear relationship where ​b​ = 0, or to put it another way, where the line passes through the origin (0, 0)

Ratio and proportions are used in real-life situations, such as determining amounts to be used in recipes or finding the mileage per gallon of gas. In general, ratios and proportions describe relationships between and among quantities. As math advances, they are central to developing concepts and skills related to slope, constant rate of change, and similar figures, all of which are fundamental to algebraic concepts and skills. For example, ratios and proportions are found in geometry and trigonometry, in relationships found in triangles, including trigonometric ones, such as sine, cosine, and tangent, found in later algebraic instruction.

Ratios and Proportions are used a lot in science as well.