Una cantidad es la asignación, usualmente numérica, de una magnitud matemática a una propiedad medible que admite grados de comparación y representa o bien un contaje del número de elementos de un conjunto, o bien el resultado de una medición física de una magnitud. Así las cantidades pueden ser comparadas en términos de "más", "menos" o "igual" (o no ser comparables), y generalmente pueden ser representadas por diferentes sistemas de unidades (la masa se puede medir en kilogramos o en onzas). Un gran número de propiedades pueden ser representadas por cantidades escalares, aunque algunas magnitudes físicas requieren el uso de cantidades vectoriales más complejas. Una cantidad escalar es el valor numérico que resulta de una medición (de una magnitud) que se expresa con números acompañado por unidades, de la forma siguiente: Cantidad = Magnitud x Unidades Por ejemplo: 20 kg, 1 m, 60 s, son resultado de medir las magnitudes masa, longitud y tiempo. Igualmente ciertas magnitudes físicas como la cantidad de movimiento, o la velocidad requieren ser representadas por objetos matemáticos como vectores que no son simplemente valores numéricos.

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Quantity is a property that can exist as a magnitude or multitude. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value in terms of a unit of measurement. Quantity is among the basic classes of things along with quality, substance, change, and relation. Some quantities are such by their inner nature (as number), while others are functioning as states (properties, dimensions, attributes) of things such as heavy and light, long and short, broad and narrow, small and great, or much and little. A small quantity is sometimes referred to as a quantulum. Two basic divisions of quantity, magnitude and multitude, imply the principal distinction between continuity (continuum) and discontinuity. Under the name of multitude come what is discontinuous and discrete and divisible into indivisibles, all cases of collective nouns: army, fleet, flock, government, company, party, people, chorus, crowd, mess, and number. Under the name of magnitude come what is continuous and unified and divisible into divisibles, all cases of non-collective nouns: the universe, matter, mass, energy, liquid, material, animal, plant, tree. Along with analyzing its nature and classification, the issues of quantity involve such closely related topics as the relation of magnitudes and multitudes, dimensionality, equality, proportion, the measurements of quantities, the units of measurements, number and numbering systems, the types of numbers and their relations to each other as numerical ratios. Thus quantity is a property that exists in a range of magnitudes or multitudes. Mass, time, distance, heat, and angular separation are among the familiar examples of quantitative properties. Two magnitudes of a continuous quantity stand in relation to one another as a ratio which is a real number.

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