These words refer to the total size, sum or extent of something measurable. In general use, quantity suggests that something is considered in its totality, in terms of mass or bulk: a sufficient quantity of food to last a week. Quantity , itself, is that property of a thing that admits of exact measurement. In order to express a quantity in precise terms, it is necessary to divide it into units of some kind, measuring it in terms of magnitude, volume, size, sum, weight or length. A quantity of water in a storage tank, for example, can be measured in terms of gallons. Amount is often very close to quantity in meaning but may differ significantly in its use. It implies that the thing thought of in sum or in the aggregate can be broken down into separate units or parts that can be measured, counted, or otherwise specified: a large quantity of gravel; a limited amount of time; a large amount of money; a considerable amount of work. Amount rather than quantity is the word generally used of immeasurable or intangible things: He went to a considerable amount of trouble. It is quantity , however, and not amount , that is considered in terms of quality, indicating the contrast between bulk or output and the immaterial value of excellence.

When used collectively, number points to a collection of individual things that can be counted, referring to things that are physically or symbolically separate, not merely separable into units. Where amount emphasizes the whole, number focuses on the parts: an amount of money; a number of coins. Whereas quantity stresses measurement in bulk, number stresses individual items: a number of peaches; the quantity of peaches in a bushel case. Number , like quantity , may be either specific or vague. It may refer to an indefinite or unspecified quantity .

• When he moved, a number of his books had to be left behind; A limited number of seats are still available.

Or it may refer to a specific sum or total count of units or individuals: The number of students absent is 6. When number is preceded by "the," it is used with a singular verb: The number of jobs is increasing. When preceded by "a," it is used with a plural verb: A number of graduates plan to apply. Number , when used with "a," and otherwise unqualified, can imply that the amount referred to is relatively large: A number of people signed the petition. Both number and quantity , when used in the plural, can indicate a large amount or group, or a sizeable collection.

• Quantities of surplus materials are available; Numbers of people complained when the proposed shut-down was announced.