A more thorough explanation:

ACRE

An acre is a unit of measurement used to describe an area of land. It is equal to 43,560 square feet. For example, if you have a rectangular piece of land that is 208 feet wide and 209 feet long, it would be one acre in size.

Another way the term acre is used is to describe a layer of material that is spread over one acre of land. For example, a foot acre is a layer of coal, water, or other material that is one foot deep and covers one acre of land. This method is used to value coal land for tax purposes and to measure the volume and capacity of reservoirs.

Example 1: John inherited a piece of land from his grandfather that is 2 acres in size. He plans to build a house on one acre and use the other acre for farming.

Example 2: The city is planning to build a new reservoir to provide water to the growing population. They need to know the volume and capacity of the reservoir, so they measure it in foot acres.

The examples illustrate how the term acre is used to describe the size of a piece of land and to measure the volume and capacity of reservoirs. In example 1, John inherited a piece of land that is 2 acres in size, which means it is 87,120 square feet. In example 2, the city needs to know the volume and capacity of the reservoir, so they measure it in foot acres, which is a layer of material that is one foot deep and covers one acre of land.