A foot is a unit of measurement for length, width or height, but it can also be used to measure area and volume when it is squared or cubed. The measurement for the foot was originally based on an ancient measurement of people's natural feet. When referring to measurements in feet, most often we mean linear feet, but to avoid confusion with board feet, square feet or cubic feet (and to make it especially clear what kind of feet we are talking about), we specify linear feet.

...
Height is measured in linear feet.

Linear Feet

A linear foot measures exactly 12 inches or 30.48 cm. Linear feet are used to measure the length, width or height of an object, animal or person. Rulers and yardsticks work well for small, straight-edged surfaces. However, a more flexible measuring tape is better for measuring people, animals and larger objects. Convert linear feet to meters by multiplying them by 0.9144.

Board Feet

Board feet, which are abbreviated bd ft, fbm or BF, are used to measure the volume of lumber or boards. A board foot is equal to the volume of a one-linear-foot length of a standard board that measures 12 inches wide by one inch thick, for a total of 144 cubic inches. Calculate the board feet from the dimensions of a stack of lumber by multiplying height, width and length, and then multiplying your total by 12.

Square Feet

Square feet, which are abbreviated ft2 or sq ft, are used to measure the area of an object. One square foot is equal to 144 square inches or 929.0304 square centimeters. To find the area of a square or rectangular object in square feet, measure the object's length and the width, and then multiply the length by the width to find the total area of the object in square feet.

Cubic Feet

Cubic feet are used to measure volume and can be abbreviated ft3, cu ft, or cf. A cubic foot, which holds about 7.4085 U.S. liquid gallons or 6.2288 British imperial gallons, equals 1,728 inches cubed or 28.31685 metric liters. To measure things in cubic feet, measure the height, width and length in linear feet. Convert any extra inches to decimals in linear feet by dividing the inches by 12. For example, if your width is six feet six inches, you need to convert the six inches into linear feet. Do so by dividing the inches by 12. Your width measurement to use when you calculate the volume in linear feet will be 6.5 feet. Multiply the height, width and length by each other to get your volume in cubic feet.