Foot and feet have their own rules when they are used in combination with numbers to form expressions for units of measure. You can say a four-foot plank but not a four-feet plank; you can also say a plank four feet (less frequently, four foot) long and a plank four feet six inches long (or four foot six inches long).

But when you combine foot with numbers greater than one to refer to simple distance, use only the plural feet: a ledge 20 feet (not foot) away. At that speed, a car moves 88 feet (not foot) in a second