Circumference is a linear distance. So if the diameter is in feet, then the circumference is also in feet (not square feet or cubic feet)
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Example:
Let's say we had a circle with diameter d = 10 feet
The circumference is approximately...
C = pi*d
C = 3.14*10
C = 31.4
The circumference is roughly 31.4 feet
The circumference is essentially the distance around the circle's edge. We can use a flexible ruler or a string to help measure out this distance, or you can imagine laying the circle's edge to be placed along a straight line, which is another way to help measure the perimeter.