Because it is a length and so must have units of length.
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just units
A circle with a radius of 5 units has a circumference of 31.42 units.
A circle with a radius of 45 units has a circumference of 282.74 units.
A circle with a radius of 5 units has a circumference of 31.42 units.
A circle with a radius of 3.3 units has a circumference of 20.73 units.