What is the circumference of a 15 inch diameter circle?

You do pi times 2 times radius

47.1 inches

For this we have to use the formula for the circumference of a circle:

2rπ = 2 x radius x pi

Since 2 x radius is the same as the diameter we can simplify this to give:

d x π = diameter x pi

Now, we input our known value (the diameter) and solve!

15 x π =

15π =

47.1238898

So your answer (correct to 2 decimal places) would be 47.12 inches!