The circumference of a circle is found with the formula C= π*d = 2*π*r. Thus pi equals a circle's circumference divided by its diameter. Plug your numbers into a calculator: the result should be roughly 3.14.
p.s. if you’re using a calculator do don’t just multiply by 3.14. you can press shift and then a button that looks like 10^x that will give you the actual pie value and so you can multiply and get accurate results
