convert diameter to circumference Formula required

[mnmhenry]

Can anyone help with a formula that will convert diameter to circumference (and vice versa). I'm after something simple so, say A1 is to be Diam and B1 is to hold formula and display answer.

Thanks again..
...Mark

 

[IML]

if Diameter is in a1, your formula for Circumerence would be =pi()*a1