Size of Buoy

I have just had new 5/8", 18ft risers fitted to my fore and aft mooring. I want to calculate the size of buoys that I need to fit. Can anyone help?

Your buoy needs to have a volume of approx 1.5 litres for every kg of weight of your chain. Volume of buoy,if it's spherical, in litres is approx 0.004 times the cube of the radius in cm.

But I'd go for something a little larger

Charles, Thanks for your help.