I have to do this in Dutch because I have no idea what’s it called in English, sorry.
It not really important, and will be used in a project which has little to do with this explanation.
Omtrek = pi * diameter = 2 * pi * straal
Oppervlakte = 1/4 * pi * diameter2 = pi * straal2
(more about circles here)
“omtrek” = circumference
“oppervlakte” = area (read more: circle area)
“diameter” = diameter (diameter)
“straal” = radius
(look more math stuff up at at Math Open Reference)