[TowerTalk] I need help with VHF link...

[Merlin-7 KI4ILB]

 I am looking for a way to figure out how high a tower needs to be to have a line of sight shot on another tower. In other words I need to know how high 2 antennas need to be, on different towers (VHF) in flat terrain to be able to make a link between the two.

Does anyone know of a web site or such that has the formula to do that...better yet one I could plug the numbers into?
 Thanks
 Joe KI4ILB