Antenna Range

There are some calculators available online. I linked to them a while back. Link...

The key thing you'll need to know is the transmit power and the recieve sensitivty of the radios. (only if you want to calculate your actual link strength)

If you are trying to compare 2 antenna, dBi should be directly comparable. i.e. an omni with 15 dBi will have roughly the same range as a directional with 15 dBi. The areas of coverage will be significantly different though. If I remember right, for each 3 dBi of gain the range will roughly double. A 18 dBi antenna will have twice the range as a 15 dBi antenna.