[glc]

http://www.testeq.com/charts/pwrcnv.lasso

Looks like 18 dBm is between 50 and 100 milliwatts. dBm is power expressed logarithmically, with 0 dBm being 1 milliwatt.

Here's something on dBi.

http://www.radarproblems.com/chapter....dir/c06p6.htm