FM signals transmit "Line of Sight". There is no way to tell you how far away your signal will be heard based solely on the transmitter output power. Please do not try to order a station that will cover (fill in the blank) Kilometers. This is not the way it is done. Placing your transmitting antenna on a hill without obstruction will allow your station to be heard farther away. If your antenna is too high on a mountain top, the signal may skip over the town in the valley below unless the antenna is designed with "beam tilt". Building a proper radio station is complicated and therefore you may require the services of a broadcast consulting engineer.