The calculator converts from conducted power (EIRP) at the output of the transmitter in dBm to dBuV/m.
Typically this measurement is made at 3 meters for the purposes of FCC testing. However you can specify any distance in the calculator.
Use this calculator to determine if you will pass the FCC limit test by making a measurement of the transmit power (EIRP) at the RF IN/OUT port of your device.
For example if the FCC limit is specified to be 55 dBuV/m at a 3m distance, then the maximum EIRP is not to exceed -40.2 dBm.
Note that there is usually a frequency dependence to this limit (i.e. the limit will vary with frequency) so the EIRP should be taken account based on this.