Calculation of output power of USRP
You should provide a link to a data sheet and a guide to what you are talking about.
If you are using a $US1000+ device you'd usually try and know the sort of thing you are asking before playing with it. Deacronyming SDR may help. Or not. 

FCC regs that apply are unknown as you have given no data on use etc.

Adding an external antenna may affect FCC acceptability deep ending on whether mrp or merp is relevant. 

Datasheet here

0 dBm is 1 mW*.
10 dBm is 1 mW &times; 10(dBm stated/10) higher in power = 10 mW.  


*In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance, while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance.Wikipedia - dBm 


  10 dBm &rarr; 10 mW.
  
  Knowledge base.
  http://www.ettus.com/kb/detail/usrp-b200-and-b210-faq


Pretty pictures

Related:

http://www.law.cornell.edu/cfr/text/47/74.1235

FCC regs: ERP, MERP, wow, yoiks ... 

More yoiks

Agh &amp; More agh




Overview
The USRP B210 provides a fully integrated, single-board, Universal Software Radio Peripheral (USRP™) platform with continuous frequency coverage from 70 MHz – 6 GHz. Designed for low-cost experimentation, it combines a single chip direct-conversion transceiver providing up to 56MHz of real-time bandwidth, an open and reprogrammable Spartan6 FPGA, and fast SuperSpeed USB 3.0 connectivity with convenient bus-power. Full support for the USRP Hardware Driver™ (UHD) software allows you to immediately begin developing with GNU Radio, prototype your own GSM base station with OpenBTS, and seamless transition code from the USRP B210 to higher performance, industry-ready USRP platforms.