So, if you add a gain of 3 dB to a power of 10 dBm, you obtain 13 dBm. ... You can't add dBm to dBm. If, for example, you have a circuit or a device that combines the power of one signal of 10 dBm (10 mW) with the power of another signal of 13 dBm (20 mW), the result is 10 mW + 20 mW = 30 mW which is 14.8 dBm.