What power level does a receiver minimum discernible signal of -100 dBm represent?
Why dBm is power referenced to 1 milliwatt, so 0 dBm = 1 mW = 10^-3 W. Each 10 dB step changes power by a factor of 10, so -100 dBm is 10 decades below 1 mW: 10^-10 mW, which equals 10^-13 W. Since 1 picowatt is 10^-12 W, that works out to 0.1 pW, an extremely weak but typical MDS figure for a good HF receiver.
Watch out The choice of 0.001 microwatt is 1 nanowatt, or 10^-9 W, which is only -60 dBm; it is off by four decades because the count of 10 dB steps was cut short.
0 dBm = 1 mW; every 10 dB moves the decimal one place. -100 dBm = 10 decades down = 0.1 pW.