Why is the noise floor calculated from kT?

The noise floor of a receiver is -174dbm.
but why it is caculated from kT instead of 4kT?
Thanks!

Why 4kT? The 4 is for resistor. Equivalent noise generator: sqrt(4*k*T*R)


kT
k: Boltzmann constant
T: 290 K => 174 dBm

It's apply to B=1 Hz bandwidth.

http://en.wikipedia.org/wiki/Thermal_noise

The available noise power from any resistor is kt = -174dBm/Hz.

The open-circuit noise voltage density for a resistor is: sqrt(4ktR). To deliver the available power to a load the load needs to be a conjagate match. So given a source of value R and a noise-less load of value R the voltage across the load is 1/2 the open circuit noise: 1/2*sqrt(4kTR). What is the power delivered? It is P = V^2/R = (1/2*sqrt(4kTR)^2/R = kT. The noisy resistor delivered kT to the noiseless resistor.

In a radio receiver the antenna can be considered the source and the radio the load. The available noise power that can be delivered is kT - but only to a matched load.