Resistor Noise Calculator

Use our resistor noise calculator to find out how much RMS noise signal resistors will generate.

Thermal noise voltage895.0925 nV√(4k_BTRB) — RMS, and present in every resistor whether current flows or not

Noise voltage density895.0925 pV/√Hzthe bandwidth-independent figure, which is how datasheets quote it

Available noise power4.006 fWk_BTB — independent of the resistance entirely, which is why a matched source always delivers the same noise power

In dBm-113.973 dBmthe famous −174 dBm/Hz floor at 290 K, plus 10log₁₀(bandwidth)

Noise floor per hertz-173.97 dBm/Hzat 290.2 K — the reference temperature for noise figures is 290 K, giving −174

Noise current17.9019 nAthe same noise seen as a current through the resistance

Noise factor F1.99526the linear form of a 3 dB noise figure

Equivalent noise temperature288.63 K290(F − 1) — the temperature a resistor would need to produce the same added noise

Output noise power799.2927 fWinput noise × F × gain

Noise added by the amplifier3.987 fWreferred to the input

Two identical stages cascaded3.0216 dBbarely worse than one stage — by Friis's formula the second stage's noise is divided by the first's gain, which is why the FIRST amplifier in a chain determines the system noise figure

At four times the bandwidth1.7902 µVdouble the noise voltage — it goes as the square root of bandwidth, so halving the bandwidth buys only 3 dB

At four times the resistance1.7902 µValso double — and this is why low-noise front ends use low source impedances

The formula

V_n = √(4k_BTRB); NF = 10log₁₀(F)

Noise power does not depend on resistance

Johnson noise voltage grows as the square root of resistance, but the available noise power is k_BTB and contains no resistance at all. A matched source therefore delivers the same noise power whatever its impedance, which is why radio engineers work in noise power and why the −174 dBm/Hz floor at 290 K is a universal reference.

The noise exists whether or not current flows. It is thermal agitation of charge carriers, so it depends only on temperature, resistance and bandwidth — not on the material, the construction or the applied voltage. Only cooling or narrowing the bandwidth reduces it.

Both levers are weak because the relationship is a square root. Halving the bandwidth improves the noise voltage by only 3 dB, and cutting the source resistance fourfold gains 6 dB.

Friis's formula is the design lesson: in a cascade, each stage's noise contribution is divided by the total gain ahead of it. The first amplifier therefore dominates the system noise figure almost entirely, which is why a low-noise preamplifier goes at the antenna and not in the receiver.