Noise Figure Calculator

Work out noise figure instantly with clear inputs, formula shown and shareable results.

About Noise Figure Calculator

Friis' formula shows why the first stage dominates: Ftotal = F1 + (F2 - 1)/G1. With 15 dB of first-stage gain, a 6 dB second stage adds only about 0.5 dB to the system figure. The noise floor is -174 dBm/Hz plus 10log10(bandwidth) plus the noise figure.

Formula

Friis cascade

Ftotal = F1 + (F2 - 1)/G1 (linear ratios); noise floor = -174 dBm/Hz + 10log10(BW) + NF

Worked example

With the default values below, Noise Figure Calculator returns Cascaded noise figure of 1.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Stage 1 noise figure: 1.2 dB
Stage 1 gain: 15 dB
Stage 2 noise figure: 6 dB
Bandwidth: 20 MHz

Results

Cascaded noise figure: 1.50 dB
Noise floor: -99.49 dBm
Equivalent noise temperature: 119.6 K
Stage 2 contribution: 0.3000 dB

Step by step

1Values used
2Friis cascade
3Cascaded noise figure= 1.50 dB
4Noise floor= -99.49 dBm
5Equivalent noise temperature= 119.6 K
6Stage 2 contribution= 0.3000 dB

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does the first amplifier matter most?

Its noise is amplified by every following stage, while later stages' noise is divided by the gain ahead of them. That is why the LNA goes at the antenna.

Where does -174 dBm/Hz come from?

It is kTB expressed in dBm for a 1 Hz bandwidth at 290 K, the thermal noise floor of any resistive source at room temperature.