# Noise Figure Calculator

URL: https://speedycalculator.com/calculators/physics/noise-figure/
Last updated: 2026-10-30

> The noise figure calculator finds the total noise figure of up to four cascaded amplifier stages with Friis's formula, the noise figure from input and output SNR, and converts between noise factor and noise figure.

## Inputs

- Calculate
- Stage 1 noise figure (dB)
- Stage 1 gain (dB)
- Stage 2 noise figure (dB)
- Stage 2 gain (dB)
- Stage 3 noise figure (dB)
- Stage 3 gain (dB)
- Stage 4 noise figure (dB)
- Stage 4 gain (dB)
- Input SNR (dB)
- Output SNR (dB)
- Noise factor F (linear)

## Results

- Noise figure
- Noise factor (linear)
- Equivalent noise temperature
- Total gain

## Formula

```
F_total = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁G₂) + … (linear) · NF = 10·log₁₀F · NF = SNR_in − SNR_out (dB)
```

## Explanation

Every amplifier adds noise. The first stage of a receiver dominates the total noise figure, which is why satellite dishes put a low-noise amplifier (LNA) right at the antenna.

### How to use the Noise Figure Calculator

- **Calculate**
- **Stage 1 noise figure (dB)**
- **Stage 1 gain (dB)**
- **Stage 2 noise figure (dB)**
- **Stage 2 gain (dB)**
- **Stage 3 noise figure (dB)**
- **Stage 3 gain (dB)**
- **Stage 4 noise figure (dB)**
- **Stage 4 gain (dB)**
- **Input SNR (dB)**
- **Output SNR (dB)**
- **Noise factor F (linear)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose the mode and enter the values.

### Inputs explained

FieldWhat to enterCalculateChoose one: Cascaded stages (Friis), From input and output SNR (dB), Noise factor ↔ noise figureStage 1 noise figure (dB)A numberStage 1 gain (dB)A numberStage 2 noise figure (dB)A numberStage 2 gain (dB)A numberStage 3 noise figure (dB)A numberStage 3 gain (dB)A numberStage 4 noise figure (dB)A numberStage 4 gain (dB)A numberInput SNR (dB)A numberOutput SNR (dB)A numberNoise factor F (linear)A number

### What each result means

- **Noise factor (linear)** = 10^([Noise figure]÷10)
- **Equivalent noise temperature** = 290×(10^([Noise figure]÷10) - 1)
- **Total gain** = [Stage 1 gain (dB)] + [Stage 2 gain (dB)] + [Stage 3 gain (dB)] + [Stage 4 gain (dB)]

### Example values

How the main result changes when **Stage 1 noise figure** changes and everything else stays at the example values above:

Stage 1 noise figure (dB)Noise figure (dB)0.50.56450.750.81111.05761.251.30441.51.551322.0458

## Worked example

Inputs — Calculate: Cascaded stages (Friis); Stage 1 noise figure (dB): 1; Stage 1 gain (dB): 20; Stage 2 noise figure (dB): 4; Stage 2 gain (dB): 15; Stage 3 noise figure (dB): 8; Stage 3 gain (dB): 10; Stage 4 noise figure (dB): 0; Stage 4 gain (dB): 0.

Results — **Noise figure = 1.0576 dB**; Noise factor (linear) = 1.27572; Equivalent noise temperature = 79.9598 K; Total gain = 45 dB.

## Key terms

- **Friis formula**: Total noise factor of cascaded stages.

## FAQ

**Why does the first stage matter most?**
Later stages' noise is divided by the gain before them.

**What is the difference between noise factor and figure?**
Figure is the factor in decibels.

**What is a good LNA noise figure?**
Below 1 dB.

**What is noise temperature?**
An equivalent way to express noise: T = 290 K × (F − 1).


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)