Thermal Noise Floor Calculator
Calculate a receiver's thermal noise floor and noise power spectral density from the bandwidth, noise figure and temperature. The noise floor is the baseline that any wanted signal must rise above to be received.
Enter Values
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How to use this calculator
- Enter the receiver (or channel) bandwidth in hertz.
- Enter the receiver noise figure in dB (leave at 0 for an ideal receiver) and the temperature in kelvin (290 K is the standard reference).
- Read the noise power density in dBm/Hz and the total noise floor in dBm for that bandwidth.
How it works
Thermal (Johnson-Nyquist) noise power is k·T·B watts, where k = 1.380649×10⁻²³ J/K is Boltzmann's constant. In decibels: N (dBm) = 10·log10(k·T·1000) + 10·log10(B) + NF. The first term is the noise density in dBm/Hz (−174 dBm/Hz at 290 K), 10·log10(B) scales it for bandwidth, and the noise figure NF adds the receiver's own excess noise.
Worked example
Worked example. For B = 1 MHz, NF = 5 dB at T = 290 K: density = 10·log10(1.380649e−23 × 290 × 1000) = −173.98 dBm/Hz; add 10·log10(10⁶) = 60 dB and the 5 dB noise figure to get N = −173.98 + 60 + 5 = −108.98 dBm.
Common mistakes
- Entering bandwidth in kHz or MHz instead of hertz — the 10·log10(B) term needs B in Hz.
- Forgetting the noise figure, which can add several dB and raise the real noise floor above the theoretical −174 dBm/Hz baseline.
- Using −174 dBm/Hz at a temperature other than 290 K; the density shifts with temperature (it is 10·log10(k·T·1000)).
Frequently asked questions
Where does −174 dBm/Hz come from?
It is 10·log10(k·T·1000) at the standard reference temperature T = 290 K, giving about −173.98 dBm/Hz — the thermal noise power in a 1 Hz bandwidth expressed in dBm.
How does the noise floor relate to sensitivity?
Receiver sensitivity ≈ noise floor + the minimum signal-to-noise ratio the demodulator needs. Lower bandwidth and a lower noise figure both push the noise floor down and improve sensitivity.
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Tip: Enter any known values to calculate the remaining results.
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