How to use this calculator
- Enter or choose values are.
- Enter or choose signal.
- Enter or choose noise.
- The sNR updates as you type. Use Copy result or Share to keep it.
Method
SNR(dB) = 10 log₁₀(P_s ÷ P_n) = 20 log₁₀(V_s ÷ V_n); from dBm: SNR = L_s − L_n
- P_s, P_n
- signal and noise power
- V_s, V_n
- signal and noise RMS voltage across the same impedance
- L_s, L_n
- signal and noise levels in dBm
Worked example
- Power ratio = 10 ÷ 0.01 = 1,000.
- SNR = 10 × log₁₀(1,000) = 10 × 3 = 30 dB.
A 10 mW signal over 0.01 mW of noise has an SNR of 30 dB, a power ratio of 1,000.
What the result means
Every 10 dB is a further factor of 10 in power. As a rough guide, voice links need 10–20 dB, CD audio has a theoretical 96 dB, and a positive SNR at least means the signal is stronger than the noise.
Assumptions and limits
- Signal and noise are measured over the same bandwidth; voltages are RMS across the same impedance.
- If the measured “signal” includes noise (signal + noise), the true SNR is slightly lower, noticeably so below about 10 dB.
Questions
Why 20 log for voltage?
Power is proportional to voltage squared, and log(V²) = 2 log V, so 10 log of the power ratio equals 20 log of the voltage ratio.
Can SNR be negative?
Yes. Negative SNR means the noise is stronger than the signal; spread-spectrum systems such as GPS still work below 0 dB.