Use this calculator to find signal-to-noise ratio in linear form and dB from power, amplitude, or already-logarithmic levels.
Advanced options
How to use our Signal-to-Noise Ratio Calculator
- Choose Input type based on what you measured: power, amplitude, or levels already in dB.
- Select Units for entered values that match that mode, such as W or mW for power, V RMS or mV RMS for amplitude, or dB-style units for levels already in dB.
- Enter Signal value (selected unit) and Noise value (selected unit) using the same unit and the same reference.
- If you want voltage-to-power style conversions, enter Impedance (ohms). If you only need SNR itself, the default impedance may not matter.
- Open Advanced options if you want a different Number display format, extra unit checks, or a warning threshold with Warn if SNR is below (dB).
- Click Calculate to see SNR (linear ratio) and SNR (decibels).
- Sanity-check the result: if signal is 100 times noise in power, SNR should be 20 dB; if signal is 100 times noise in amplitude, SNR should be 40 dB.
- Use Signal (converted) and Noise (converted) to catch unit mix-ups, and read Notes and warnings for mode, reference, or impedance issues.
Definitions
Signal-to-noise ratio (SNR): A comparison between signal strength and noise strength. Bigger SNR means the useful signal stands out more clearly from the noise.[2]
SNR (linear ratio): The plain ratio signal divided by noise. For example, 100 means the signal is 100 times the noise on the chosen basis.[2]
SNR (decibels): The same idea shown on a logarithmic dB scale. A positive value means signal is stronger than noise, 0 dB means equal, and a negative value means signal is weaker.[3]
Power: A quantity such as watts or milliwatts. Power ratios use the 10 log rule when converted to dB.[3]
Amplitude: A size measurement such as V RMS. Amplitude ratios use the 20 log rule when both measurements are across the same impedance.[3]
Levels already in dB: Inputs that are already logarithmic, such as dBm, dBV, dBu, or generic dB. When both levels use the same reference, SNR in dB is found by subtraction.[3]
Impedance (ohms): The load resistance used when converting between voltage-based and power-based quantities. It matters for conversions, not for a same-type ratio by itself.
Common mistakes and quick fixes
Mistake: Choosing Input type as Power but entering volts in Signal value (selected unit) and Noise value (selected unit) .
Fix: Switch Input type to Amplitude or change Units for entered values to a power unit that matches your measurements.
Mistake: Entering signal in mW and noise in W while both fields say selected unit .
Fix: Set one shared choice in Units for entered values and convert both numbers to that same unit before calculating.
Mistake: Using 0 or a negative number for Noise value (selected unit) in a ratio-based calculation.
Fix: Enter a noise value greater than 0. The calculator needs positive signal and noise values to compute SNR (linear ratio) and SNR (decibels) .
Mistake: Forgetting that Levels already in dB mode expects the same reference for both entries, like both in dBm or both in dBV.
Fix: Make sure Signal value (selected unit) and Noise value (selected unit) use the same reference under Units for entered values .
Mistake: Expecting Impedance (ohms) to change the SNR result in a simple same-type ratio.
Fix: Use Impedance (ohms) mainly for conversion help in Signal (converted) and Noise (converted) , not to change the basic ratio when signal and noise are already comparable.
Mistake: Thinking a negative SNR (decibels) must be an error.
Fix: It can be valid. A negative dB result means the signal is weaker than the noise on the chosen basis.
Limitations & Key Assumptions / Boundary Conditions
- The calculator assumes Signal value (selected unit) and Noise value (selected unit) describe the same kind of measurement and the same reference system.
- In Amplitude mode, the 20 log rule is appropriate only when both amplitudes are measured across the same impedance.
- In Levels already in dB mode, subtraction is valid only when signal and noise use the same dB reference, such as both dBm or both dBV.
- Impedance (ohms) affects optional conversions between voltage and power units, but it does not change the basic SNR if the ratio is already formed from matching quantities.
- Conversions that depend on impedance need an impedance greater than 0. If impedance is missing or invalid, SNR may still be computed while some converted outputs are unavailable.
- Very large or very small ratios can lead to rounded display values, especially when scientific notation formatting is selected.
- This tool reports mathematical SNR from the values you enter. Real measurements can differ because of bandwidth, averaging method, detector type, instrument noise floor, or non-RMS measurements.
Methodology
Core idea
SNR compares signal to noise as a ratio.[2] The calculator first checks that the chosen mode matches the entered unit style, then computes the ratio in linear form and in dB.
Formulas used
SNR_linear = signal / noise
SNR_dB = 10 * log10(P_signal / P_noise)
SNR_dB = 20 * log10(A_signal / A_noise)
SNR_dB = L_signal_dB - L_noise_dB
When to use 10 or 20
Use the 10 log rule for power quantities such as W, mW, or dBm.[3] Use the 20 log rule for amplitude quantities such as V RMS when both values are measured across the same impedance, because power is proportional to amplitude squared.[3]
Direct dB mode
If your inputs are already in dB and share the same reference, the dB SNR is just signal level minus noise level.[3] After that, the linear ratio is recovered from the dB result.
SNR_linear = 10^(SNR_dB / 10)
Conversions used for helper outputs
The extra conversion cards are only for checking units. They do not replace the main SNR formulas.
P_W = P_mW / 1000
P_dBm = 10 * log10(P_mW)
P_mW = 10^(P_dBm / 10)
P_W = V_rms^2 / R
V_rms = sqrt(P_W * R)
dBV = 20 * log10(V_rms / 1)
dBu = 20 * log10(V_rms / 0.7745966692)
dBm = 10 * log10(1000 * V_rms^2 / R)
Mini-example
Suppose Input type is Power, Signal value (selected unit) is 1 W, and Noise value (selected unit) is 0.01 W. The linear SNR is 1 / 0.01 = 100. Then the dB SNR is 10 x log10(100) = 20 dB.[3]
Now suppose Input type is Amplitude, with 1 V RMS signal and 0.01 V RMS noise across the same impedance. The linear ratio is still 100, but the dB result is 20 x log10(100) = 40 dB.[3]
For direct dB levels, if signal is 0 dB and noise is -90 dB with the same reference, then SNR (decibels) is 90 dB, and the linear ratio is 10^(90/10) = 31,622.7766.[3]
Assumptions used by this calculator
The math assumes valid positive linear inputs for ratio-based modes, finite dB inputs for direct dB mode, matching references for dB subtraction, and matching measurement type for signal and noise. If a requested conversion needs impedance, the calculator uses Impedance (ohms) only for that conversion step, not to alter a same-type SNR ratio.