Signal to Noise Ratio Calculator
Signal-to-noise ratio (SNR) compares wanted signal power to background noise power.
Enter Values
How to use this calculator
- Enter the signal power in milliwatts (mW).
- Enter the noise power (noise floor) in the same unit, mW.
- Read the SNR in decibels and the equivalent linear signal-to-noise ratio.
How it works
Signal-to-noise ratio (SNR) compares wanted signal power to background noise power. As a plain ratio it is S/N = signal power / noise power. Because signal levels span many orders of magnitude, engineers express SNR on a logarithmic decibel scale: SNR (dB) = 10 x log10(signal power / noise power). Both powers must be in the same unit (mW here), so the unit cancels and only the ratio matters.
Every 10 dB step means a 10x change in power ratio, and roughly every 3 dB means a 2x change. A larger positive SNR means the signal stands well above the noise, allowing higher data rates and fewer errors; an SNR near or below 0 dB means the signal is buried in noise. Note that this power-based form uses the 10 x log10 factor — if you start from a voltage or amplitude ratio instead, use 20 x log10 because power is proportional to voltage squared.
Worked example
Wi-Fi receiver: 0.5 mW signal, 0.002 mW noise. A receiver measures a signal power of 0.5 mW against a noise floor of 0.002 mW. The linear ratio is 0.5 / 0.002 = 250 : 1. SNR (dB) = 10 x log10(250) = 23.98 dB — a healthy margin that supports fast, reliable data rates.
Common mistakes
- Using 20 x log10 with power values. The 20x factor is for voltage or amplitude ratios; for power ratios use 10 x log10.
- Mixing units — entering signal in mW and noise in W. Both inputs must be in the same unit so the ratio is correct.
- Confusing SNR with dBm. dBm is an absolute power level referenced to 1 mW, whereas SNR is a relative ratio between two powers.
Frequently asked questions
What is a good signal-to-noise ratio?
It depends on the system, but higher is better. For Wi-Fi, roughly 20 dB or more is considered good and 40 dB excellent, while below about 10 dB connections become slow and unreliable. Voice and data links each define their own minimum acceptable SNR.
Why do you use 10 x log10 and not 20 x log10?
This calculator takes power values, and SNR in decibels for power is 10 x log10(signal/noise). The 20 x log10 form applies only when you input a voltage or amplitude ratio, because power is proportional to voltage squared, which doubles the log factor.
Can SNR be negative?
Yes. A negative SNR in dB means the noise power is greater than the signal power (a ratio below 1:1), so the signal is buried in noise and usually cannot be recovered without special processing.
Does the unit of power matter?
Only that both inputs use the same unit. Because SNR is a ratio, the unit cancels out, so mW/mW, W/W, or any consistent pair gives the same decibel result.
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Tip: Enter any known values to calculate the remaining results.
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