Decibel Calculator
Convert a power or amplitude ratio to decibels.
Results update as you type.
How it works
The decibel is a logarithmic unit for expressing ratios, especially of power and amplitude, and this calculator converts a ratio into decibels. You enter the measured value and the reference value, choose whether the quantities are power-like or amplitude-like, and it returns the level in decibels. The decibel is not an absolute unit but a comparison: it tells you how many times larger or smaller one value is than another, on a scale that compresses huge ratios into convenient numbers. For power quantities, such as electrical power or sound intensity, the level is ten times the logarithm of the ratio, so a tenfold power increase is ten decibels and a hundredfold is twenty. For amplitude quantities, such as voltage, current, or sound pressure, the factor is twenty instead of ten, because power scales with the square of amplitude, so a tenfold voltage increase is twenty decibels. A positive result indicates a gain, a negative one a loss or attenuation, and zero means the two values are equal. Decibels are ubiquitous in audio, electronics, telecommunications, and signal processing for expressing gain, loss, signal-to-noise ratio, and dynamic range. Choosing the correct power-or-amplitude type is essential, since using the wrong factor doubles or halves the result. Enter the measured value, reference, and ratio type to compute decibels.
dB = 10·log₁₀(P/P₀) or 20·log₁₀(A/A₀)
The decibel expresses a ratio on a logarithmic scale. For power quantities it is ten times the base-10 logarithm of the measured value divided by the reference. For amplitude quantities like voltage or sound pressure it is twenty times that logarithm, because power is proportional to amplitude squared. A positive result is a gain, and a negative one is a loss.
Source:Signal processing
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter measured value, reference value and ratio type.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Level updates instantly as you type — no button to press.
Frequently asked questions
What is a decibel?
A decibel is a logarithmic unit that expresses the ratio between two values, most often of power or amplitude. It is a relative measure, not an absolute one, describing how many times larger or smaller one quantity is than a reference. Its logarithmic nature compresses very large ratios into small, manageable numbers.
Why is there a 10 for power but 20 for amplitude?
Power is proportional to the square of amplitude, such as voltage or sound pressure. Because the logarithm of a squared quantity doubles, converting an amplitude ratio to decibels uses a factor of twenty, while a power ratio uses ten. This keeps decibel values consistent whether you measure power directly or through amplitude.
What does a negative decibel value mean?
A negative decibel value means the measured value is smaller than the reference, indicating a loss or attenuation rather than a gain. For example, a signal reduced to one tenth of its power is minus ten decibels. Zero decibels means the measured and reference values are equal, with no gain or loss.
How much is 3 dB or 6 dB?
A change of about three decibels in power corresponds to roughly doubling or halving the power, and six decibels is about four times. For amplitude, six decibels is a doubling. These round figures are common rules of thumb in audio and electronics for quickly gauging how much a signal has changed.
Are decibels an absolute unit?
Plain decibels express only a ratio, so they are relative. Absolute decibel measurements use a defined reference, indicated by a suffix, such as dBm for milliwatts, dBW for watts, or dB SPL for sound pressure. This calculator computes the ratio; to get an absolute level, use the appropriate reference value as your reference input.