Decibel Conversion Calculator

Switch between dB, ratios, watts, dBm, and dBW. See instant answers with a responsive workspace. Download results, inspect formulas, and compare realistic reference values.

Calculator

Choose a decibel conversion family, enter values, and calculate instantly.

Conversion Family

Absolute Input Scale

Measured Value

Reference Value

Absolute Input Value

Decimal Precision

Reset

Plotly Graph

The graph updates after each calculation and helps visualize how decibel values change across ratios or power levels.

Formula Used

Power ratio to decibel
dB = 10 × log₁₀(P₂ / P₁)

Use this when you compare power quantities such as transmitted power, received power, or intensity values.

Amplitude ratio to decibel
dB = 20 × log₁₀(A₂ / A₁)

Use this for voltage, current, pressure, or field quantities when both values share the same impedance condition.

dBm to milliwatts
mW = 10^{(dBm / 10)}

Since dBm is referenced to 1 milliwatt, it is convenient for radio, audio, and instrumentation work.

dBW to watts
W = 10^{(dBW / 10)}

dBW is referenced to 1 watt. You can also convert with dBm = dBW + 30.

How to Use This Calculator

Select the conversion family that matches your problem.
For ratio conversions, enter measured and reference values.
For absolute power conversions, choose the input scale first.
Enter the value you already know.
Choose the decimal precision you want in the results.
Click Convert Now to display the result above the form.
Review the result table and graph.
Download the output as CSV or PDF when needed.

Example Data Table

Scenario	Known Value	Equivalent Result	Note
Power ratio	P₂ / P₁ = 10	10 dB	Ten times more power.
Power ratio	P₂ / P₁ = 0.1	-10 dB	Ten times less power.
Amplitude ratio	A₂ / A₁ = 2	6.0206 dB	Common voltage gain example.
Absolute power	0 dBm	1 mW	Standard reference point.
Absolute power	30 dBm	1 W	Very common RF conversion.
Absolute power	0 dBW	1 W	Watt-based reference point.

Frequently Asked Questions

1) What is a decibel?

A decibel is a logarithmic way to express a ratio. It helps compare very large or very small changes in power, voltage, pressure, or intensity using a compact scale.

2) Why does power use 10 in the formula?

Power is directly proportional to the physical quantity being compared, so the decibel expression uses 10 × log10 of the power ratio.

3) Why does voltage or amplitude use 20 in the formula?

Voltage, current, and similar amplitudes often relate to power by a square law. That is why amplitude ratios use 20 × log10 instead of 10 × log10.

4) What is the difference between dBm and dBW?

dBm is referenced to 1 milliwatt. dBW is referenced to 1 watt. The scales differ by 30 dB, so dBm = dBW + 30.

5) Can decibels be negative?

Yes. A negative decibel value means the measured quantity is smaller than the reference quantity. It does not mean the physical quantity itself is negative.

6) When should I use power ratio conversion?

Use power ratio conversion when both values represent power-related quantities, such as watts, acoustic intensity, or received transmitter power.

7) When should I use amplitude ratio conversion?

Use amplitude ratio conversion for voltage, current, sound pressure, or field quantities when the compared conditions keep impedance consistent.

8) Why is a logarithmic graph useful here?

Decibel relationships span wide ranges. A logarithmic view makes tiny and huge ratios easier to compare, which improves interpretation for engineering and laboratory work.

This single-file calculator is ready to save as decibel_conversion_c.php.