Decibel Calculator
Choose the conversion, type the number and see the answer. Power ratios use 10 log, voltage ratios use 20 log, and dBm is relative to a milliwatt.
A ratio is the output divided by the input, for example 2 for doubling. Negative decibels mean a loss.
Worked examples
Doubling power
Power ratio 2
3.0103 dB
10 x log10(2).
One watt
1000 milliwatts
30 dBm
10 x log10(1000).
About Decibel Calculator
The decibel is a way of comparing two levels on a logarithmic scale. It is used for sound, radio signals, audio equipment and cables because the ranges involved are huge. For power the number of decibels is 10 times the base ten logarithm of the ratio. For voltage, current or sound pressure it is 20 times the logarithm, because power goes with the square of voltage. This calculator converts in both directions for both kinds of ratio.
It also handles the dBm scale, which measures power against one milliwatt, so 0 dBm is 1 milliwatt, 30 dBm is 1 watt and minus 30 dBm is 1 microwatt. Pick the conversion, type the number and the answer appears with the formula that was used. A few results are worth remembering: 3 decibels is double the power, 6 decibels is double the voltage, and 10 decibels is ten times the power.
How to use Decibel Calculator
- Choose the conversion, for example power ratio to decibels or dBm to milliwatts.
- Type the value, such as a ratio of 2 or a level of 30 dBm.
- Read the answer and the formula shown underneath.
- Change the conversion to check the reverse direction.
Common uses
- Working out the gain or loss of an amplifier or cable
- Converting radio transmitter power between watts and dBm
- Checking audio level changes in decibels
- Understanding data sheets that quote dB figures
Frequently asked questions
What is the decibel formula?
For power, dB = 10 x log10(P2 / P1). For voltage or amplitude, dB = 20 x log10(V2 / V1). Both give the same number when the voltages are across the same resistance.
What does a minus decibel value mean?
A negative value is a loss: the output is smaller than the input. Minus 3 dB is roughly half the power, and minus 20 dB is one hundredth.
What is dBm?
dBm is power in decibels relative to one milliwatt. 0 dBm is 1 mW, 10 dBm is 10 mW, 20 dBm is 100 mW and 30 dBm is 1 W.
Why use 20 for voltage and 10 for power?
Power is proportional to the square of the voltage across a fixed resistance, and the logarithm of a square is twice the logarithm, so the multiplier doubles.