Electronics / Signals
Decibel Calculator (dB, dBm, dBW, dBV, dBu)
Convert between decibels and power or voltage ratios, and between dBm or dBW and watts, or dBV or dBu and volts, in either direction.
Decibel Calculator (dB, dBm, dBW, dBV, dBu): A decibel compares two values on a logarithmic scale: 10 × log10 of a power ratio, or 20 × log10 of a voltage ratio, because power goes with the square of voltage. Units with a reference compare against a fixed value: dBm against 1 mW, dBW against 1 W, dBV against 1 V, and dBu against 0.7746 V. Runs 100% locally in your browser with zero server file uploads.
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Power ratios use 10 × log10, so doubling the power is +3 dB; voltage ratios use 20 × log10, because power goes with the square of voltage, so doubling the voltage is +6 dB. dBm is power against 1 milliwatt (30 dBm = 1 W), dBW against 1 watt, dBV voltage against 1 volt, and dBu against 0.775 volts, the voltage that puts 1 mW into 600 Ω.
Quick reference
Power: +3 dB ≈ 2×, +10 dB = 10×, +20 dB = 100×, −3 dB ≈ half. Voltage: +6 dB ≈ 2×, +20 dB = 10×. Radio power: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W.
Sound levels in dB SPL use 20 µPa as the reference; the sound level meter estimates them with a microphone.
Adding gains and losses
Because decibels are logarithms, gains and losses along a chain simply add: an amplifier of +20 dB after a cable losing 3 dB gives +17 dB overall, a power ratio of about 50.
To find the voltage a divider produces from a signal, see the voltage divider calculator.
How to use it
- Choose what to convert: a power or voltage ratio, or a referenced unit.
- Choose from or to decibels.
- Type the value and read the result.
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Frequently asked questions
What does +3 dB mean?
Double the power, or about 1.41 times the voltage. +10 dB is ten times the power, and +6 dB double the voltage.
How many watts is 20 dBm?
100 mW; 30 dBm is 1 W and 0 dBm is 1 mW.
What is the difference between dBu and dBV?
dBV is against 1 V; dBu against 0.775 V. So +4 dBu, the professional audio line level, is about 1.23 V, and −10 dBV, the consumer line level, is about 0.32 V.
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