Decibel Calculator (dB, dBm, Watts)
Convert power and voltage ratios to dB, dBm to watts, and add decibel levels
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Converts power and voltage ratios to decibels and back, converts dBm, dBW, dBV, dBu and dBµV to watts and volts, adds several dB levels and sums the gains of a signal chain. Use it for RF signal levels, audio levels and amplifier gain.
How to use
- On "Ratio ↔ dB", type in any of "Decibels", "Power ratio" or "Voltage or amplitude ratio" and the others follow. For an input and an output value, use "Decibels from two values".
- On "dBm, W, V", set the "Impedance" and type a value in one box, such as dBm, watts, volts or dBu.
- On "Add levels", enter the levels separated by commas or new lines to see the "Combined level".
- On "Gain chain", enter the "Input level" and the gain or loss of each stage in dB to see the "Output level".
How it works
- For power ratios, dB = 10 × log10(P2 ÷ P1); for voltage, current or sound pressure ratios, dB = 20 × log10(V2 ÷ V1), because power is proportional to voltage squared.
- Levels are added as powers of unrelated sources: L = 10 × log10(10^(L1/10) + 10^(L2/10) + …). Two identical signals in phase add 6 dB instead of 3 dB.
- Voltages are RMS values. 0 dBu is the voltage that gives 1 mW in 600 Ω, about 0.7746 V, and 0 dBV is 1 V. With the impedance empty, power and voltage are not converted into each other.
Questions and answers
- How do I convert dBm to mW or watts?
- mW = 10^(dBm ÷ 10). 0 dBm is 1 mW, 20 dBm is 100 mW and 30 dBm is 1 W. dBW is the dBm value minus 30.
- How much is 3 dB?
- About 2 times the power (1.995) or about 1.41 times the voltage. About 2 times the voltage is 6 dB, and every 10 dB is 10 times the power.
- What do two 90 dB sources add up to?
- About 93 dB, not 180 dB. Doubling the number of equal sources adds 10 × log10(2) ≈ 3 dB.
- Why do voltage ratios use 20 × log10?
- Power is proportional to voltage squared, and log10(x²) = 2 × log10(x). Across the same impedance, both methods therefore give the same dB value.